| 1. |
- Brunner, M., et al.
(författare)
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Molecular line study of the S-type AGB star W Aquilae: ALMA observations of CS, SiS, SiO and HCN
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
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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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| 2. |
- Bujarrabal, V., et al.
(författare)
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High-resolution observations of the symbiotic system R Aqr Direct imaging of the gravitational effects of the secondary on the stellar wind
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- We have observed the symbiotic stellar system R Aqr, aiming to describe the gravitational interaction between the white dwarf (WD) and the wind from the Mira star, the key phenomenon driving the symbiotic activity and the formation of nebulae in such systems. We present high-resolution ALMA maps of the (CO)-C-12 and (CO)-C-13 J = 3-2 lines, the 0.9 mm continuum distribution, and some high-excitation molecular lines in R Aqr. The maps, which have resolutions ranging between 40 mas and less than 20 mas probe the circumstellar regions at suborbital scales as the distance between the stars is similar to 40 mas. Our observations show the gravitational effects of the secondary on the stellar wind. The AGB star was identified in our maps from the continuum and molecular line data, and we estimated the probable position of the secondary from a new estimation of the orbital parameters. The (preliminary) comparison of our maps with theoretical predictions is surprisingly satisfactory and the main expected gravitational effects are directly mapped for the first time. We find a strong focusing in the equatorial plane of the resulting wind, which shows two plumes in opposite directions that have different velocities and very probably correspond to the expected double spiral due to the interaction. Our continuum maps show the very inner regions of the nascent bipolar jets, at scales of some AU. Continuum maps obtained with the highest resolution show the presence of a clump that very probably corresponds to the emission of the ionized surroundings of the WD and of a bridge of material joining both stars, which is likely material flowing from the AGB primary to the accretion disk around the WD secondary.
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| 3. |
- Danilovich, Taissa, 1987, et al.
(författare)
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Detailed Modelling of the Circumstellar Envelope of the S-type AGB Star W Aquilae
- 2015
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Ingår i: Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna JUL 28-AUG 01, 2014. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; 497, s. 219-220
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Konferensbidrag (refereegranskat)abstract
- We present new Herschel(1) HIFI (de Graauw et al. 2010) and PACS (Poglitsch et al. 2010) sub-millimeter and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances.
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| 4. |
- Danilovich, T., et al.
(författare)
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Sulphur-bearing molecules in AGB stars
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 606
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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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| 5. |
- Danilovich, T., et al.
(författare)
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Sulphur-bearing molecules in AGB stars : II. Abundances and distributions of CS and SiS
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 617
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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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| 6. |
- De Beck, Elvire, 1985, et al.
(författare)
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Search for aluminium monoxide in the winds of oxygen-rich AGB stars
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 598
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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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| 7. |
- De Nutte, R., et al.
(författare)
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Nucleosynthesis in AGB Stars Traced by Oxygen Isotopic Ratios
- 2015
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Ingår i: Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna, 28- July - 1 Aug. 2014: (Astronomical Society of the Pacific Conference Series). - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; 497, s. 289-294, s. 289-294
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Konferensbidrag (refereegranskat)abstract
- Isotopic ratios are by far the best diagnostic tracers of the stellar origin of elements, as they are very sensitive to the precise conditions in the nuclear burning regions. They allow us to give direct constraints on stellar evolution models and on the progenitor mass. However, up to now different isotopic ratios have been well constrained for only a handful of Asymptotic Giant Branch (AGB) stars. We present new data on isotopologue lines of a well-selected sample of AGB stars, covering the three spectral classes of C-, S- and M-type stars. We report on the first efforts made in determining accurate isotopologue fractions, focusing on oxygen isotopes which are a crucial tracer of the poorly constrained extra mixing processes in stellar atmospheres.
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| 8. |
- De Nutte, R., et al.
(författare)
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Nucleosynthesis in AGB stars traced by oxygen isotopic ratios I. Determining the stellar initial mass by means of the O-17/O-18 ratio
- 2017
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 600
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We seek to investigate the O-17/O-18 ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period.Methods. Circumstellar (CO)-C-13-O-16, (CO)-C-12-O-17, and (CO)-C-12-O-18 line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface O-17/O-18 abundance ratio.Results. Stellar evolution models predict the O-17/O-18 ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 M-circle dot. This makes the measured ratio a probe of the initial stellar mass.Conclusions. Observed O-17/O-18 ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the O-17/O-18 ratio to the stellar pulsation period.
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| 9. |
- Doan, Lam
(författare)
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Companion wind shaping in binaries involving an AGB star
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stars of initial masses between 0.8-8 M☉ will become "asymptotic giant branch" (AGB) stars during the final stages of their evolution. During this phase, the stars are characterized by low velocity and high-density winds. An AGB star can lose a substantial fraction of its mass through the stellar wind and thereby avoid ending up as a supernova explosion. The AGB stars, therefore, play an important role in enriching the interstellar medium (ISM) with chemical elements and in contributing dust and gas to the ISM. The mass-loss rate on the AGB is a decisive parameter for the lifetime of this evolutionary phase and the fate of low- and intermediate-mass stars. An accurate mass-loss-rate estimation provides an important constraint for wind models aimed to better understand the wind-driving mechanism, as well as for stellar evolution.The stellar wind is driven by radiation pressure on dust grains and blows away dust and gas from the central star. This creates an extended envelope which is expected to be spherical because of the isotropic radiation field of the central star, and the connection between the radiation field and the wind. However, there is growing observational evidence of asymmetrical morphology, e.g., torii, jets, bipolar outflows, in AGB circumstellar envelopes (CSEs). Moreover, proto-planetary nebulae (proto-PNe) and PNe, the next evolutionary phase after the AGB phase, show a wide range of asymmetrical morphologies. In many cases, an embedded binary system has been detected in the gas envelopes. This is pointing to the gravitational effect of the companion as important for the envelope shaping mechanism.The work that this thesis is based on, studies two interesting examples of (post) AGB stars which show complex morphologies of their CSEs. The S-star π1 Gruis shows a CSE structure consisting of an equatorial low-velocity expanding spiral and a fast bipolar outflow. The circumstellar environment of the post-AGB (or post red giant branch, post-RGB) star HD 101584 shows an equatorial density enhancement and a high-velocity bipolar outflow. Same conclusions are drawn for both cases; that the radiation pressure on the dust cannot support the observed energetic outflows, and that interaction with the companions are proposed to shape the envelopes and accelerate the gas.The thesis gives a brief introduction on AGB stars and wind shaping mechanisms of AGB CSEs. The thesis also presents the principles of interferometry, the data reduction methods, and the radiative transfer calculations used in the studies. Results from the included papers are also discussed.
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| 10. |
- Doan, Lam, et al.
(författare)
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The extended molecular envelope of the asymptotic giant branchstar π1 Gruis as seen by ALMA : I. Large-scale kinematic structure and CO excitation properties
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Tidskriftsartikel (refereegranskat)abstract
- Context. The S-type asymptotic giant branch (AGB) star pi(1) Gru has a known companion at a separation of 2 ''.7 (approximate to 400 AU). Previous observations of the circumstellar envelope (CSE) show strong deviations from spherical symmetry. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive.Aims. The aim is to make a 3D model of the CSE and to constrain the density and temperature distribution using new spatially resolved observations of the CO rotational lines.Methods. We have observed the J = 3-2 line emission from (CO)-C-12 and (CO)-C-13 using the compact arrays of the Atacama Large Millimeter/submillimeter Array (ALMA). The new ALMA data, together with previously published (CO)-C-12 J = 2-1 data from the Submillimeter Array (SMA), and the (CO)-C-12 J = 5-4 and J = 9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL.Results. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s(-1) along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s(-1); and a fast, bipolar outflow with velocity proportionally increasing from 14 km s(-1) at the base up to 100 km s(-1) at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7 x 10(-7) M-circle dot yr(-1). The total mass and linear momentum of the fast outflow are estimated at 7.3 x 10(-4) M-circle dot and 9.6 x 10(37) g cm s(-1), respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a (CO)-C-12/(CO)-C-13 abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed.
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| 11. |
- Kerschbaum, F., et al.
(författare)
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Rings and filaments : The remarkable detached CO shell of U Antliae
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Our goal is to characterize the intermediate age, detached shell carbon star U Antliae morphologically and physically in order to study the mass-loss evolution after a possible thermal pulse.Methods. High spatial resolution ALMA observations of unprecedented quality in thermal CO lines allow us to derive first critical spatial and temporal scales and constrain modeling efforts to estimate mass-loss rates for both the present day as well as the ejection period of the detached shell.Results. The detached shell is remarkably thin, overall spherically symmetric, and shows a barely resolved filamentary substructure possibly caused by instabilities in the interaction zone of winds with different outflow velocities. The expansion age of the detached shell is of the order of 2700 yr and its overall width indicates a high expansion-velocity and high mass-loss period of only a few hundred years at an average mass-loss rate of approximate to 10(-5) M-circle dot yr(-1). The post-high-mass-loss-rate-epoch evolution of U Ant shows a significant decline to a substantially lower gas expansion velocity and a mass-loss rate amounting to 4 x 10(-8) M-circle dot yr(-1), at present being consistent with evolutionary changes as predicted for the period between thermal pulses.
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| 12. |
- Khouri, Theo, 1985, et al.
(författare)
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ALMA observations of the vibrationally excited rotational CO transition v=1, J=3-2 towards five AGB stars
- 2016
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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
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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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| 13. |
- Khouri, Theo, 1985, et al.
(författare)
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High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 620
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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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| 14. |
- Lykou, F., et al.
(författare)
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Dissecting the AGB star L-2 Puppis : a torus in the making
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 576
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The circumstellar environment of L-2 Pup, an oxygen-rich semiregular variable, was observed to understand the evolution of mass loss and the shaping of ejecta in the late stages of stellar evolution. Methods. High-angular resolution observations from a single 8 m telescope were obtained using aperture masking in the near-infrared (1.64, 2.30 and 3.74 mu m) on the NACO/VLT, both in imaging and polarimetric modes. Results. The aperture-masking images of L-2 Pup at 2.30 mu m show a resolved structure that resembles a toroidal structure with a major axis of similar to 140 milliarcseconds (mas) and an east-west orientation. Two clumps can be seen on either side of the star, similar to 65 mas from the star, beyond the edge of the circumstellar envelope (estimated diameter is similar to 27 mas), while a faint, hook-like structure appear toward the northeast. The patterns are visible both in the imaging and polarimetric mode, although the latter was only used to measure the total intensity (Stokes I). The overall shape of the structure is similar at the 3.74 mu m pseudo-continuum (dust emission), where the clumps appear to be embedded within a dark, dusty lane. The faint, hook-like patterns are also seen at this wavelength, extending northeast and southwest with the central, dark lane being an apparent axis of symmetry. We interpret the structure as a circumstellar torus with inner radius of 4.2 au. With a rotation velocity of 10 kms(-1) as suggested by the SiO maser profile, we estimate a stellar mass of 0.7 M-circle dot.
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| 15. |
- Lykou, F., et al.
(författare)
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Unraveling Disks in AGB Stars
- 2015
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Ingår i: EAS Publications Series. - : EDP Sciences. - 1633-4760 .- 1638-1963. - 9782759819072 ; 71-72, s. 217-222
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Konferensbidrag (refereegranskat)abstract
- It is commonly accepted that asymmetries found in the post-AGB stars and planetary nebulae should originate as early as during the AGB phase. We present results from our high-angular resolution observing programs, with an aperture masking technique on the VLT, of a sample of evolved stars that were known to present asymmetries at larger spatial scales (e.g. jets, torii and/or bipolar nebulae). Disk-like structures have been found in the vicinity of at least two of these stars.
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| 16. |
- Maercker, Matthias, 1979, et al.
(författare)
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A detailed view of the gas shell around R Sculptoris with ALMA
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 586
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Tidskriftsartikel (refereegranskat)abstract
- Context. During the asymptotic giant branch (AGB) phase, stars undergo thermal pulses - short-lived phases of explosive helium burning in a shell around the stellar core. Thermal pulses lead to the formation and mixing-up of new elements to the stellar surface. They are hence fundamental to the chemical evolution of the star and its circumstellar envelope. A further consequence of thermal pulses is the formation of detached shells of gas and dust around the star, several of which have been observed around carbon-rich AGB stars. Aims. We aim to determine the physical properties of the detached gas shell around R Sculptoris, in particular the shell mass and temperature, and to constrain the evolution of the mass-loss rate during and after a thermal pulse. Methods. We analyse (CO)-C-12(1-0), (CO)-C-12(2-1), and (CO)-C-12(3-2) emission, observed with the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 0 and complemented by single-dish observations. The spatial resolution of the ALMA data allows us to separate the detached shell emission from the extended emission inside the shell. We perform radiative transfer modelling of both components to determine the shell properties and the post-pulse mass-loss properties. Results. The ALMA data show a gas shell with a radius of 19.'' 5 expanding at 14.3 km s(-1). The different scales probed by the ALMA Cycle 0 array show that the shell must be entirely filled with gas, contrary to the idea of a detached shell. The comparison to single-dish spectra and radiative transfer modelling confirms this. We derive a shell mass of 4.5 x 10(-3) M-circle dot with a temperature of 50 K. Typical timescales for thermal pulses imply a pulse mass-loss rate of 2.3 x 10(-5) M-circle dot yr(-1). For the post-pulse mass-loss rate, we find evidence for a gradual decline of the mass-loss rate, with an average value of 1.6 x 10(-5) M-circle dot yr(-1). The total amount of mass lost since the last thermal pulse is 0.03 M-circle dot, a factor four higher compared to classical models, with a sharp decline in mass-loss rate immediately after the pulse. Conclusions. We find that the mass-loss rate after a thermal pulse has to decline more slowly than generally expected from models of thermal pulses. This may cause the star to lose significantly more mass during a thermal pulse cycle, which affects the lifetime on the AGB and the chemical evolution of the star, its circumstellar envelope, and the interstellar medium.
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| 17. |
- McDonald, I., et al.
(författare)
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ALMA reveals sunburn : CO dissociation around AGB stars in the globular cluster 47 Tucanae
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 453:4, s. 4324-4336
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Tidskriftsartikel (refereegranskat)abstract
- Atacama Large Millimetre Array observations show a non-detection of carbon monoxide around the four most luminous asymptotic giant branch (AGB) stars in the globular cluster 47 Tucanae. Stellar evolution models and star counts show that the mass-loss rates from these stars should be similar to 1.2-3.5x10(-7) M-circle dot yr(-1). We would naively expect such stars to be detectable at this distance (4.5 kpc). By modelling the ultraviolet radiation field from post-AGB stars and white dwarfs in 47 Tuc, we conclude that CO should be dissociated abnormally close to the stars. We estimate that the CO envelopes will be truncated at a few hundred stellar radii from their host stars and that the line intensities are about two orders of magnitude below our current detection limits. The truncation of CO envelopes should be important for AGB stars in dense clusters. Observing the CO (3-2) and higher transitions and targeting stars far from the centres of clusters should result in the detections needed to measure the outflow velocities from these stars.
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| 18. |
- Mohamed, S., et al.
(författare)
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3d Models of Symbiotic Binaries
- 2015
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Ingår i: EAS Publications Series. - : EDP Sciences. - 1633-4760 .- 1638-1963. - 9782759819072 ; 71-72, s. 81-86
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Konferensbidrag (refereegranskat)abstract
- Symbiotic binaries consist of a cool, mass-losing giant and an accreting, compact companion. We present 3D Smoothed Particle Hydrodynamics (SPH) models of two such interacting binaries, RS Oph and Mira AB. RS Oph is also a recurrent nova system, thus we model multiple quiescent mass transfer-nova outburst cycles. The resulting circumstellar structures of both systems are highly complex with the formation of spirals, arcs, shells, equatorial and bipolar outflows. We compare the models to recent observations and discuss the implications of our results for related systems, e.g., bipolar nebulae and jets, chemically peculiar stars, and the progenitors of Type Ia supernovae.
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| 19. |
- Montez, Rodolfo, Jr., et al.
(författare)
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A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars
- 2017
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 841:1
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the near-UV (NUV) bandpass (lambda(eff) similar to 2310 angstrom), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the far-UV (FUV) bandpass (lambda(eff) similar to 1528 angstrom). We find that NUV emission is correlated with optical to near-infrared emission, leading to higher detection fractions among the brightest, and hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission and possibly the FUV emission are anticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.
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| 20. |
- Olofsson, Hans, 1952, et al.
(författare)
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ALMA view of the circumstellar environment of the post-common-envelope-evolution binary system HD 101584
- 2015
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 576
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We study the circumstellar evolution of the binary HD 101584, consisting of a post-AGB star and a low-mass companion, which is most likely a post-common-envelope-evolution system. Methods. We used ALMA observations of the (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 lines and the 1.3 mm continuum to determine the morphology, kinematics, masses, and energetics of the circumstellar environment. Results. The circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic jet, approximate to 150 km s(-1). We conjecture that the circumstellar morphology is related to an event that took place approximate to 500 yr ago, possibly a capture event where the companion spiraled in towards the AGB star. However, the kinetic energy of the accelerated gas exceeds the released orbital energy, and, taking into account the expected energy transfer efficiency of the process, the observed phenomenon does not match current common-envelope scenarios. This suggests that another process must augment, or even dominate, the ejection process. A significant amount of material resides in an unresolved region, presumably in the equatorial plane of the binary system.
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| 21. |
- Olofsson, Hans, 1952, et al.
(författare)
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An ALMA View of the Complex Circumstellar Environment of the Post-AGB Object HD 101584
- 2015
-
Ingår i: 4th ALMA Science Conference on Revolution in Astronomy with ALMA: The Third Year, Tokyo, Japan, 8-11 December. - 9781583818831 - 9781583818824 ; 499, s. 319-322
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Konferensbidrag (refereegranskat)abstract
- We use (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 lines and 1.3 mm continuum ALMA observations to study the circumstellar evolution of the binary HD 101584, a post-AGB star and a low-mass companion, which is-most likely a post-common envelope-evolution system. It is inferred that the circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic, approximate to 150 km s(-1), jet. Significant amount of material resides in an unresolved central region. It is proposed that the circumstellar morphology is related to an event which took place approximate to 500 yr ago, possibly a capture event where the companion spiraled in towards the AGB star. However, the kinetic energy of the accelerated gas exceeds the released orbital energy. Hence, the observed phenomenon does not match current common-envelope scenarios, and another process must augment, or even dominate, the ejection process.
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| 22. |
- Olofsson, H., et al.
(författare)
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An ALMA view of the post-AGB object HD101584
- 2016
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Ingår i: 11th Pacific Rim Conference On Stellar Astrophysics. - : IOP PUBLISHING LTD.
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Konferensbidrag (refereegranskat)abstract
- ALMA cycles 1 and 3 observations of CO isotopologues and 1.3mm continuum are used in a study of the circumstellar environment of the binary HD 101584, a post-AGB star and a low-mass companion that is most likely a post-common-envelope-evolution system. These data are supplemented with new information from OH maser emission. It is inferred that the large- scale circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic, >= 150 km s(-1), jet. Significant amount of material still resides in the central region. It is proposed that the circumstellar morphology is related to an event which took place <= 500 yr ago, possibly a capture event where the companion spiralled in towards the AGB star. Several observed features remain to be explained, and may hint to a more complicated scenario.
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| 23. |
- Olofsson, Hans, 1952, et al.
(författare)
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First detection of methanol towards a post-AGB object, HD 101584
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603, s. L2-
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Tidskriftsartikel (refereegranskat)abstract
- The circumstellar environments of objects on the asymptotic giant branch and beyond are rich in molecular species. Nevertheless, methanol has never been detected in such an object, and is therefore often taken as a clear signpost for a young stellar object. However, we report the first detection of CH3OH in a post-AGB object, HD 101584, using ALMA. Its emission, together with emissions from CO, SiO, SO, CS, and H2CO, comes from two extreme velocity spots on either side of the object where a high-velocity outflow appears to interact with the surrounding medium. We have derived molecular abundances, and propose that the detected molecular species are the effect of a post-shock chemistry where circumstellar grains play a role. We further provide evidence that HD 101584 was a low-mass, M-type AGB star.
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| 24. |
- Olofsson, Hans, 1952, et al.
(författare)
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HD 101584: circumstellar characteristics and evolutionary status
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 623
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Tidskriftsartikel (refereegranskat)abstract
- Context. There is growing evidence that red giant evolution is often affected by an interplay with a nearby companion, in some cases taking the form of a common-envelope evolution. Aims. We have performed a study of the characteristics of the circumstellar environment of the binary object HD 101584, that provides information on a likely evolutionary scenario. Methods. We have obtained and analysed ALMA observations, complemented with observations using APEX, of a large number of molecular lines. An analysis of the spectral energy distribution has also been performed. Results. Emissions from 12 molecular species (not counting isotopologues) have been observed, and most of them mapped with angular resolutions in the range 0 ''.1-0 ''.6. Four circumstellar components are identified: (i) a central compact source of size approximate to 0 ''.15, (ii) an expanding equatorial density enhancement (a flattened density distribution in the plane of the orbit) of size approximate to 3 '', (iii) a bipolar high-velocity outflow (approximate to 150 km s(-1)), and (iv) an hourglass structure. The outflow is directed almost along the line of sight. There is evidence of a second bipolar outflow. The mass of the circumstellar gas is approximate to 0.5 [D/1 kpc](2) M-circle dot, about half of it lies in the equatorial density enhancement. The dust mass is approximate to 0.01 [D/1 kpc](2) M-circle dot, and a substantial fraction of this is in the form of large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is approximate to 770 [D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated gas are estimated to be 7 x 10(45) [D/1 kpc](2) erg and 10(39) [D/1 kpc](2) g cm s(-1), respectively. Conclusions. We provide good evidence that the binary system HD 101584 is in a post-common-envelope-evolution phase, that ended before a stellar merger. Isotope ratios combined with stellar mass estimates suggest that the primary star's evolution was terminated already on the first red giant branch (RGB). Most of the energy required to drive the outflowing gas was probably released when material fell towards the companion.
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| 25. |
- Olofsson, Hans, 1952, et al.
(författare)
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The Circumstellar (CO)-C-12/(CO)-C-13 Ratio in AGB Stars of Different Chemical Type
- 2015
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Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; , s. 295-300
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Konferensbidrag (refereegranskat)abstract
- We have investigated the evolution of the C-12/C-13 ratio along the AGB through the circumstellar isotopologue (CO)-C-12/(CO)-C-13 abundance ratio for a significant number of M-, S-, and C-type AGB stars. The isotopologue ratios are estimated through a detailed radiative transfer analysis of single-dish radio-line observations. The circumstellar (CO)-C-12/(CO)-C-13 ratio differs between the three spectral types; the median values are 13, 26, and 34 for the M-, S-type, and carbon AGB stars, respectively, consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence. The spread of the abundance ratio among carbon stars is much larger than for the other chemical types. We find no correlation between the isotopologue ratio and the mass-loss rate.
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| 26. |
- Paladini, C., et al.
(författare)
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Large granulation cells on the surface of the giant star π1 Gruis
- 2018
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 553:7688, s. 310-
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Tidskriftsartikel (refereegranskat)abstract
- Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs1,2. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun3—a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells3, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns4. These properties can be inferred from geometric model fitting5,6,7, but this indirect method does not provide information about the physical origin of the convective cells5,6,7. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type8,9 S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection10,11,12.
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| 27. |
- Ramstedt, Sofia, et al.
(författare)
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CO envelope of the symbiotic star R Aquarii seen by ALMA
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- The symbiotic star R Aqr is part of a small sample of binary AGB stars observed with the Atacama Large Millimeter/submillimeter Array (ALMA). The sample stars are: R Aqr, Mira, W Aql, and π1Gru. The sample covers a range in binary separation and wind properties, where R Aqr is the source with the smallest separation. The R Aqr binary pair consists of an M-type AGB star and a white dwarf at a separation of 45 mas, equivalent to about 10 AU at 218 pc. The aim of the ALMA study is to investigate the dependence of the wind shaping on the binary separation and to provide constraints for hydrodynamical binary interaction models. R Aqr is particularly interesting as the source with the smallest separation and a complex circumstellar environment that is strongly affected by the interaction between the two stars and by the high-energy radiation resulting from this interaction and from the hot white dwarf companion. The CO(J = 3 →2) line emission has been observed with ALMA at ∼0.5′′ spatial resolution. The CO envelope around the binary pair is marginally resolved, showing what appears to be a rather complex distribution. The outer radius of the CO emitting region is estimated from the data and found to be about a factor of 10 larger than previously thought. This implies an average mass-loss rate during the past ∼100 yr of 2×10-7M∗yr-1, a factor of 45 less than previous estimates. The channel maps are presented and the molecular gas distribution is discussed and set into the context of what was previously known about the system from multiwavelength observations. Additional molecular line emission detected within the bandwidth covered by the ALMA observations is also presented. Because of the limited extent of the emission, firm conclusions about the dynamical evolution of the system will have to wait for higher spatial resolution observations. However, the data presented here support the assumption that the mass-loss rate from the Mira star strongly varies and is focused on the orbital plane.
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| 28. |
- Ramstedt, Sofia, et al.
(författare)
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Shaping the Outflows of Binary AGB Stars
- 2015
-
Ingår i: Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Conference on Why Galaxies Care About AGB Stars III: A Closer Look in Space and Time, Vienna jul 28-aug 01, 2014. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; 497, s. 187-192, s. 187-192
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Konferensbidrag (refereegranskat)abstract
- The required conditions for stars to evolve into planetary nebulae (PNe) continue to puzzle. Since PNe are found in a wide variety of shapes, processes that could sculpt circumstellar envelopes (CSEs) are being investigated. A binary companion will have a strong gravitational effect, but known binary AGB stars are rare. We have observed the CO emission from a small sample of known binary AGB stars (R Aqr, Mira, W Aql, and pi(1). Gru) with ALMA. The stars cover a decisive range in binary separation, necessary to provide essential constraints for 3-D models of the binary interaction. They have previously been observed with Herschel/PACS and VLTI/MIDI allowing the circumstellar morphology to be studied from the very inner CSE out to the very largest scales. The circumstellar gas distribution will strongly depend on how the mass is initially lost from the primary. These observations will therefore not only help us understand the important processes for the binary interaction, but will also provide crucial information needed to understand the mass-loss mechanisms of the primary. The first set of observations has been delivered and preliminary results are presented.
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| 29. |
- Ramstedt, Sofia, et al.
(författare)
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The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit
- 2017
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
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Tidskriftsartikel (refereegranskat)abstract
- Context. Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3 -> 2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results. The estimated average mass-loss rate of W Aql is (M) over dot = 3.0 x 10(-6) M-circle dot yr(-1) and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10 0 0 of the CSE is asymmetric with arc-like structures at separations of 23" scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions. The CO(J = 3 -> 2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of similar to 10" and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yr) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.
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| 30. |
- Ramstedt, Sofia, et al.
(författare)
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The Outflows of Binary AGB Stars
- 2015
-
Ingår i: 4th ALMA Science Conference on Revolution in Astronomy with ALMA: The Third Year, Tokyo, Japan, 8-11 December. - 9781583818831 - 9781583818824 ; 499, s. 331-334
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Konferensbidrag (refereegranskat)abstract
- The required conditions for stars to evolve into planetary nebulae (PNs) continues to puzzle. Since PNs are found in a wide variety of shapes, processes that could sculpt circumstellar envelopes (CSEs) are being investigated. A binary companion will have a strong gravitational effect, but known binary AGB stars are rare. Using ALMA in Cycle 1 and 2, we have observed a small sample of well-studied, binary AGB stars, covering a decisive range in separation, in order to determine the influence of a companion on the circumstellar morphology of the AGB primary. The first steps toward interpreting and analyzing the data have been taken, and the results will be compared to 3D Smoothed Particle Hydrodynamics (SPH) models of the gravitational interaction.
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| 31. |
- Saberi, Maryam, 1988, et al.
(författare)
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Detection of CI line emission towards the oxygen-rich AGB star omi Ceti
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 612
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Tidskriftsartikel (refereegranskat)abstract
- We present the detection of neutral atomic carbon CI( 3 P 1 - 3 P 0 ) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 × 10 -5 predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field. However, the CI emission is shifted by ∼4 km s -1 from the stellar velocity. Based on this velocity shift, we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion. The velocity shift could, therefore, be the result of the orbital velocity of the binary companion around omi Cet. In this case, the CI column density is estimated to be 1.1 × 10 19 cm -2 . This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement. However, this hypothesis can be confirmed by high-angular resolution observations.
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| 32. |
- Sanchez, Enmanuel, et al.
(författare)
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First Detection of Ultraviolet Emission from a Detached Dust Shell : Galaxy Evolution Explorer Observations of the Carbon Asymptotic Giant Branch Star U Hya
- 2015
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 798:2, s. L39-
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Tidskriftsartikel (refereegranskat)abstract
- We present the discovery of an extended ring of ultraviolet (UV) emission surrounding the asymptotic giant branch (AGB) star U Hya in archival observations performed by the Galaxy Evolution Explorer. This is the third discovery of extended UV emission from a carbon AGB star and the first from an AGB star with a detached shell. From imaging and photometric analysis of the FUV and NUV images, we determined that the UV ring has a radius of similar to 110 '', thus indicating that the emitting material is likely associated with the detached shell seen in the infrared. We find that scattering of the central point source of NUV and FUV emission by the dust shell is negligible. Moreover, we find that scattering of the interstellar radiation field by the dust shell can contribute at most similar to 10% of the FUV flux. Morphological and photometric evidence suggests that shocks caused by the star's motion through space and, possibly, shock-excited H-2 molecules are the most likely origins of the UV flux. In contrast to previous examples of extended UV emission from AGB stars, the extended UV emission from U Hya does not show a bow-shock-like structure, which is consistent with a lower space velocity and lower interstellar medium density. This suggests the detached dust shell is the source of the UV-emitting material and can be used to better understand the formation of detached shells.
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| 33. |
- Vlemmings, Wouter, 1974, et al.
(författare)
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Resolving the stellar activity of the Mira AB binary with ALMA
- 2015
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 577
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We present the size, shape, and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (similar to 94.6 GHz) and Band 6 (similar to 228.7 GHz), from the binary Mira A (o Ceti) and Mira B. Methods. The Mira AB system was observed with ALMA at a spatial resolution down to similar to 25 mas. The extended atmosphere of Mira A and the wind around Mira B sources were resolved, and we derived the sizes of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Bands 3 and 6 were also derived. Results. The spectral index of Mira A is found to change from 1.71 +/- 0.05 within Band 3 to 1.54 +/- 0.04 between Bands 3 and 6. The spectral index of Mira B is 1.3 +/- 0.2 in Band 3, in good agreement with measurements at longer wavelengths; however, it rises to 1.72 +/- 0.11 between the bands. For the first time, the extended atmosphere of a star is resolved at these frequencies, and for Mira A the diameter is similar to 3.8 x 3.2 AU in Band 3 (with brightness temperature T-b similar to 5300 K) and similar to 4.0 x 3.6 AU in Band 6 (T-b similar to 2500 K). Additionally, a bright hotspot similar to 0.4 AU, with T-b similar to 10 000 K, is found on the stellar disk of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be similar to 2.4 AU. Conclusions. The emission around Mira B is consistent with emission from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions with brightness temperatures in Band 3 significantly higher than expected, potentially owing to shock heating. The hotspot is very likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.
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| 34. |
- Vlemmings, Wouter, 1974, et al.
(författare)
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Rotation of the asymptotic giant branch star R Doradus
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 613
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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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| 35. |
- Vlemmings, Wouter, 1974, et al.
(författare)
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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
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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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