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Sökning: WFRF:(de Beck Elvire 1985)

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1.
  • Decin, L., et al. (författare)
  • (Sub)stellar companions shape the winds of evolved stars
  • 2020
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 369:6509, s. 1497-1500
  • Tidskriftsartikel (refereegranskat)abstract
    • Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe.
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2.
  • Danilovich, T., et al. (författare)
  • ATOMIUM: halide molecules around the S-type AGB star W Aquilae
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 655
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. W Aql is a nearby S-type star, with well-known circumstellar parameters, making it an ideal object for in-depth study of less common molecules. Aims. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results. Methods. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of (AlCl)-Cl-35, (AlCl)-Cl-37, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species. Results. AlCl is found in clumps very close to the star, with emission confined within 0 ''.1 of the star. AlF emission is more extended, with faint emission extending 0 ''.2 to 0 ''.6 from the continuum peak. We find peak abundances, relative to H-2, of 1.7 x 10(-7) for (AlCl)-Cl-35, 7 x 10(-8) for (AlCl)-Cl-37, and 1 x 10(-7) for AlF. From the PACS spectra, we find abundances of 9.7 x 10(-8) and <= 10(-8), relative to H-2, for HCl and HF, respectively. Conclusions. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen in the wind.
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3.
  • Gottlieb, C. A., et al. (författare)
  • ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
  • 2022
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 660
  • Tidskriftsartikel (refereegranskat)abstract
    • This overview paper presents atomium, a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of atomium is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (0.025-0.050), medium (0.13-0.24), and low (~1) angular resolution. The sensitivity per ~1.3 km s-1 channel was 1.5-5 mJy beam-1, and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO2, and HCN; secondary molecules include SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result - the wind kinematics of the atomium sources. Our analysis suggests that the atomium sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12CO v = 0 J = 2 - 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The atomium data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models.
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4.
  • Roelfsema, P. R., et al. (författare)
  • In-orbit performance of Herschel-HIFI
  • 2012
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims: In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described.Methods: The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources.Results: The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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5.
  • Danilovich, Taissa, 1987, et al. (författare)
  • Chemical tracers of a highly eccentric AGB–main-sequence star binary
  • 2024
  • Ingår i: Nature Astronomy. - 2397-3366.
  • Tidskriftsartikel (refereegranskat)abstract
    • Binary interactions have been proposed to explain a variety of circumstellar structures seen around evolved stars, including asymptotic giant branch (AGB) stars and planetary nebulae. Studies resolving the circumstellar envelopes of AGB stars have revealed spirals, disks and bipolar outflows, with shaping attributed to interactions with a companion. Here we use a combined chemical and dynamical analysis to reveal a highly eccentric and long-period orbit for W Aquilae, a binary system containing an AGB star and a main-sequence companion. Our results are based on anisotropic SiN emission, the detections of irregular NS and SiC emission towards the S-type star, and density structures observed in the CO emission. These features are all interpreted as having formed during periastron interactions. Our astrochemistry-based method can yield stringent constraints on the orbital parameters of long-period binaries containing AGB stars, and will be applicable to other systems.
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6.
  • De Marco, O., et al. (författare)
  • The messy death of a multiple star system and the resulting planetary nebula as observed by JWST
  • 2022
  • Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 6:12, s. 1421-1432
  • Tidskriftsartikel (refereegranskat)abstract
    • Planetary nebulae—the ejected envelopes of red giant stars—provide us with a history of the last, mass-losing phases of 90% of stars initially more massive than the Sun. Here we analyse images of the planetary nebula NGC 3132 from the James Webb Space Telescope (JWST) Early Release Observations. A structured, extended hydrogen halo surrounding an ionized central bubble is imprinted with spiral structures, probably shaped by a low-mass companion orbiting the central star at about 40–60 au. The images also reveal a mid-infrared excess at the central star, interpreted as a dusty disk, which is indicative of an interaction with another closer companion. Including the previously known A-type visual companion, the progenitor of the NGC 3132 planetary nebula must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionization and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Furthermore, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star with excellent precision: 2.86 ± 0.06 M⊙. These results serve as pathfinders for future JWST observations of planetary nebulae, providing unique insight into fundamental astrophysical processes including colliding winds and binary star interactions, with implications for supernovae and gravitational-wave systems.
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7.
  • Decin, L., et al. (författare)
  • Study of the aluminium content in AGB winds using ALMA Indications for the presence of gas-phase (Al2O3)n clusters
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 608, s. A55-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O 34) can be the potential agents of the broad 11 mu m feature in the SED and in the interferometric data and we propose potential formation mechanisms for these large clusters.
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8.
  • Decin, L., et al. (författare)
  • Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L4-
  • Tidskriftsartikel (refereegranskat)abstract
    • During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of (H2O)-O-16 and the rarer isotopologues (H2O)-O-17 and (H2O)-O-18 in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 x 10(-5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of (CO)-C-12, (CO)-C-13, (SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.
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9.
  • Khouri, Theo, 1985, et al. (författare)
  • Inner dusty envelope of the AGB stars W Hydrae, SW Virginis, and R Crateris using SPHERE/ZIMPOL
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The asymptotic giant branch (AGB) marks the final evolutionary stage of stars with initial masses between ∼0.8 and 8 M⊙. During this phase, stars undergo copious mass loss, which contributes significantly to the enrichment of the interstellar medium. The well-accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the density-enhanced and extended AGB stellar atmospheres. The details of the mass-loss process are not yet well understood, however. For oxygen-rich AGB stars, which are the focus of this study, the dust grains that drive the wind are expected to scatter visible light very efficiently because their sizes are relative large. Aims. We study the distribution of dust in the inner wind of oxygen-rich AGB stars to advance our understanding of the wind-driving process. Methods. We observed light scattered off dust grains that form around three oxygen-rich AGB stars (W Hya, SW Vir, and R Crt) with mass-loss rates between 10-7 and 10-6 M⊙ yr-1 using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL with three filters centred at 0.65, 0.75 and 0.82 μm. We compared the observed morphologies and the spectral dependence of the scattered light between the three sources and determined the radial profile, per image octant, of the dust density distribution around the closest target, W Hya. Results. We find the distribution of dust to be asymmetric for the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be inferred from the ZIMPOL data, but we speculate that it might be the consequence of a circumstellar disc or of the action of strong magnetic fields. The dust grains polarise light more efficiently at 0.65 μm for R Crt and SW Vir and at 0.82 μm for W Hya. This indicates that at the time of the observations, the grains around SW Vir and R Crt had sizes <0.1 μm, while those around W Hya were larger, with sizes ≳0.1 μm. The asymmetric distribution of dust around R Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. We fit radiative transfer models to the radial profile of the polarised light observed around W Hya and find a steep dust density profile, with steepness varying considerably with direction. We find the wind-acceleration region of W Hya to extend to at least ∼7 R∗. This is in agreement with theoretical predictions of wind acceleration up to ∼12 R∗, and highlights that ZIMPOL observations probe the crucial region around AGB stars where dust forms and is accelerated.
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10.
  • Khouri, Theo, 1985, et al. (författare)
  • Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 591
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. On the asymptotic giant branch (AGB) low-and intermediate-mass stars eject a large fraction of their envelope, but the mechanism driving these outflows is still poorly understood. For oxygen-rich AGB stars, the wind is thought to be driven by radiation pressure caused by scattering of radiation off dust grains. Aims. We study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We focus on investigating the spatial distribution of the dust grains that scatter light and whether these grains can be responsible for driving the outflow of this star. Methods. We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study R Dor and its inner envelope in a novel way. We present observations in filters V, cntH alpha, and cnt820 and investigate the surface brightness distribution of the star and of the polarised light produced in the inner envelope. Thanks to second-epoch observations in cntH alpha, we are able to see variability on the stellar photosphere. We study the polarised-light data using a continuum-radiative-transfer code that accounts for direction-dependent scattering of photons off dust grains. Results. We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntH alpha, the filters where molecular opacity is stronger, while in cnt820 the surface brightness is closer to being axisymmetric. The second-epoch observations in cntH alpha show that the morphology of R Dor has changed completely in a timespan of 48 days to a more axisymmetric and compact configuration. This variable morphology is probably linked to changes in the opacity provided by TiO molecules in the extended atmosphere. The observations show polarised light coming from a region around the central star. The inner radius of the region from where polarised light is seen varies only by a small amount with azimuth. The value of the polarised intensity, however, varies by between a factor of 2.3 and 3.7 with azimuth for the different images. We fit the radial profile of the polarised intensity using a spherically symmetric model and a parametric description of the dust density profile, rho(r) = rho(degrees)r(-n). On average, we find exponents of -4.5 +/- 0.5 that correspond to a much steeper density profile than that of a wind expanding at constant velocity. The dust densities we derive imply an upper limit for the dust-to-gas ratio of similar to 2 x 10(-4) at 5.0 R-*. Considering all the uncertainties in observations and models, this value is consistent with the minimum values required by wind-driving models for the onset of a wind, of similar to 3.3 x 10(-4). However, if the steep density profile we find extends to larger distances from the star, the dust-to-gas ratio will quickly become too small for the wind of R Dor to be driven by the grains that produce the scattered light.
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11.
  • Alcolea, J., et al. (författare)
  • HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star
  • 2013
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559, s. (article nr.) A93-
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims.The study of the molecular gas in the circumstellar envelopes of evolved stars is normally undertaken by observing lines ofCO (and other species) in the millimetre-wave domain. In general, the excitation requirements of the observed lines are low at thesewavelengths, and therefore these observations predominantly probe the cold outer envelope while studying the warm inner regions ofthe envelopes normally requires sub-millimetre (sub-mm) and far-infrared (FIR) observational data.Methods.To gain insight into the physical conditions and kinematics of the warm (100–1000 K) gas around the red hyper-giantVY CMa, we performed sensitive high spectral resolution observations of molecular lines in the sub-mm/FIR using the HIFI instru-ment of theHerschelSpace Observatory. We observed CO, H2O, and other molecular species, sampling excitation energies from afew tens to a few thousand K. These observations are part of theHerschelguaranteed time key program HIFISTARS.Results.We detected theJ=6–5,J=10–9, andJ=16–15 lines of12CO and13CO at∼100, 300, and 750 K above the groundstate (and the13COJ=9–8 line). These lines are crucial for improving the modelling of the internal layers of the envelope aroundVY CMa. We also detected 27 lines of H2O and its isotopomers, and 96 lines of species such as NH3,SiO,SO,SO2HCN, OH andothers, some of them originating from vibrationally excited levels. Three lines were not unambiguously assigned.Conclusions.Our observations confirm that VY CMa’s envelope must consist of two or more detached components. The molecularexcitation in the outer layers is significantly lower than in the inner ones, resulting in strong self-absorbed profiles in molecular linesthat are optically thick in this outer envelope, for instance, low-lying lines of H2O. Except for the most abundant species, CO and H2O,most of the molecular emission detected at these sub-mm/FIR wavelengths arise from the central parts of the envelope. The spectrumof VY CMa is very prominent in vibrationally excited lines, which are caused by the strong IR pumping present in the central regions.Compared with envelopes of other massive evolved stars, VY CMa’s emission is particularly strong in these vibrationally excitedlines, as well as in the emission from less abundant species such as H13CN, SO, and NH3.
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12.
  • Andriantsaralaza, Miora, et al. (författare)
  • DEATHSTAR: Nearby AGB stars with the Atacama Compact Array: II. CO envelope sizes and asymmetries: The S-type stars
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 653
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. We aim to constrain the sizes of, and investigate deviations from spherical symmetry in, the CO circumstellar envelopes (CSEs) of 16 S-type stars, along with an additional 7 and 4 CSEs of C-type and M-type AGB stars, respectively. Methods. We map the emission from the CO J = 2-1 and 3-2 lines observed with the Atacama Compact Array (ACA) and its total power (TP) antennas, and fit with a Gaussian distribution in the uv- and image planes for ACA-only and TP observations, respectively. The major axis of the fitted Gaussian for the CO(2-1) line data gives a first estimate of the size of the CO-line-emitting CSE. We investigate possible signs of deviation from spherical symmetry by analysing the line profiles and the minor-to-major axis ratio obtained from visibility fitting, and by investigating the deconvolved images. Results. The sizes of the CO-line-emitting CSEs of low-mass-loss-rate (low-MLR) S-type stars fall between the sizes of the CSEs of C-stars, which are larger, and those of M-stars, which are smaller, as expected because of the differences in their respective CO abundances and the dependence of the photodissociation rate on this quantity. The sizes of the low-MLR S-type stars show no dependence on circumstellar density, as measured by the ratio of the MLR to terminal outflow velocity, irrespective of variability type. The density dependence steepens for S-stars with higher MLRs. While the CO(2-1) brightness distribution size of the low-density S-stars is in general smaller than the predicted photodissociation radius (assuming the standard interstellar radiation field), the measured size of a few of the high-density sources is of the same order as the expected photodissociation radius. Furthermore, our results show that the CO CSEs of most of the S-stars in our sample are consistent with a spherically symmetric and smooth outflow. For some of the sources, clear and prominent asymmetric features are observed which are indicative of intrinsic circumstellar anisotropy. Conclusions. As the majority of the S-type CSEs of the stars in our sample are consistent with a spherical geometry, the CO envelope sizes obtained in this paper will be used to constrain detailed radiative transfer modelling to directly determine more accurate MLR estimates for the stars in our sample. For several of our sources that present signs of deviation from spherical symmetry, further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures. This will provide further insight into the mass-loss process and its related chemistry in S-type AGB stars.
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13.
  • Belitsky, Victor, 1955, et al. (författare)
  • SEPIA - A new single pixel receiver at the APEX telescope
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 612
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. We describe the new Swedish-ESO PI Instrument for APEX (SEPIA) receiver, which was designed and built by the Group for Advanced Receiver Development (GARD), at Onsala Space Observatory (OSO) in collaboration with ESO. It was installed and commissioned at the APEX telescope during 2015 with an ALMA Band 5 receiver channel and updated with a new frequency channel (ALMA Band 9) in February 2016. Aim. This manuscript aims to provide, for observers who use the SEPIA receiver, a reference in terms of the hardware description, optics and performance as well as the commissioning results. Methods. Out of three available receiver cartridge positions in SEPIA, the two current frequency channels, corresponding to ALMA Band 5, the RF band 158-211 GHz, and Band 9, the RF band 600-722 GHz, provide state-of-the-art dual polarization receivers. The Band 5 frequency channel uses 2SB SIS mixers with an average SSB noise temperature around 45 K with IF (intermediate frequency) band 4-8 GHz for each sideband providing total 4 × 4 GHz IF band. The Band 9 frequency channel uses DSB SIS mixers with a noise temperature of 75-125 K with IF band 4-12 GHz for each polarization. Results. Both current SEPIA receiver channels are available to all APEX observers.
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14.
  • Cernicharo, J., et al. (författare)
  • A high-resolution line survey of IRC+10216 with Herschel/HIFI First results: Detection of warm silicon dicarbide (SiC2)
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L8-
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5-636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-to-noise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of similar to 2 x 10(-7) relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of similar to 204 K and a source-averaged column density of similar to 6.4 x 10(15) cm(-2). Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.
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15.
  • Danilovich, Taissa, 1987, et al. (författare)
  • Sulphur-bearing molecules in AGB stars. Part I. The occurrence of hydrogen sulphide
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 606
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Sulphur is a relatively abundant element in the local Galaxy that is known to form a variety of molecules in the circumstellar envelopes of AGB stars. The abundances of these molecules vary based on the chemical types and mass-loss rates of AGB stars. Aims. Through a survey of (sub-) millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H2S in AGB circumstellar envelopes and determine its abundance, where possible. Methods. We surveyed 20 AGB stars with a range of mass-loss rates and different chemical types using the Atacama Pathfinder Experiment (APEX) telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO2, and H2S. Here we present our results for H2S, including detections, non-detections, and detailed radiative transfer modelling of the detected lines. We compared results based on various descriptions of the molecular excitation of H2S and different abundance distributions, including Gaussian abundances, where possible, and two different abundance distributions derived from chemical modelling results. Results. We detected H2S towards five AGB stars, all of which have high mass-loss rates of. M >= 5 x 10(-6) M-circle dot yr(-1) and are oxygen rich. H2S was not detected towards the carbon or S-type stars that fall in a similar mass-loss range. For the stars in our sample with detections, we find peak o-H2S abundances relative to H-2 between 4 x 10(-7) and 2.5 x 10(-5). Conclusions. Overall, we conclude that H2S can play a significant role in oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a key role in stars of other chemical types or in lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H2S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.
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16.
  • De Beck, Elvire, 1985, et al. (författare)
  • Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 523
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims: We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods: The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results: We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 \times 10^-8 M_ȯ yr^-1 up to 8 \times 10^-5 M_ȯ yr^-1. For red supergiants they reach values between 2 \times 10^-7 M_ȯ yr^-1 and 3 \times 10^-4 M_ȯ yr^-1. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ~3 \times 10^-5 M_ȯ yr^-1 for periods exceeding 850 days. Conclusions: Appendices are only available in electronic form at http://www.aanda.org
  •  
17.
  • Decin, L., et al. (författare)
  • ALMA-resolved salt emission traces the chemical footprint and inner wind morphology of VY Canis Majoris
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 592:A76
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. At the end of their lives, most stars lose a significant amount of mass through a stellar wind. The specific physical and chemical circumstances that lead to the onset of the stellar wind for cool luminous stars are not yet understood. Complex geometrical morphologies in the circumstellar envelopes prove that various dynamical and chemical processes are interlocked and that their relative contributions are not easy to disentangle. Aims. We aim to study the inner-wind structure (R
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18.
  • Harwit, M., et al. (författare)
  • Polarisation observations of VY Canis Majoris H2O 532-441 620.701 GHz maser emission with HIFI
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L51-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight. Aims. We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H2O. Methods. In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 km s(-1), which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut fur Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission. Results. We report the first astronomical detection to date of H2O maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings.
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19.
  • Humphreys, E. M., et al. (författare)
  • Simultaneous 183 GHz H2O maser and SiO observations towards evolved stars using APEX SEPIA Band 5
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603, s. A77-
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. The aim is to investigate the use of 183 GHz H2O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to observe the 183 GHz H2O line towards two red supergiant (RSG) and three asymptotic giant branch (AGB) stars. Simultaneously, we observed the J = 4-3 line for (SiO)-Si-28 nu = 0, 1, 2 and 3, and for (SiO)-Si-29 nu = 0 and 1. We compared the results with simulations and radiative transfer models for H2O and SiO, and examined data for the individual linear orthogonal polarizations. Results. We detected the 183 GHz H2O line towards all the stars with peak flux densities >100 Jy, including a new detection from VYCMa. Towards all five targets, the water line had indications of being caused by maser emission and had higher peak flux densities than for the SiO lines. The SiO lines appear to originate from both thermal and maser processes. Comparison with simulations and models indicate that 183 GHz maser emission is likely to extend to greater radii in the circumstellar envelopes than SiO maser emission and to similar or greater radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent blue-shifted feature in the WHya 183 GHz spectrum is amplifying the stellar continuum, and is located at a similar distance from the star as mainline OH maser emission. We note that the coupling of an SiO maser model to a hydrodynamical pulsating model of an AGB star yields qualitatively similar simulated results to the observations. From a comparison of the individual polarizations, we find that the SiO maser linear polarization fraction of several features exceeds the maximum fraction allowed under standard maser assumptions and requires strong anisotropic pumping of the maser transition and strongly saturated maser emission. The low polarization fraction of the H2O maser however, fits with the expectation for a non-saturated maser. Conclusions. 183 GHz H2O masers can provide strong probes of the mass loss process of evolved stars. Higher angular resolution observations of this line using ALMA Band 5 will enable detailed investigation of the emission location in circumstellar envelopes and can also provide information on magnetic field strength and structure.
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20.
  • Khouri, Theo, 1985, et al. (författare)
  • ALMA observations of the vibrationally excited rotational CO transition v=1, J=3-2 towards five AGB stars
  • 2016
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 463:1, s. L74-L78
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the serendipitous detection with the Atacama Large Millimeter/submillimeter Array (ALMA) of the vibrationally excited pure-rotational CO transition, J = 3 - 2 towards five asymptotic giant branch (AGB) stars, Cet, R Aqr, R Scl, W Aql, and pi(1) Gru. The observed lines are formed in the poorly understood region located between the stellar surface and the region where the wind starts, the so-called warm molecular layer. We successfully reproduce the observed lines profiles using a simple model. We constrain the extents, densities, and kinematics of the region where the lines are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate infall of material on to the stars. The line profiles of Cet and R Scl show variability. The serendipitous detection towards these five sources shows that vibrationally excited rotational lines can be observed towards a large number of nearby AGB stars using ALMA. This opens a new possibility for the study of the innermost regions of AGB circumstellar envelopes.
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21.
  • Khouri, Theo, 1985, et al. (författare)
  • An empirical view of the extended atmosphere and inner envelope of the asymptotic giant branch star R Doradus I. Physical model based on CO lines
  • 2024
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 685
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The mass loss experienced on the asymptotic giant branch (AGB) at the end of the lives of low- and intermediate-mass stars is widely accepted to rely on radiation pressure acting on newly formed dust grains. Dust formation happens in the extended atmospheres of these stars, where the density, velocity, and temperature distributions are strongly affected by convection, stellar pulsation, and heating and cooling processes. The interaction between these processes and how that affects dust formation and growth is complex. Hence, characterising the extended atmospheres empirically is paramount to advance our understanding of the dust formation and wind-driving processes. Aims. We aim to determine the density, temperature, and velocity distributions of the gas in the extended atmosphere of the AGB star R Dor. Methods. We acquired observations using ALMA towards R Dor to study the gas through molecular line absorption and emission. We modelled the observed 12CO v = 0, J = 2−1, v = 1, J = 2−1, and 3−2 and 13CO v = 0, J = 3−2 lines using the 3D radiative transfer code LIME to determine the density, temperature, and velocity distributions up to a distance of four times the radius of the star at sub-millimetre wavelengths. Results. The high angular resolution of the sub-millimetre maps allows for even the stellar photosphere to be spatially resolved. By analysing the absorption against the star, we infer that the innermost layer in the near-side hemisphere is mostly falling towards the star, while gas in the layer above that seems to be mostly outflowing. Interestingly, the high angular resolution of the ALMA Band 7 observations reveal that the velocity field of the gas seen against the star is not homogenous across the stellar disc. The gas temperature and density distributions have to be very steep close to the star to fit the observed emission and absorption, but they become shallower for radii larger than ∼1.6 times the stellar sub-millimetre radius. While the gas mass in the innermost region is hundreds of times larger than the mass lost on average by R Dor per pulsation cycle, the gas densities just above this region are consistent with those expected based on the mass-loss rate and expansion velocity of the large-scale outflow. Our fits to the line profiles require the velocity distribution on the far side of the envelope to be mirrored, on average, with respect to that on the near side. Using a sharp absorption feature seen in the CO v = 0, J = 2−1 line, we constrained the standard deviation of the stochastic velocity distribution in the large-scale outflow to be .0.4 km s−1. We characterised two blobs detected in the CO v = 0, J = 2−1 line and found densities substantially larger than those of the surrounding gas. The two blobs also display expansion velocities that are high relative to that of the large-scale outflow. Monitoring the evolution of these blobs will lead to a better understanding of the role of these structures in the mass-loss process of R Dor.
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22.
  • Khouri, Theo, 1985, et al. (författare)
  • Detection of highly excited OH towards AGB stars: A new probe of shocked gas in the extended atmospheres
  • 2019
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 623
  • Tidskriftsartikel (refereegranskat)abstract
    • We characterise the gas in the extended atmospheres of the oxygen-rich asymptotic giant branch (AGB) stars W Hya and R Dor using high angular resolution ALMA observations. Methods. We report the detection and investigate the properties of high-excitation Λ-doubling line emission of hydroxyl (OH). Results. The OH lines are produced very close to the central stars and seem optically thin and with no maser effect. We analyse the molecular excitation using a population diagram and find rotational temperatures of ∼2500 K and column densities of ∼10 19 cm -2 for both sources. For W Hya, we observe emission from vibrationally excited H 2 O arising from the same region as the OH emission. Moreover, CO v = 1, J = 3-2 emission also shows a brightness peak in the same region. Considering optically thin emission and the rotational temperature derived for OH, we find a CO column density ∼15 times higher than that of OH, within an area of (92 × 84) mas 2 centred on the OH emission peak. These results should be considered tentative because of the simple methods employed. The observed OH line frequencies differ significantly from the predicted transition frequencies in the literature, and provide the possibility of using OH lines observed in AGB stars to improve the accuracy of the Hamiltonian used for the OH molecule. We predict stronger OH Λ-doubling lines at millimetre wavelengths than those we detected. These lines will be a good probe of shocked gas in the extended atmosphere and are possibly even suitable as probes of the magnetic field in the atmospheres of close-by AGB stars through the Zeeman effect.
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23.
  • Khouri, Theo, 1985, et al. (författare)
  • High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 620
  • Tidskriftsartikel (refereegranskat)abstract
    • The outflows of oxygen-rich asymptotic giant branch (AGB) stars are thought to be driven by radiation pressure due to the scattering of photons on relatively large grains, with sizes of tenths of microns. The details of the formation of dust in the extended atmospheres of these stars and, therefore, the mass-loss process, is still not well understood. Aims. We aim to constrain the distribution of the gas and the composition and properties of the dust grains that form in the inner circumstellar environment of the archetypal Mira variable o Cet. Methods. We obtained quasi-simultaneous observations using ALMA and SPHERE/ZIMPOL on the Very Large Telescope (VLT) to probe the distribution of gas and large dust grains, respectively. Results. The polarized light images show dust grains around Mira A, but also around the companion, Mira B, and a dust trail that connects the two sources. The ALMA observations show that dust around Mira A is contained in a high-gas-density region with a significant fraction of the grains that produce the polarized light located at the edge of this region. Hydrodynamical and wind-driving models show that dust grains form efficiently behind shock fronts caused by stellar pulsation or convective motions. The distance at which we observe the density decline (a few tens of au) is, however, significantly larger than expected for stellar-pulsation-induced shocks. Other possibilities for creating the high-gas-density region are a recent change in the mass-loss rate of Mira A or interactions with Mira B. We are not able to determine which of these scenarios is correct. We constrained the gas density, temperature, and velocity within a few stellar radii from the star by modelling the CO v = 1, J = 3-2 line. We find a mass (~3.8 ± 1.3) × 104 M to be contained between the stellar millimetre photosphere, R338 GHz, and 4 R338 GHz. Our best-fit models with lower masses also reproduce the 13CO v = 0, J = 3-2 line emission from this region well. We find TiO2 and AlO abundances corresponding to 4.5% and <0.1% of the total titanium and aluminium expected for a gas with solar composition. The low abundance of AlO allows for a scenario in which Al depletion into dust happens already very close to the star, as expected from thermal dust emission observations and theoretical calculations of Mira variables. The relatively large abundance of aluminium for a gas with solar composition allows us to constrain the presence of aluminium oxide grains based on the scattered light observations and on the gas densities we obtain. These models imply that aluminium oxide grains could account for a significant fraction of the total aluminium atoms in this region only if the grains have sizes 0.02 μm. This is an order of magnitude smaller than the maximum sizes predicted by dust-formation and wind-driving models. Conclusions. The study we present highlights the importance of coordinated observations using different instruments to advance our understanding of dust nucleation, dust growth, and wind driving in AGB stars.
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24.
  • Khouri, Theo, 1985, et al. (författare)
  • Imaging the dust and the gas around Mira using ALMA and SPHERE/ZIMPOL
  • 2018
  • Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 14:S343, s. 31-35
  • Konferensbidrag (refereegranskat)abstract
    • The mass-loss mechanism of asymptotic giant branch stars has long been thought to rely on two processes: Stellar pulsations and dust formation. The details of the mass-loss mechanism have remained elusive, however, because of the overall complexity of the dust formation process in the very dynamical pulsation-enhanced atmosphere. Recently, our understanding of AGB stars and the associated mass loss has evolved significantly, thanks both to new instruments which allow sensitive and high-angular-resolution observations and the development of models for the convective AGB envelopes and the dust formation process. ALMA and SPHERE/ZIMPOL on the VLT have been very important instruments in driving this advance in the last few years by providing high-angular resolution images in the sub-mm and visible wavelengths, respectively. I will present observations obtained using these instruments at the same epoch (2.5 weeks apart) of the AGB star Mira that resolve even the stellar disk. The ALMA data reveals the distribution and dynamics of the gas around the star, while the polarised light imaged using SPHERE shows the distribution of the dust grains expected to drive the outflows. Moreover, the observations show a central source surrounded by asymmetric distributions of gas and dust, with complementary structures seen in the two components. We model the observed CO v = 1, J = 3-2 line to determine the density, temperature and velocity of gas close to the star. This model is then used to estimate the abundance of AlO. Our results show that only a very small fraction of aluminium (‰20.1%) is locked in AlO molecules. We also calculate models to fit the observed polarised light based on the gas densities we find. The low level of visible-light polarisation detected using ZIMPOL implies that, at the time of the observations, aluminium atoms are either not efficiently depleted into dust or the aluminium-oxide grains are relatively small (‰20.02μm).
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25.
  • Maercker, Matthias, 1979, et al. (författare)
  • Investigating dust properties in AGB wind-ISM interaction regions
  • 2022
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 663
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. In this paper, we aim to constrain the dust mass and grain sizes in the interaction regions between the stellar winds and the interstellar medium (ISM) around asymptotic giant branch (AGB) stars. By describing the dust in these regions, we aim to shed light on the role of evolved low-mass stars in the origin of dust in galaxies. Methods. We use images in the far-infrared (FIR) at 70 and 160 μm to derive dust temperatures and dust masses in the wind-ISM interaction regions around a sample of carbon-rich and oxygen-rich AGB stars. The dust temperature and mass are determined in two ways: first, directly from the data using the ratio of the measured fluxes and assuming opacities for dust with a constant grain size of 0.1 μm, and then using three-dimensional dust-radiative transfer models spatially constrained by the observations. Each of the radiative transfer models contains one constant grain size, which is varied between 0.01 and 5.0 μm. Results. We find that the observed dust mass in the wind-ISM interaction regions is consistent with mass accumulated from the stellar winds. For the carbon-rich sources, adding the spatial constraints in the radiative transfer models results in preferentially larger grain sizes (-2 μm). For the oxygen-rich sources, the spatial constraints result in overly high temperatures in the models, making it impossible to fit the observed FIR ratio irrespective of the grain size used, indicating a more complex interplay of grain properties and the stellar radiation field. Conclusions. Our results have implications for how likely it is for the grains to survive the transition into the ISM, and the properties of dust particles that later act as seeds for grain growth in the ISM. However, our results for the oxygen-rich sources show that the derivation of dust properties is not straight forward, requiring more complex modelling.
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26.
  • Maercker, Matthias, 1979, et al. (författare)
  • Probing the dynamical and kinematical structures of detached shells around AGB stars
  • 2024
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 687
  • Tidskriftsartikel (refereegranskat)abstract
    • Context . The chemical evolution of asymptotic giant branch (AGB) stars is driven by repeated thermal pulses (TPs). The duration of a TP is only a few hundred years, whereas an inter-pulse period lasts 104−105 yr. Direct observations of TPs are hence unlikely. However, the detached shells seen in CO line emission that are formed as a result of a TP provide indirect constraints on the changes experienced by the star during the pulse. Aims. We aim to resolve the spatial and kinematic sub-structures in five detached-shell sources to provide detailed constraints for hydrodynamic models that describe the formation and evolution of the shells. Methods . We used observations of the 12CO (1−0) emission towards five carbon-AGB stars with ALMA (Atacama Large Millimeter/submillimeter Array), including previously published observations of the carbon AGB star U Ant. The data have angular resolutions of 000 .3 to 100 and a velocity resolution of 0.3 km s−1. This enabled us to quantify spatial and kinematic structures in the shells. Combining the ALMA data with single-dish observations of the 12CO (1−0) to 12CO (4−3) emission towards the sources, we used radiative transfer models to compare the observed structures with previous estimates of the shell masses and temperatures. Results . The observed emission is separated into two distinct components: a more coherent, bright outer shell and a more filamentary, fainter inner shell. The kinematic information shows that the inner sub-shells move at a higher velocity relative to the outer sub-shells. The observed sub-structures reveal a negative velocity gradient outwards across the detached shells, confirming the predictions from hydrodynamical models. However, the models do not predict a double-shell structure, and the CO emission likely only traces the inner and outer edges of the shell, implying a lack of CO in the middle layers of the detached shell. Previous estimates of the masses and temperatures are consistent with originating mainly from the brighter subshell, but the total shell masses are likely lower limits. Also, additional structures in the form of partial shells outside the detached shell around V644 Sco, arcs within the shell of R Scl, and a partially filled shell for DR Ser indicate a more complicated evolution of the shells and mass-loss process throughout the TP cycle than previously assumed. Conclusions . The observed spatial and kinematical splittings of the shells appear consistent with results from the hydrodynamical models, provided the CO emission does not trace the H2 density distribution in the shell but rather traces the edges of the shells. The hydrodynamical models predict very different density profiles depending on the evolution of the shells and the different physical processes involved in the wind-wind interaction (e.g. heating and cooling processes). It is therefore not possible to constrain the total shell mass based on the CO observations alone. Additional features outside and inside the shells complicate the interpretation of the data. Complementary observations of, for example, CI as a dissociation product of CO would be necessary to understand the distribution of CO compared to H2, in addition to new detailed hydrodynamical models of the pre-pulse, pulse, and post-pulse wind. Only a comprehensive combination of observations and models will allow us to constrain the evolution of the shells and the changes in the star during the thermal-pulse cycle.
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27.
  • Maercker, Matthias, 1979, et al. (författare)
  • Properties of dust in the detached shells around U Antilae, DR Serpentis, and V644 Scorpii
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 620
  • Tidskriftsartikel (refereegranskat)abstract
    • Asymptotic giant branch (AGB) stars experience strong mass loss driven by dust particles formed in the upper atmospheres. The dust is released into the interstellar medium, and replenishes galaxies with synthesised material from the star. The dust grains further act as seeds for continued dust growth in the diffuse medium of galaxies. As such, understanding the properties of dust produced during the asymptotic giant branch phase of stellar evolution is important for understanding the evolution of stars and galaxies. Recent observations of the carbon AGB star R Scl have shown that observations at far-infrared and submillimetre wavelengths can effectively constrain the grain sizes in the shell, while the total mass depends on the structure of the grains (solid vs. hollow or fluffy). Aims. We aim to constrain the properties of the dust observed in the submillimetre in the detached shells around the three carbon AGB stars U Ant, DR Ser, and V644 Sco, and to investigate the constraints on the dust masses and grain sizes provided by far-infrared and submm observations. Methods. We observed the carbon AGB stars U Ant, DR Ser, and V644 Sco at 870 μm using LABOCA on APEX. Combined with observations from the optical to far-infrared, we produced dust radiative transfer models of the spectral energy distributions (SEDs) with contributions from the stars, present-day mass-loss and detached shells. We assume spherical, solid dust grains, and test the effect of different total dust masses and grain sizes on the SED, and attempted to consistently reproduce the SEDs from the optical to the submm. Results. We derive dust masses in the shells of a few 10-5Mâ. The best-fit grain radii are comparatively large, and indicate the presence of grains between 0.1 μm and 2 μm. The LABOCA observations suffer from contamination from 12CO (3-2), and hence gives fluxes that are higher than the predicted dust emission at submm wavelengths. We investigate the effect on the best-fitting models by assuming different degrees of contamination and show that far-infrared and submillimetre observations are important to constrain the dust mass and grain sizes in the shells. Conclusions. Spatially resolved observations of the detached shells in the far-infrared and submillimetre effectively constrain the temperatures in the shells, and hence the grain sizes. The dust mass is also constrained by the observations, but additional observations are needed to constrain the structure of the grains.
  •  
28.
  • Menten, K. M., et al. (författare)
  • Herschel/HIFI deepens the circumstellar NH3 enigma
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L7-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Circumstellar envelopes (CSEs) of a variety of evolved stars have been found to contain ammonia (NH3) in amounts that exceed predictions from conventional chemical models by many orders of magnitude. Aims. The observations reported here were performed in order to better constrain the NH3 abundance in the CSEs of four, quite diverse, oxygen-rich stars using the NH3 ortho J(K) = 1(0)-0(0) ground-state line. Methods. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH3 J(K) = 1(0)-0(0) transition near 572.5 GHz, simultaneously with the ortho-H2O J(Ka,Kc) = 1(1,0)-1(0,1) transition, toward VY CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative transfer modeling with the goal to derive the NH3 abundance in these objects' CSEs. For the last two stars, Very Large Array imaging of NH3 radio-wavelength inversion lines were used to provide further constraints, particularly on the spatial extent of the NH3-emitting regions. Results. We find remarkably strong NH3 emission in all of our objects with the NH3 line intensities rivaling those of the ground state H2O line. The NH3 abundances relative to H-2 are very high and range from 2 x 10(-7) to 3 x 10(-6) for the objects we have studied. Conclusions. Our observations confirm and even deepen the circumstellar NH3 enigma. While our radiative transfer modeling does not yield satisfactory fits to the observed line profiles, it does lead to abundance estimates that confirm the very high values found in earlier studies. New ways to tackle this mystery will include further Herschel observations of more NH3 lines and imaging with the Expanded Very Large Array.
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29.
  • O Gorman, Eamon, 1984, et al. (författare)
  • ALMA Observations of Anisotropic Dust Mass Loss around VY CMa
  • 2015
  • Ingår i: 4th ALMA Science Conference on Revolution in Astronomy with ALMA: The Third Year, Tokyo, Japan, 8-11 December. - 9781583818831 ; 499, s. 335-336
  • Konferensbidrag (refereegranskat)abstract
    • We present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, thus allowing us to trace dust on spatial scales down to 11 R-star (71 AU). Two prominent dust components are detected and resolved. We find that at least 17% of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of 5 x 10(-6) M-circle dot yr(-1). The anisotropic morphology of the dust indicates a continuous, directed mass loss over a few decades, suggesting that this mass loss cannot be driven by large convection cells alone.
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30.
  • Planquart, L., et al. (författare)
  • An impressionist view of V Hydrae: When MATISSE paints asymmetric giant blobs
  • 2024
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 687
  • Tidskriftsartikel (refereegranskat)abstract
    • Context . Asymptotic giant branch (AGB) stars enrich the interstellar medium through their mass loss. The mechanism(s) shaping the circumstellar environment of mass-losing stars is not clearly understood so far. Aims. Our purpose is to study the effect of binary companions located within the first 10 stellar radii from the primary AGB star. In this work, we target the mass-losing carbon star V Hydrae (V Hya) and search for signatures of its companion in the dust-forming region of the atmosphere. Methods . The star was observed in the L and N bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of the photosphere and surroundings of V Hya using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used the 1D radiative transfer code DUSTY to model the spectral energy distribution. Results . The star is dominated by dust emission in the L- and N-bands. The MATISSE reconstructed images show asymmetric and elongated structures in both infrared bands. In the L band, we detected an elongated shape of approximately 15 mas that likely is of photospheric origin. In the N band, we found a 20 mas extension northeast from the star and perpendicular to the L-band elongated axis. The position angle and the size of the N-band extension match the prediction of the companion position at the MATISSE epoch. By comparing MATISSE N-band with MIDI data, we deduce that the elongation axis in the N-band has rotated since the previous interferometric measurements 13 yr ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. Conclusions . The MATISSE images unveil the presence of a dust enhancement at the position of the companion. This opens new doors for further analyses of the binary interaction with an AGB component.
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31.
  • Ramstedt, Sofia, et al. (författare)
  • DEATHSTAR: Nearby AGB stars with the Atacama Compact Array I. CO envelope sizes and asymmetries: A new hope for accurate mass-loss-rate estimates
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 640
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. This is the first publication from the DEATHSTAR project. The overall goal of the project is to reduce the uncertainties of the observational estimates of mass-loss rates from evolved stars on the Asymptotic Giant Branch (AGB). Aim. The aim in this first publication is to constrain the sizes of the (CO)-C-12 emitting region from the circumstellar envelopes around 42 mostly southern AGB stars, of which 21 are M-type and 21 are C-type, using the Atacama Compact Array (ACA) at the Atacama Large Millimeter/submillimeter Array. The symmetry of the outflows is also investigated. Methods. Line emission from (CO)-C-12 J = 2 -> 1 and 3 -> 2 from all of the sources were mapped using the ACA. In this initial analysis, the emission distribution was fit to a Gaussian distribution in the uv-plane. A detailed radiative transfer analysis will be presented in a future publication. The major and minor axis of the best-fit Gaussian at the line center velocity of the (CO)-C-12 J = 2 -> 1 emission gives a first indication of the size of the emitting region. Furthermore, the fitting results, such as the Gaussian major and minor axis, center position, and the goodness of fit across both lines, constrain the symmetry of the emission distribution. For a subsample of sources, the measured emission distribution is compared to predictions from previous best-fit radiative transfer modeling results. Results. We find that the CO envelope sizes are, in general, larger for C-type than for M-type AGB stars, which is as expected if the CO/H-2 ratio is larger in C-type stars. Furthermore, the measurements show a relation between the measured (Gaussian) (CO)-C-12 J = 2 -> 1 size and circumstellar density that, while in broad agreement with photodissociation calculations, reveals large scatter and some systematic differences between the different stellar types. For lower mass-loss-rate irregular and semi-regular variables of both M- and C-type AGB stars, the (CO)-C-12 J = 2 -> 1 size appears to be independent of the ratio of the mass-loss rate to outflow velocity, which is a measure of circumstellar density. For the higher mass-loss-rate Mira stars, the (CO)-C-12 J = 2 -> 1 size clearly increases with circumstellar density, with larger sizes for the higher CO-abundance C-type stars. The M-type stars appear to be consistently smaller than predicted from photodissociation theory. The majority of the sources have CO envelope sizes that are consistent with a spherically symmetric, smooth outflow, at least on larger scales. For about a third of the sources, indications of strong asymmetries are detected. This is consistent with what was found in previous interferometric investigations of northern sources. Smaller scale asymmetries are found in a larger fraction of sources. Conclusions. These results for CO envelope radii and shapes can be used to constrain detailed radiative transfer modeling of the same stars so as to determine mass-loss rates that are independent of photodissociation models. For a large fraction of the sources, observations at higher spatial resolution will be necessary to deduce the nature and origin of the complex circumstellar dynamics revealed by our ACA observations.
  •  
32.
  • Saberi, Maryam, 1988, et al. (författare)
  • CO and HCN isotopologue ratios in the outflows of AGB stars
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 638
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the isotopologue ratio in the circumstellar envelope (CSE) may deviate considerably from the stellar atmospheric value. Aims. In this paper, we aim to examine how the (CO)-C-12/(CO)-C-13 and (HCN)-C-12/(HCN)-C-13 abundance ratios vary radially due to chemical reactions in the outflows of asymptotic giant branch (AGB) stars and the effect of excitation and optical depth on the resulting line intensity ratios. We study both carbon-rich (C-type) and oxygen-rich (O-type) CSEs. Methods. We performed chemical modeling to derive radial abundance distributions of our selected species in the CSEs over a wide range of mass-loss rates (10(-8) < M < 10(-4)M(circle dot) yr(-1)). We used these as input in a non-local thermodynamic equilibrium radiative transfer code to derive the line intensities of several ground-state rotational transitions. We also test the influence of stellar parameters, physical conditions in the outflows, the intensity of the interstellar radiation field, and the importance of considering the chemical networks in our model results. Results. We quantified deviations from the atmospheric value for typical outflows. We find that the circumstellar value of (CO)-C-12/(CO)-C-13 can deviate from its atmospheric value by up to 25-94% and 6-60% for C- and O-type CSEs, respectively, in radial ranges that depend on the mass-loss rate. We show that variations of the intensity of the interstellar radiation field and the gas kinetic temperature can significantly influence the CO isotopologue abundance ratio in the outer CSEs of both C-type and O-type. On the contrary, the (HCN)-C-12/(HCN)-C-13 abundance ratio is stable throughout the CSEs for all tested mass-loss rates. The radiative transfer modeling shows that the integrated line intensity ratio (ICO)-C-12/(ICO)-C-13 of different rotational transitions varies significantly for stars with mass-loss rates in the range from 10(-7) to 10(-6)M(circle dot) yr(-1) due to combined chemical and excitation effects. In contrast, the excitation conditions for the HCN isotopologues are the same for both isotopologues. Conclusions. We demonstrate the importance of using the isotopologue abundance profiles from detailed chemical models as inputs to radiative transfer models in the interpretation of isotopologue observations. Previous studies of circumstellar CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar (CO)-C-12/(CO)-C-13 abundance ratio. The use of the HCN molecule as a tracer of C isotope ratios is affected by fewer complicating problems, but we note that the corrections for high optical depths are very large in the case of high-mass-loss-rate C-type CSEs; and in O-type CSEs the (HCN)-C-13 lines may be too weak to detect.
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33.
  • Tafoya Martinez, Daniel, 1981, et al. (författare)
  • (Sub)millimeter emission lines of molecules in born-again stars
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 600
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Born-again stars provide a unique possibility to study the evolution of the circumstellar envelope of evolved stars in human timescales. Up until now, most of the observations of the circumstellar material in these stars have been limited to studying the relatively hot gas and dust. In other evolved stars, the emission from rotational transitions of molecules, such as CO, is commonly used to study the cool component of their circumstellar envelopes. Thus, the detection and study of molecular gas in born-again stars is of great importance when attempting to understand their composition and chemical evolution. In addition, the molecular emission is an invaluable tool for exploring the physical conditions, kinematics, and formation of asymmetric structures in the circumstellar envelopes of these evolved stars. However, up until now, all attempts to detect molecular emission from the cool material around born-again stars have failed. Aims. We searched for emission from rotational transitions of molecules in the hydrogen-deficient circumstellar envelopes of born-again stars to explore the chemical composition, kinematics, and physical parameters of the relatively cool gas. Methods. We carried out observations using the APEX and IRAM 30 m telescopes to search for molecular emission toward four well-studied born-again stars, V4334 Sgr, V605 Aql, A30, and A78, that are thought to represent an evolutionary sequence. Results. For the first time, we detected emission from HCN and (IICN)-C-13 molecules toward V4334 Sgr, and CO emission in V605 Aql. No molecular emission was detected above the noise level toward A30 and A78. The detected lines exhibit broad linewidths greater than or similar to 150 km s(-1), which indicates that the emission comes from gas ejected during the born-again event, rather than from the old planetary nebula. A first estimate of the (HCN)-C-12/(HCN)-C-13 abundance ratio in the circumstellar environment of V4334 Sgr is approximate to 3, which is similar to the value of the C-12/C-13 ratio measured from other observations. We derived a rotational temperature of T-rot = 13 +/- 1 K, and a total column density of N-HCN = 1.6 +/- 0.1 x 10(16) cm(-2) for V4334 Sgr. This result sets a lower limit on the amount of hydrogen that was ejected into the wind during the born-again event of this source. For V605 Aql, we obtained a lower limit for the integrated line intensities I-12CO/I-13CO > 4.
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34.
  • Unnikrishnan Nair, Ramlal, 1997, et al. (författare)
  • Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars: I. ALMA 3 mm spectral surveys
  • 2024
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 684
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Asymptotic giant branch (AGB) stars are major contributors to the chemical enrichment of the interstellar medium through nucleosynthesis and extensive mass loss. Direct measures of both processes can be obtained by studying their circumstellar envelopes in molecular line emission. Most of our current knowledge of circumstellar chemistry, in particular in a C-rich environment, is based on observations of the carbon star IRC +10216. Aims. We aim to obtain a more generalised understanding of the chemistry in C-rich AGB circumstellar envelopes by studying a sample of three carbon stars, IRAS 15194-5115, IRAS 15082-4808, and IRAS 07454-7112, and to observationally test the archetypal status often attributed to IRC +10216. Methods. We performed spatially resolved, unbiased spectral surveys in ALMA Band 3 (85-116 GHz). We estimated the sizes of the molecular emitting regions using azimuthally averaged radial profiles of the line brightness distributions. We derived abundance estimates, using a population diagram analysis for molecules with multiple detected lines, and using single-line analytical calculations for the others. Results. We identify a total of 132 rotational transitions from 49 molecular species. There are two main morphologies of the brightness distributions: centrally peaked (CS, SiO, SiS, HCN) and shell-like (CN, HNC, C2H, C3H, C4H, C3N, HC5N, c-C3H2). The brightness distributions of HC3N and SiC2 have both a central and a shell component. The qualitative behaviour of the brightness distributions of all detected molecules, in particular their relative locations with respect to the central star, is the same for all three stars, and consistent with those observed towards IRC +10216. Of the shell distributions, the cyanopolyynes peak at slightly smaller radii than the hydrocarbons, and CN and HNC show the most extended emission. The emitting regions for each species are the smallest for IRAS 07454-7112, consistent with this object having the lowest circumstellar density within our sample. We find that, within the uncertainties of the analysis, the three stars present similar abundances for most species, and also compared to IRC +10216. We find, tentatively, that SiO is more abundant in our three stars compared to IRC+10216, and that the hydrocarbons are under-abundant in IRAS 07454-7112 compared to the other stars and IRC +10216. Our estimated 12C/13C ratios match well the literature values for the three sources and our estimated silicon and sulphur isotopic ratios are very similar across the three stars and IRC +10216. Conclusions. The observed circumstellar chemistry appears very similar across our sample and compared to that of IRC +10216, both in terms of the relative location of the emitting regions and molecular abundances. This implies that, to a first approximation, the chemical models tailored to IRC +10216 are, at least, able to reproduce the observed chemistry in C-rich envelopes across roughly an order of magnitude in wind density.
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35.
  • Vlemmings, Wouter, 1974, et al. (författare)
  • Rotation of the asymptotic giant branch star R Doradus
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 613
  • Tidskriftsartikel (refereegranskat)abstract
    • High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 x 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of omega(R) sin i = (3.5 +/- 0.3) x 10(-9) rad s(-1), which indicates a rotational velocity of vertical bar nu(rot) sin i vertical bar = 1.0 +/- 0.1 km s(-1) at the stellar surface (R-* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Phi = 7 +/- 6 degrees. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.
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36.
  • Vlemmings, Wouter, 1974, et al. (författare)
  • The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
  • 2017
  • Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 1:12, s. 848-853
  • Tidskriftsartikel (refereegranskat)abstract
    • Our current understanding of the chemistry and mass-loss processes in Sun-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere 1 . Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions 2 , but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimetre continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hydrae. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time longer than commonly assumed. A shocked hot layer will be an important ingredient in current models of stellar convection, pulsation and chemistry at the late stages of stellar evolution.
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37.
  • Andrews, Holly, 1994, et al. (författare)
  • Multiple components in the molecular outflow of the red supergiant NML Cyg
  • 2022
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 510:1, s. 383-398
  • Tidskriftsartikel (refereegranskat)abstract
    • Despite their large impact on stellar and galactic evolution, the properties of outflows from red supergiants are not well characterized. We used the Onsala 20m telescope to perform a spectral survey at 3 and 4mm (68-116 GHz) of the red supergiant NML Cyg, alongside the yellow hypergiant IRC + 10420. Our observations of NML Cyg were combined with complementary archival data to enable a search for signatures of morphological complexity in the circumstellar environment, using emission lines from 15 molecular species. The recovered parameters imply the presence of three distinct, coherent, and persistent components, comprised of blue-shifted and red-shifted components, in addition to an underlying outflow centred at the stellar systemic velocity. Furthermore, to reproduce (CO)-C-12 emission with 3D radiative transfer models required a spherical outflow with three superposed conical outflows, one towards and one away from the observer, and one in the plane of the sky. These components are higher in density than the spherical outflow by up to an order of magnitude. We hence propose that NML Cyg's circumstellar environment consists of a small number of high-density large-scale coherent outflows embedded in a spherical wind. This would make the mass-loss history similar to that of VY CMa, and distinct from mu Cep, where the outflow contains many randomly distributed smaller clumps. A possible correlation between stellar properties, outflow structures, and content is critical in understanding the evolution of massive stars and their environmental impact.
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38.
  • Andriantsaralaza, Miora, et al. (författare)
  • DEATHSTAR—CO Envelope Size and Asymmetry of Nearby AGB Stars
  • 2022
  • Ingår i: Galaxies. - : MDPI AG. - 2075-4434. ; 10:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Low-and intermediate-mass stars evolve into asymptotic giant branch (AGB) stars near the end of their lives, losing mass through slow and massive winds. The ejected material creates a chemically-rich expanding envelope around the star, namely the circumstellar envelope (CSE). Investigating the anisotropy of the mass-loss phenomenon on the AGB is crucial in gaining a better understanding of the shaping of the CSE during the transition from AGB star to planetary nebula (PN). We investigate possible signs of deviation from spherical symmetry in the CO-emitting CSEs of 70 AGB stars by analysing their emission maps in CO J = 2 − 1 and 3 − 2 observed with the Atacama Compact Array, as part of the DEATHSTAR project. We find that about one third of the sources are likely aspherical, as they exhibit large-scale asymmetries that are unlikely to have been created by a smooth wind. Further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures.
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39.
  • Andriantsaralaza, Miora, et al. (författare)
  • Distance estimates for AGB stars from parallax measurements
  • 2022
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 667
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Estimating the distances to asymptotic giant branch (AGB) stars using optical measurements of their parallaxes is not straightforward because of the large uncertainties introduced by their dusty envelopes, their large angular sizes, and their surface brightness variability. Aims. This paper aims to assess the reliability of the distances derived with Gaia DR3 parallaxes for AGB stars, and provide a new distance catalogue for a sample of similar to 200 nearby AGB stars. Methods. We compared the parallaxes from Gaia DR3 with parallaxes measured with maser observations with very long baseline interferometry (VLBI) to determine a statistical correction factor for the DR3 parallaxes using a sub-sample of 33 maser-emitting oxygen-rich nearby AGB stars. We then calculated the distances of a total of similar to 200 AGB stars in the DEATHSTAR project using a Bayesian statistical approach on the corrected DR3 parallaxes and a prior based on the previously determined Galactic distribution of AGB stars. We performed radiative transfer modelling of the stellar and dust emission to determine the luminosity of the sources in the VLBI sub-sample based on the distances derived from maser parallaxes, and derived a new bolometric period-luminosity relation for Galactic oxygen-rich Mira variables. Results. We find that the errors on the Gaia DR3 parallaxes given in the Gaia DR3 catalogue are underestimated by a factor of 5.44 for the brightest sources (G < 8 mag). Fainter sources (8 <= G < 12) require a lower parallax error inflation factor of 2.74. We obtain a Gaia DR3 parallax zero-point offset of -0.077 mas for bright AGB stars. The offset becomes more negative for fainter AGB stars. After correcting the DR3 parallaxes, we find that the derived distances are associated with significant, asymmetrical errors for more than 40% of the sources in our sample. We obtain a PL relation of the form Mbol = (- 3.31 +/- 0.24) [log P - 2.5]+(-4.317 +/- 0.060) for the oxygen-rich Mira variables in the Milky Way. A new distance catalogue based on these results is provided for the sources in the DEATHSTAR sample. Conclusions. The corrected Gaia DR3 parallaxes can be used to estimate distances for AGB stars using the AGB prior, but we confirm that one needs to be careful when the uncertainties on parallax measurements are larger than 20%, which can result in model-dependent distances and source-dependent offsets. We find that a RUWE (re-normalised unit weight error) below 1.4 does not guarantee reliable distance estimates and we advise against the use of only the RUWE to measure the quality of Gaia DR3 astrometric data for individual AGB stars.
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40.
  • Battersby, C., et al. (författare)
  • The Origins Space Telescope
  • 2018
  • Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 2:8, s. 596-599
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The Origins Space Telescope, one of four large Mission Concept Studies sponsored by NASA for review in the 2020 US Astrophysics Decadal Survey, will open unprecedented discovery space in the infrared, unveiling our cosmic origins.
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41.
  • Brunner, M., et al. (författare)
  • Molecular line study of the S-type AGB star W Aquilae: ALMA observations of CS, SiS, SiO and HCN
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. With the outstanding spatial resolution and sensitivity of the Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than the abundant CO can be observed and resolved in circumstellar envelopes (CSEs) around evolved stars, such as the binary S-type asymptotic giant branch (AGB) star W Aquilae. Aims. We aim to constrain the chemical composition of the CSE and determine the radial abundance distribution, the photospheric peak abundance, and isotopic ratios of a selection of chemically important molecular species in the innermost CSE of W Aql. The derived parameters are put into the context of the chemical evolution of AGB stars and are compared with theoretical models. Methods . We employ one-dimensional radiative transfer modeling - with the accelerated lambda iteration (ALI) radiative transfer code-of the radial abundance distribution of a total of five molecular species (CS, SiS, (SiS)-Si-30, (SiO)-Si-29 and (HCN)-C-13) and determine the best fitting model parameters based on high-resolution ALMA observations as well as archival single-dish observations. The additional advantage of the spatially resolved ALMA observations is that we can directly constrain the radial profile of the observed line transitions from the observations. Results . We derive abundances and e-folding radii for CS, SiS, (SiS)-Si-30, (SiO)-Si-29 and (HCN)-C-13 and compare them to previous studies, which are based only on unresolved single-dish spectra. Our results are in line with previous results and are more accurate due to resolution of the emission regions.
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42.
  • Cernicharo, J., et al. (författare)
  • Discovery of time variation of the intensity of molecular lines in IRC+10216 in the submillimeter and far-infrared domains
  • 2014
  • Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 796:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species toward the archetypical circumstellar envelope of IRC+10216. The observations have been carried out with the Heterodyne Instrument for the Far-Infrared (HIFI) instrument on board Herschel and with the IRAM30 m telescope. They cover several observing periods spreading over three years. The line intensity variations for molecules produced in the external layers of the envelope most likely result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations must take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The submillimeter and far infrared lines of asymptotic giant branch stars can no longer be considered as safe intensity calibrators.
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43.
  • Danilovich, T., et al. (författare)
  • Sulphur-bearing molecules in AGB stars. Part 2. II. Abundances and distributions of CS and SiS
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities. Aims. By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances. Methods. We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. Results. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (M > 5 x 10(-6) M-circle dot yr(-1)). SiS is detected towards the highest mass-loss rate sources of all chemical types (M >= similar to 8 x 10(-7) M-circle dot yr(-1)). We find CS peak fractional abundances ranging from similar to 4 x 10(-7) to similar to 2 x 10(-5) for the carbon stars, from similar to 3 x 10(-8) to similar to 1 x 10(-7) for the oxygen-rich stars, and from similar to 1 x 10(-7) to similar to 8 x 10(-6) for the S-type stars. We find SiS peak fractional abundances ranging from similar to 9 x 10(-6) to similar to 2 x 10(-5) for the carbon stars, from similar to 5 x 10(-7) to similar to 2 x 10(-6) for the oxygen-rich stars, and from similar to 2 x 10(-7) to similar to 2 x 10(-6) for the S-type stars. Conclusions. Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.
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44.
  • Danilovich, Taissa, 1987, et al. (författare)
  • Sulphur molecules in the circumstellar envelopes of M-type AGB stars
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 588
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. The sulphur compounds SO and SO2 have not been widely studied in the circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting and modelling a large number of SO and SO2 lines in the low mass-loss rate M-type AGB star R Dor, and modelling the available lines of those molecules in a further four M-type AGB stars, we aim to determine their circumstellar abundances and distributions. Methods. We use a detailed radiative transfer analysis based on the accelerated lambda iteration method to model circumstellar SO and SO2 line emission. We use molecular data files for both SO and SO2 that are more extensive than those previously available. Results. Using 17 SO lines and 98 SO2 lines to constrain our models for R Dor, we find an SO abundance of (6.7 +/- 0.9) x 10(6) and an SO2 abundance of 5 x 10(6) with both species having high abundances close to the star. We also modelled (SO)-S-34 and found an abundance of (3.1 +/- 0.8) x 10(7), giving an (SO)-S-32/(SO)-S-34 ratio of 21.6 +/- 8.5. We derive similar results for the circumstellar SO and SO2 abundances and their distributions for the low mass-loss rate object W Hya. For the higher mass-loss rate stars, we find shell-like SO distributions with peak abundances that decrease and peak abundance radii that increase with increasing mass-loss rate. The positions of the peak SO abundance agree very well with the photodissociation radii of H2O. We also modelled SO2 in two higher mass-loss rate stars but our models for these were less conclusive. Conclusions. We conclude that for the low mass-loss rate stars, the circumstellar SO and SO2 abundances are much higher than predicted by chemical models of the extended stellar atmosphere. These two species may also account for all the available sulphur. For the higher mass-loss rate stars we find evidence that SO is most efficiently formed in the circumstellar envelope, most likely through the photodissociation of H2O and the subsequent reaction between S and OH. The S-bearing parent molecule does not appear to be H2S. The SO2 models for the higher mass-loss rate stars are less conclusive, but suggest an origin close to the star for this species. This is not consistent with current chemical models. The combined circumstellar SO and SO2 abundances are significantly lower than that of sulphur for these higher mass-loss rate objects.
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45.
  • De Beck, Elvire, 1985, et al. (författare)
  • ALMA observations of TiO2 around VY Canis Majoris
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Titanium dioxide, TiO2, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO2 has been detected only in the complex environment of the red supergiant VY CMa. Aims. We aim to constrain the distribution and excitation of TiO2 around VY CMa in order to clarify its role in dust formation. Methods. We analyse spectra and channel maps for TiO2 extracted from ALMA science verification data. Results. We detect 15 transitions of TiO2, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO2 emission likely traces gas exposed to the stellar radiation field. An accelerating bipolar-like structure is found, oriented roughly east-west, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. Conclusions. We find that a significant fraction of TiO2 remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.
  •  
46.
  • De Beck, Elvire, 1985, et al. (författare)
  • ALMA observations of TiO2 around VY CMa
  • 2016
  • Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 728:2, s. Art. no. UNSP 022009-
  • Konferensbidrag (refereegranskat)abstract
    • Titanium dioxide, TiO2, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. We present and discuss the detections of 15 emission lines of TiO2 with ALMA in the complex environment of the red supergiant VY CMa. The observations reveal a highly clumpy, anisotropic outflow in which the TiO2 emission likely traces gas exposed to the stellar radiation field. We find evidence for a roughly east-west oriented, accelerating bipolar-like structure, of which the blue component runs into and breaks up around a solid continuum component. We see a distinct tail to the south-west for some transitions, consistent with features seen in the optical and near-infrared. We find that a significant fraction of TiO2 remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.
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47.
  • De Beck, Elvire, 1985, et al. (författare)
  • Circumstellar environment of the M-type AGB star R Doradus APEX spectral scan at 159.0-368.5GHz
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 615
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Our current insights into the circumstellar chemistry of asymptotic giant branch (AGB) stars are largely based on studies of carbon-rich stars and stars with high mass-loss rates. Aims. In order to expand the current molecular inventory of evolved stars we present a spectral scan of the nearby, oxygen-rich star R Dor, a star with a low mass-loss rate (similar to 2 x 10(-7) M-circle dot yr(-1)). Methods. We carried out a spectral scan in the frequency ranges 159.0-321.5 GHz and 338.5-368.5 GHz (wavelength range 0.8-1.9 mm) using the SEPIA/Band-5 and SHeFI instruments on the APEX telescope and we compare it to previous surveys, including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss rate (similar to 5 x 10(-6) M-circle dot yr(-1)). Results. The spectrum of R Dor is dominated by emission lines of SO2 and the different isotopologues of SiO. We also detect CO, H2O, HCN, CN, PO, PN, SO, and tentatively TiO2, AlO, and NaCl. Sixteen out of approximately 320 spectral features remain unidentified. Among these is a strong but previously unknown maser at 354.2 GHz, which we suggest could pertain to H2SiO, silanone. With the exception of one, none of these unidentified lines are found in a similarly sensitive survey of IK Tau performed with the IRAM 30m telescope. We present radiative transfer models for five isotopologues of SiO ((SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, (SiO)-O-17, (SiO)-O-18), providing constraints on their fractional abundance and radial extent. We derive isotopic ratios for C, O, Si, and S and estimate that, based on our results for O-17/O-18, R Dor likely had an initial mass in the range 1.3-1.6 M-circle dot, in agreement with earlier findings based on models of H2O line emission. From the presence of spectral features recurring in many of the measured thermal and maser emission lines we tentatively identify up to five kinematical components in the outflow of R Dor, indicating deviations from a smooth, spherical wind.
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48.
  • De Beck, Elvire, 1985, et al. (författare)
  • Search for aluminium monoxide in the winds of oxygen-rich AGB stars
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 598
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Aluminium monoxide (AlO) is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina (Al2O3) clusters and dust seeds. The presence of AlO gas in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygenrich asymptotic giant branch star o Cet (Mira A). The central role aluminium might play in dust formation and wind driving, in combination with these first detections of AlO at long wavelengths, shows the need for a wider search for this molecule in the winds of evolved stars. Aims. The detection at long wavelengths of emission in rotational transitions of AlO towards asymptotic giant branch stars can help constrain the presence and location of AlO gas in the outflows and ultimately also the efficiency of the depletion process. Methods. In search of AlO, we mined data obtained with APEX, the IRAM 30m telescope, Herschel / HIFI, SMA, and ALMA, which were primarily aimed at studying other species around asymptotic giant branch stars. We report here on observations of AlO towards a sample of eight oxygen-rich asymptotic giant branch stars in different rotational transitions, up to seven for some stars. Results. We present definite detections of one rotational transition of AlO for o Cet and R Aqr, and tentative detections of one transition for R Dor and o Cet and two transitions for IK Tau and W Hya. The presented spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R Aqr, and IK Tau, we find that the AlO (N = 9 8) emission likely traces the inner parts of the wind, out to only a few tens of AU, where the gas has not yet been accelerated to its terminal velocity. This is in agreement with recently published results from a detailed study on o Cet. Conclusions. The conclusive detections of AlO emission in the case of o Cet and R Aqr confirm the presence of AlO in the gas phase in outflows of asymptotic giant branch stars. The tentative detections further support this. Since most of the observations presented in this study were obtained with stronger emission from other species than AlO in mind, observations with higher sensitivity in combination with high angular resolution will improve our understanding of the presence and behaviour of AlO. From the current data sets we cannot firmly conclude whether there is a direct correlation between the wind properties and the detection rate of AlO emission. We hope that this study can serve as a stimulus to perform sample studies in search of AlO in oxygen-rich outflows.
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49.
  • De Beck, Elvire, 1985, et al. (författare)
  • SMA Spectral Line Imaging Survey at 279-355 GHz of the Oxygen-rich AGB Star IK Tau
  • 2015
  • Ingår i: Astronomical Society of the Pacific Conference Series; Why galaxies care about AGB stars III: A closer look in space and time; Vienna, Austria; Jul 28–Aug 1, 2015. - 9781583818794 ; 497, s. 73-78
  • Konferensbidrag (refereegranskat)abstract
    • Dedicated, unbiased spectral scans of asymptotic giant branch stars have so far been published only for a few carbon-rich stars, with a strong focus on the nearby and bright IRC +10216. We present results from a spectral survey of the circumstellar envelope of the oxygen-rich AGB star IK Tau obtained with the Submillimeter Array (SMA) at similar to 0 ''.9 angular resolution in the frequency range 279 - 355 GHz, expanding the molecular inventory for M-type evolved stars and filling an observational gap. The survey shows over 140 emission lines, belonging to more than 30 species. The emission of AlO and of several vibrationally excited species traces the acceleration of the wind. Isotopic ratios for carbon, silicon, and sulfur will be derived from the observed emission of isotopologues of CO, SiO, SiS, HCN, SO, and SO2. This will allow us to constrain the AGB nucleosynthesis of IK Tau. We highlight the first detection of PO and PN around an oxygen-rich AGB star, detected at unexpectedly high abundances, and emphasise the importance of unbiased spectral surveys of AGB stars and the need for updated chemical models.
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50.
  • de Beck, Elvire, 1985, et al. (författare)
  • The surprisingly carbon-rich environment of the S-type star W Aql
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 642
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O approximate to 0.98. It has previously been reported to have circumstellar molecular abundances intermediate between those of M-type and C-type AGB stars, which respectively have C/O < 1 and C/O > 1. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes is currently rather limited.Aims. We aim to assess the reported intermediate status of W Aql by analysing the line emission of molecules that have never before been observed towards this star.Methods. We performed observations in the frequency range 159-268 GHz with the SEPIA/B5 and PI230 instruments on the APEX telescope. We made abundance estimates through direct comparison to available spectra towards a number of well-studied AGB stars and based on rotational diagram analysis in the case of one molecule.Results. From a compilation of our abundance estimates and those found in the literature for two M-type (R Dor, IK Tau), two S-type (chi Cyg, W Aql), and two C-type stars (V Aql, IRC +10 216), we conclude that the circumstellar environment of W Aql appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the circumstellar environment of C-type stars. This conclusion, based on the chemistry of the gaseous component of the circumstellar environment, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species which are typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the circumstellar environment, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars.Conclusions. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the circumstellar environment of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.
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