| 1. |
- Alcolea, J., et al.
(författare)
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HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559, s. (article nr.) A93-
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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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| 2. |
- Bujarrabal, V., et al.
(författare)
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Herschel/HIFI observations of high-J CO transitions in the protoplanetary nebula CRL 618
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L3-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas components in the protoplanetary nebula CRL 618. These components are particularly important for understanding the evolution of the nebula. Methods. We performed Herschel/HIFI observations of several CO lines in the far-infrared/sub-mm in the protoplanetary nebula CRL 618. The high spectral resolution provided by HIFI allows measurement of the line profiles. Since the dynamics and structure of the nebula is well known from mm-wave interferometric maps, it is possible to identify the contributions of the different nebular components (fast bipolar outflows, double shells, compact slow shell) to the line profiles. The observation of these relatively high-energy transitions allows an accurate study of the excitation conditions in these components, particularly in the warm ones, which cannot be properly studied from the low-energy lines. Results. The (CO)-C-12 J = 16-15, 10-9, and 6-5 lines are easily detected in this source. Both (CO)-C-13 J = 10-9 and 6-5 are also detected. Wide profiles showing spectacular line wings have been found, particularly in (CO)-C-12 J = 16-15. Other lines observed simultaneously with CO are also shown. Our analysis of the CO high-J transitions, when compared with the existing models, confirms the very low expansion velocity of the central, dense component, which probably indicates that the shells ejected during the last AGB phases were driven by radiation pressure under a regime of maximum transfer of momentum. No contribution of the diffuse halo found from mm-wave data is identified in our spectra, because of its low temperature. We find that the fast bipolar outflow is quite hot, much hotter than previously estimated; for instance, gas flowing at 100 km s(-1) must have a temperature higher than similar to 200 K. Probably, this very fast outflow, with a kinematic age
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| 3. |
- Bujarrabal, V., et al.
(författare)
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Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537, s. Article Number: A8-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas in protoplanetary nebulae and young planetary nebulae (PPNe, PNe). The information that the observations of the different components deliver is of particular importance for understanding the evolution of these objects. Methods. We performed Herschel/HIFI observations of intermediate-excitation molecular lines in the far-infrared/submillimeter range in a sample of ten nebulae. The high spectral resolution provided by HIFI allows the accurate measurement of the line profiles. The dynamics and evolution of these nebulae are known to result from the presence of several gas components, notably fast bipolar outflows and slow shells (that often are the fossil AGB shells), and the interaction between them. Because of the diverse kinematic properties of the different components, their emissions can be identified in the line profiles. The observation of these high-energy transitions allows an accurate study of the excitation conditions, particularly in the warm gas, which cannot be properly studied from the low-energy lines. Results. We have detected FIR/sub-mm lines of several molecules, in particular of (CO)-C-12, (CO)-C-13, and H2O. Emission from other species, like NH3, OH, (H2O)-O-18, HCN, SiO, etc., has been also detected. Wide profiles showing sometimes spectacular line wings have been found. We have mainly studied the excitation properties of the high-velocity emission, which is known to come from fast bipolar outflows. From comparison with general theoretical predictions, we find that CRL 618 shows a particularly warm fast wind, with characteristic kinetic temperature T-k greater than or similar to 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, T-k similar to 30 K. Other nebulae, like CRL 2688, show intermediate temperatures, with characteristic values around 100 K. We also discuss how the complex structure of the nebulae can affect our estimates, considering two-component models. We argue that the differences in temperature in the different nebulae can be caused by cooling after the gas acceleration (that is probably caused by shocks); for instance, CRL 618 is a case of very recent acceleration, less than similar to 100 yr ago, while the fast gas in OH 231.8+4.2 was accelerated similar to 1000 yr ago. We also find indications that the densest gas tends to be cooler, which may be explained by the expected increase of the radiative cooling efficiency with the density.
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| 4. |
- Castro-Carrizo, A., et al.
(författare)
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Mapping the 12CO J = 1-0 and J = 2-1 emission in AGB and early post-AGB circumstellar envelopes. I. The COSAS program, first sample
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 523:4, s. A59-
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Tidskriftsartikel (refereegranskat)abstract
- We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the 12CO J = 1−0 and J = 2−1 line emission in a representative sample of circumstellar envelopes around AGB and post-AGB stars. The survey was undertaken with the aim of investigating small- and large-scale morphological and kinematical properties of the molecular environment surrounding stars in the late AGB and early post-AGB phases. For this, COSAS combines the high sensitivity and spatial resolving power of the IRAM Plateau de Bure interferometer with the better capability of the IRAM 30 m telescope to map extended emission. The global sample encompasses 45 stars selected to span a range in chemical type, variability type, evolutionary state, and initial mass. COSAS provides means to quantify variations in the mass-loss rates, assess morphological and kinematical features, and to investigate the appearance of fast aspherical winds in the early post-AGB phase. This paper, which is the first of a series of COSAS papers, presents the results from the analyses of a first sample of 16 selected sources. The envelopes around late AGB stars are found to be mostly spherical, often mingled with features such as concentric arcs (R Cas and TX Cam), a broken spiral density pattern (TX Cam), molecular patches testifying to aspherical mass-loss (WX Psc, IK Tau, V Cyg, and S Cep), and also with well-defined axisymmetric morphologies and kinematical patterns (X Her and RX Boo). The sources span a wide range of angular sizes, from relatively compact (CRL 2362, OH 104.9+2.4 and CRL 2477) to very large (χ Cyg and TX Cam) envelopes, sometimes partially obscured by self-absorption features, which particularly for IK Tau and χ Cyg testifies to the emergence of aspherical winds in the innermost circumstellar regions. Strong axial structures with more or less complex morphologies are detected in four early post-AGB stars (IRAS 20028+3910, IRAS 23321+6545, IRAS 19475+3119 and IRAS 21282+5050) of the sub-sample.
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| 5. |
- Claussen, M. J., et al.
(författare)
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A Pilot Imaging Line Survey of RW LMi and IK Tau Using the Expanded Very Large Array
- 2011
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Ingår i: Astrophysical Journal Letters. - 2041-8213 .- 2041-8205. ; 739:1, s. L5-
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Tidskriftsartikel (refereegranskat)abstract
- We report on a pilot imaging line survey (36.0-37.0 GHz, with ~1 km s–1 spectral channels) with the Expanded Very Large Array for two asymptotic giant branch stars, RW LMi (= CIT6, which has a carbon-rich circumstellar envelope, CSE) and IK Tau (=NML Tau, with an oxygen-rich CSE). Radio continuum emission consistent with photospheric emission was detected from both stars. From RW LMi we imaged the HC3N (J = 4→3) emission. The images show several partial rings of emission; these multiple shells trace the evolution of the CSE from 400 to 1200 years. SiS (J = 2→1) emission was detected from both RW LMi and IK Tau. For both stars the SiS emission is centrally condensed with the peak line emission coincident with the stellar radio continuum emission. In addition, we have detected weak HC7N (J = 32→31) emission from RW LMi.
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| 6. |
- Decin, L., et al.
(författare)
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Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L4-
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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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| 7. |
- Harwit, M., et al.
(författare)
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Polarisation observations of VY Canis Majoris H2O 532-441 620.701 GHz maser emission with HIFI
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L51-
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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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| 8. |
- Justtanont, Kay, 1965, et al.
(författare)
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A HIFI preview of warm molecular gas around chi Cygni: first detection of H2O emission toward an S-type AGB star
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L6-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. A set of new, sensitive, and spectrally resolved, sub-millimeter line observations are used to probe the warm circumstellar gas around the S-type AGB star chi Cyg. The observed lines involve high rotational quantum numbers, which, combined with previously obtained lower-frequency data, make it posible to study in detail the chemical and physical properties of, essentially, the entire circumstellar envelope of chi Cyg. Methods. The data were obtained using the HIFI instrument aboard Herschel, whose high spectral resolution provides valuable information about the line profiles. Detailed, non-LTE, radiative transfer modelling, including dust radiative transfer coupled with a dynamical model, has been performed to derive the temperature, density, and velocity structure of the circumstellar envelope. Results. We report the first detection of circumstellar H2O rotational emission lines in an S-star. Using the high-J CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho-and para-H2O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H2O abundance (relative to H-2) is (1.1 +/- 0.2) x 10(-5), much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1 +/- 0.6 is close to the high-temperature equilibrium limit, consistent with H2O being formed in the photosphere.
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| 9. |
- Justtanont, Kay, 1965, et al.
(författare)
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Herschel/HIFI observations of O-rich AGB stars: molecular inventory
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 537, s. Article Number: A144 -
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form part of the guaranteed time key program HIFISTARS are presented. The aim of this program is to study the dynamical structure, mass-loss driving mechanism, and chemistry of the outflows from AGB stars as a function of chemical composition and initial mass. Methods. We used the HIFI instrument to observe nine AGB stars, mainly in the H2O and high rotational CO lines. We investigate the correlation between line luminosity, line ratio and mass-loss rate, line width and excitation energy. Results. A total of nine different molecules, along with some of their isotopologues have been identified, covering a wide range of excitation temperature. Maser emission is detected in both the ortho-and para-H2O molecules. The line luminosities of ground state lines of ortho- and para-H2O, the high-J CO and NH3 lines show a clear correlation with mass-loss rate. The line ratios of H2O and NH3 relative to CO J = 6-5 correlate with the mass-loss rate while ratios of higher CO lines to the 6-5 is independent of it. In most cases, the expansion velocity derived from the observed line width of highly excited transitions formed relatively close to the stellar photosphere is lower than that of lower excitation transitions, formed farther out, pointing to an accelerated outflow. In some objects, the vibrationally excited H2O and SiO which probe the acceleration zone suggests the wind reaches its terminal velocity already in the innermost part of the envelope, i.e., the acceleration is rapid. Interestingly, for R Dor we find indications of a deceleration of the outflow in the region where the material has already escaped from the star.
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| 10. |
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| 11. |
- Lindqvist, Michael, 1960, et al.
(författare)
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MERLIN observations of OH/IR stars
- 2008
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Ingår i: Proceedings of the 9th European VLBI Network Symposium on The role of VLBI in the Golden Age for Radio Astronomy and EVN Users Meeting.. ; , s. 81-83
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 12. |
- Lindqvist, Michael, 1960, et al.
(författare)
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MERLIN observations of OH/IR stars
- 2008
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Ingår i: Proceedings of Science. - 1824-8039. ; 72
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Konferensbidrag (refereegranskat)abstract
- We present MERLIN OH 1612 MHz observations towards 3 OH/IR stars, OH15.7+0.8, OH38.3+1.9 and IRAS 19319+2214. The main objective of the project is to investigate if their complicated OH 1612 MHz spectral properties can be related to the phenomenon of highly time-variable mass loss found towards a handful of carbon stars, where detached shells with an overall spherical symmetry are present. Despite extensive searches, no oxygen-rich star with such a shell has been found in any molecular emission. Based on the MERLIN data however, we find that the circumstellar structure of the three OH/IR stars appears to be more complicated than that of detached shells. Signs of bipolarity in the outflows of OH38.3+1.9 and IRAS 19319+2214 could indicate that these are early post-AGB objects.
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| 13. |
- Maercker, Matthias, 1979, et al.
(författare)
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A HIFI View of Circumstellar H2O Emission from the S-type AGB Star X Cyg
- 2011
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Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. - 9781583817704 ; 445, s. 613-617
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 14. |
- Maercker, Matthias, 1979-
(författare)
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Asymptotic Giant Branch stars viewed up-close and far-off : The physics, chemistry, and evolution of their circumstellar envelopes
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The asymptotic giant branch (AGB) is the last stage of stellar evolution for stars with masses between 0.8-8 M(sun). This phase is characterised by an intense mass loss, which builds up a circumstellar envelope of dust and gas(CSE). It is through this process that the AGB stars contribute to the chemical evolution of galaxies. In addition, a rich and varied chemistry is active within the CSEs.Observations of circumstellar H2O are of particular interest, as it is expected to be one of the most abundant molecules in the inner envelopes of M-type AGB stars (with C/O<1). The first part of this thesis concerns the modelling of water vapour emission lines from CSEs around M-type AGB stars. Using satellite observations and detailed radiative transfer models, H2O abundances in these stars are determined and compared with theoretical chemical models. The importance of resolved H2O line profiles and excitation through different vibrationally excited states are also demonstrated.The second part of the thesis has its focus on the detached shells of dust and gas observed around a handful of carbon AGB stars (with C/O>1), believed to be an effect of highly time-variable mass loss during a thermal pulse. The detached shells around three sources were observed in stellar light scattered by dust and gas in the shells using ground-based and space telescopes. The observations allow a separation of the scattering agents, and reveal information on the detached shells in unprecedented detail.
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| 15. |
- Maercker, Matthias, 1979, et al.
(författare)
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Circumstellar H2O in M-type AGB stars
- 2008
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Ingår i: in Proceedings of "Organic Matter in Space", Hong Kong, Feb 18-22, 2008, IAU symposium 251, eds. S. Kwok, S. Sandford, Cambridge University Press. - 9780521889827 ; , s. 163-164, s. 163-164
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type AGB star W Hya. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains.
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| 16. |
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| 17. |
- Maercker, Matthias, et al.
(författare)
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Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 479:1, s. 779-791
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Tidskriftsartikel (refereegranskat)abstract
- Context: Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type asymptotic giant branch star W Hya. This has lead to the speculation that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains. Aims: By performing a detailed radiative transfer analysis, we determine fractional abundances of circumstellar H2O in the envelopes around six M-type asymptotic giant branch stars. The models are also used to predict H2O spectral line emission for the upcoming Herschel/HIFI mission. Methods: We use Infrared Space Observatory Long Wavelength Spectrometer spectra to constrain the circumstellar fractional abundance distribution of ortho-H2O, using a non-local thermal equilibrium, and non-local, radiative transfer code based on the accelerated lambda iteration formalism. The mass-loss rates and kinetic temperature structures for the sample stars are determined through radiative transfer modelling of CO line emission based on the Monte-Carlo method. The density and temperature profiles of the circumstellar dust grains are determined through spectral energy distribution modelling using the publicly available code Dusty. Results: The determined ortho-H2O abundances lie between 2×10-4 and 1.5×10-3 relative to H2, with the exception of WX Psc, which has a much lower estimated ortho-H2O abundance of only 2×10-6, possibly indicating H2O adsorption onto dust grains or recent mass-loss-rate modulations. The estimated abundances are uncertain by, at best, a factor of a few. Conclusions: The high water abundance found for the majority of the sources suggests that either the “normal” chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H2O (e.g. from icy bodies like Kuiper belt objects) play a role. However, more detailed information on the physical structure and the velocity field of the region where the water vapour lines are formed is required to improve abundance estimates. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI. These spectrally resolved lines cover a wide range of excitation conditions and will provide valuable additional information on the physical and chemical properties of the inner stellar wind where H2O is abundant.
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| 18. |
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| 19. |
- Maercker, Matthias, 1979, et al.
(författare)
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Detached Shells of Dust and Gas around Carbon Stars
- 2011
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Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. ; 445, s. 345-346
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 20. |
- Maercker, Matthias, 1979, et al.
(författare)
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The detached dust and gas shells around the carbon star U Ant
- 2009
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context: Geometrically thin, detached shells of gas have been found around a handful of carbon stars. Aims: Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation. Methods: We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines. Results: We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of approx 43" and a width of approx 2") consists mainly of gas, while the outer shell (at a radius of approx 50" and a width of approx 7") appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells. Conclusions: Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions.
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| 21. |
- Maercker, Matthias, 1979, et al.
(författare)
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The detached dust and gas shells around the carbon star U Antliae
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 511:1, s. A37-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Geometrically thin, detached shells of gas have been found around a handful of carbon stars. The current knowledge on these shells is mostly based on CO radio line data. However, imaging in scattered stellar light adds important new information as well as allows studies of the dust shells. Aims. Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation. Methods. We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines. Results. We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of approximate to 43 '' and a width of approximate to 2 '') consists mainly of gas, while the outer shell (at a radius of approximate to 50 '' and a width of approximate to 7 '') appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2 x 10(-3) M-circle dot, while the mass of the dust shell is estimated to be 5 x 10(-5) M-circle dot. The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 km s(-1), agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells. Conclusions. Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions. The separation into a gas and a dust shell, with different widths, is most likely the effect of different dynamical evolutions of the two media after their ejection.
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| 22. |
- Menten, K. M., et al.
(författare)
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Herschel/HIFI deepens the circumstellar NH3 enigma
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L7-
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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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| 23. |
- Neufeld, D. A., et al.
(författare)
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Discovery of water vapour in the carbon star V Cygni from observations with Herschel/HIFI
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1, s. Article Number: L5-
-
Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of water vapour toward the carbon star V Cygni. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1(11)-0(00) para-water transition at 1113.3430 GHz in the upper sideband of the Band 4b receiver. The observed spectral line profile is nearly parabolic, but with a slight asymmetry associated with blueshifted absorption, and the integrated antenna temperature is 1.69 +/- 0.17 K km s(-1). This detection of thermal water vapour emission, carried out as part of a small survey of water in carbon-rich stars, is only the second such detection toward a carbon-rich AGB star, the first having been obtained by the Submillimeter Wave Astronomy Satellite toward IRC+ 10216. For an assumed ortho-to-para ratio of 3 for water, the observed line intensity implies a water outflow rate similar to 3-6 x 10(-5) Earth masses per year and a water abundance relative to H-2 of similar to 2-5 x 10(-6). This value is a factor of at least 10(4) larger than the expected photospheric abundance in a carbon-rich environment, and - as in IRC+ 10216 - raises the intriguing possibility that the observed water is produced by the vapourisation of orbiting comets or dwarf planets. However, observations of the single line observed to date do not permit us to place strong constraints upon the spatial distribution or origin of the observed water, but future observations of additional transitions will allow us to determine the inner radius of the H2O-emitting zone, and the H2O ortho-to-para ratio, and thereby to place important constraints upon the origin of the observed water emission.
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| 24. |
- Neufeld, D. A., et al.
(författare)
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Herschel/HIFI Observations of IRC+10216: Water Vapor in the Inner Envelope of a Carbon-Rich Asymptotic Giant Branch Star
- 2011
-
Ingår i: Astrophysical Journal Letters. - 2041-8213 .- 2041-8205. ; 727:2
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of observations of 10 rotational transitions of water vapor toward the carbon-rich asymptotic giant branch (AGB) star IRC+10216 (CW Leonis), carried out with Herschel's HIFI instrument. Each transition was securely detected by means of observations using the dual beam switch mode of HIFI. The measured line ratios imply that water vapor is present in the inner outflow at small distances (
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| 25. |
- Neufeld, D. A., et al.
(författare)
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The Widespread Occurrence of Water Vapor in the Circumstellar Envelopes of Carbon-Rich Asymptotic Giant Branch Stars: First Results From a Survey With Herschel/HIFI
- 2011
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Ingår i: Astrophysical Journal Letters. - 2041-8213 .- 2041-8205. ; 727:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report the preliminary results of a survey for water vapor in a sample of eight C stars with large mid-IR continuum fluxes: V384 Per, CIT 6, V Hya, Y CVn, IRAS 15194-5115, V Cyg, S Cep, and IRC+40540. This survey, performed using the HIFI instrument on board the Herschel Space Observatory, entailed observations of the lowest transitions of both ortho-and para-water: the 556.936 GHz 1(10)-1(01) and 1113.343 GHz 1(11)-0(00) transitions, respectively. Water vapor was unequivocally detected in all eight of the target stars. Prior to this survey, IRC+10216 was the only carbon-rich asymptotic giant branch (AGB) star from which thermal water emissions had been discovered, in that case with the use of the Submillimeter Wave Astronomy Satellite (SWAS). Our results indicate that IRC+10216 is not unusual, except insofar as its proximity to Earth leads to a large line flux that was detectable with SWAS. The water spectral line widths are typically similar to those of CO rotational lines, arguing against the vaporization of a Kuiper Belt analog being the general explanation for water vapor in carbon-rich AGB stars. There is no apparent correlation between the ratio of the integrated water line fluxes to the 6.3 mu m continuum flux-a ratio which measures the water outflow rate-and the total mass-loss rate for the stars in our sample.
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| 26. |
- Olofsson, Hans, 1952, et al.
(författare)
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High-resolution HST/ACS images of detached shells around carbon stars
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 515:JUN 2010, s. A270-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Overall spherically symmetric, geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch (AGB) carbon stars. Their dynamical ages lie in the range of 10(3) to 10(4) years. A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash. Aims. The detached shells carry information on their formation process, as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion. Methods. The youngest detached shells, those around the carbon stars R Scl and U Cam, are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope. Quantitative results are derived assuming optically thin dust scattering. Results. The detached dust shells around R Scl and U Cam are found to be consistent with an overall spherical symmetry. They have radii of 19 ''.2 (corresponding to a linear size of 8 x 10(16) cm) and 7 ''.7 (5 x 10(16) cm), widths of 1 ''.2 (5 x 10(15) cm) and 0 ''.6 (4 x 10(15) cm), and dust masses of 3 x 10(-6) and 3 x 10(-7) M-circle dot, respectively. The dynamical ages of the R Scl and U Cam shells are estimated to be 1700 and 700 yr, respectively, and the shell widths correspond to time scales of 100 and 50 yr, respectively. Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the R Scl shell. Average clump dust masses are estimated to be about 2 x 10(-9) M-circle dot. Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (
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| 27. |
- Ramstedt, Sofia, et al.
(författare)
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Circumstellar molecular line emission from S-type AGB stars : Mass-loss rates and SiO abundances
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:9, s. 515-527
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40 S-type AGB stars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGB stars and carbon stars to look for trends with chemical type.Methods. The circumstellar envelopes are assumed to be spherically symmetric and formed by a constant mass-loss rate. The mass-loss rates are estimated from fitting the CO observations using a non-local, non-LTE radiative transfer code based on the Monte Carlo method. In the excitation analysis, the energy balance equation is solved self-consistently simultaneously as the radiative transfer and the temperature structure of the gas is derived. Effects of dust grains are also included in the molecular excitation analysis. Once the physical properties of the circumstellar envelopes are determined, the same radiative transfer code is used to model the observed SiO lines in order to derive circumstellar abundances and the sizes of the SiO line-emitting regions.Results. We have estimated mass-loss rates of 40 S-type AGB stars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGB stars and carbon stars. The estimated mass-loss rates also correlate well with the corresponding expansion velocity of the envelope, in accordance with results for M-type AGB stars and carbon stars. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGB stars. In addition, we have estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of the sample S-type AGB stars. The derived SiO abundances are, on average, about an order of magnitude higher than predicted by stellar atmosphere thermal equilibrium chemistry, indicating that non-equilibrium chemical processes determines the abundance of SiO in the circumstellar envelope. Moreover, a comparison with the results for M-type AGB stars and carbon stars show that for a certain mass-loss rate, the circumstellar SiO abundance seems independent (although with a large scatter) of the C/O-ratio.Conclusions. In our comparison of S-type AGB stars with carbon stars and M-type AGB stars, we find no large differences in circumstellar physical properties or SiO abundances depending on the chemical type of the star.
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| 28. |
- Ramstedt, S., et al.
(författare)
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Imaging the circumstellar dust around AGB stars with PolCor
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 531
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this paper is to investigate how the new imaging Polarimeter and Coronograph (PolCor) at the Nordic Optical Telescope** (NOT) can be used in the study of circumstellar structures around AGB stars. The purpose is to prepare for a study of a larger sample. Methods. We have observed two types of AGB stars using the PolCor instrument on the NOT: the binary S-type star W Aql and two carbon stars with detached shells, U Cam and DR Ser. The polarized light traces the dust distribution around the stars. From the polarimeter images the polarized intensity, the polarization degree, and the polarization angle over the images are calculated. The location and extent of dust structures are examined in the images. The total dust mass and the dust-to-gas ratios of the detached shells are also calculated. Results. The images of the circumstellar envelope of W Aql show what seems to be an elongated structure in the south-west direction. The detached shells of U Cam and DR Ser are clearly seen in the images. This is the first time the detached shell around DR Ser has been imaged. The radii (R(sh)) and widths (Delta R(sh)) of the shells are determined and found to be R(sh) = 7 ''.9 and 7 ''.6, and Delta R(sh) = 0 ''.9 and 1 ''.2, for U Cam and DR Ser, respectively. This is consistent with previous results. The dust masses of the feature south-west of W Aql, and in the shells of U Cam and DR Ser are also estimated and found to be 1 x 10(-6), 5 x 10(-7), and 2 x 10(-6) M(circle dot), respectively. Conclusions. W Aql is a known binary and the shape of the circumstellar envelope seems to be in line with what could be expected from binary interaction on these scales. For the shells, the results are in agreement with previous investigations. Ages and formation time-scales are also estimated for the detached shells and found to be consistent with the thermal-pulse-formation scenario.
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| 29. |
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| 30. |
- Ramstedt, Sofia, et al.
(författare)
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Mass-loss properties of S-stars on the AGB
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 454:2, s. L103-L106
-
Tidskriftsartikel (refereegranskat)abstract
- We have used a detailed non-LTE radiative transfer code to model new APEX CO(J = 3 → 2) data, and existing CO radio line data, on a sample of 40 AGB S-stars. The derived mass-loss-rate distribution has a median value of 2 × 10-7~Mȯ yr-1, and resembles values obtained for similar samples of M-stars and carbon stars. Possibly, there is a scarcity of high-mass-loss-rate (≥10-5~Mȯ yr-1) S-stars. The distribution of envelope gas expansion velocities is similar to that of the M-stars, the median is 7.5 km s-1, while the carbon stars, in general, have higher gas expansion velocities. The mass-loss rate correlates well with the gas expansion velocity, in accordance with results for M-stars and carbon stars.
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| 31. |
- Ramstedt, Sofia, 1974-
(författare)
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Molecules and Dust around AGB stars : Mass-loss rates and molecular abundances
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- All stars with a stellar mass of about 0.8–8 MSun will end their lives as asymptotic giant branch (AGB) stars. Through their extensive mass loss the AGB stars constitutes an important source of nuclear processed material. They also provide us with fascinating systems where an interchange of different physical and chemical processes occur, making them excellent astrophysical laboratories.Being the most important process for the evolution of an AGB star, the mass loss is well established, but its details are less well known. On the AGB, the mass-loss rate can span several orders of magnitude, reaching 10−4 MSun yr−1 toward the end of the AGB. It is challenging to find reliable methods to estimate the mass-loss rates of individual objects. Nevertheless, it is important, since the mass-loss rate affects the derived abundances of other molecules in the circumstellar envelope, and therefore the estimates of the amount and composition of the recycled material. In the first part of the thesis the reliability of mass-loss rate estimates is evaluated using two main methods; the observations and radiative transfer analysis of CO radio line emission, and dust radiative transfer combined with a dynamical model.The second part of the thesis focuses on a particular chemical type of AGB stars; the S-type. The S-stars are believed to have approximately the same amount of carbon as oxygen in the photosphere, and to be an intermediate evolutionary stage as the star evolves from an oxygen-rich M-star into a carbon star. As possible transition objects the S-stars might give important clues to the mass-loss mechanisms and to the chemical evolution along the AGB. Using observations of circumstellar radio line emission in combination with detailed radiative transfer analysis, we have estimated mass-loss rates and abundances of chemically important molecules for a sample of 40 S-stars. The results are compared to previous results for M- and carbon stars.
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| 32. |
- Ramstedt, Sofia, et al.
(författare)
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On the reliability of mass-loss-rate estimates for AGB stars
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 487:2, s. 645-657
-
Tidskriftsartikel (refereegranskat)abstract
- Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a mass-loss-rate estimator for AGB stars. Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate-to high-mass-loss-rate AGB stars with different photospheric chemistries. Methods. Mass-loss rates for 10 intermediate-to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles. Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of similar to 3 for intermediate- to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H(2)-abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.
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| 33. |
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| 34. |
- Ramstedt, Sofia, et al.
(författare)
-
The Mass-Loss Rates and Molecular Abundances of S-type AGB Stars
- 2011
-
Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. - 9781583817704 ; 445, s. 263-268
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 35. |
- Ramstedt, Sofia, et al.
(författare)
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The physics and chemistry of circumstellar envelopes of S-stars on the AGB
- 2008
-
Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 4:S251, s. 167-168, s. 167-168
-
Tidskriftsartikel (refereegranskat)abstract
- Presented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.
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| 36. |
- Schmidt, M. R., et al.
(författare)
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Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
- 2016
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 592
-
Tidskriftsartikel (refereegranskat)abstract
- Context. A discrepancy exists between the abundance of ammonia (NH3) derived previously for the circumstellar envelope (CSE) of IRC+10216 from far-IR submillimeter rotational lines and that inferred from radio inversion or mid-infrared (MIR) absorption transitions. Aims. To address the discrepancy described above, new high-resolution far-infrared (FIR) observations of both ortho-and para-NH3 transitions toward IRC+10216 were obtained with Herschel, with the goal of determining the ammonia abundance and constraining the distribution of NH3 in the envelope of IRC+10216. Methods. We used the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel to observe all rotational transitions up to the J = 3 level (three ortho-and six para-NH3 lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. The computed emission line profiles are compared with the new HIFI data, the radio inversion transitions, and the MIR absorption lines in the nu(2) band taken from the literature. Results. We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH3. The derived NH3 abundances relative to molecular hydrogen were (2.8 +/- 0.5) x 10(-8) for ortho-NH3 and (3.2(-0.6)(+0.7)) x 10(-8) for para-NH3, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH3 inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1 sigma confidence level).
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| 37. |
- Schöier, Fredrik, 1969, et al.
(författare)
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A chemical inventory of the S-type AGB star chi Cygni based on Herschel/HIFI observations of circumstellar line emission The importance of non-LTE chemical processes in a dynamical region
- 2011
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 530
-
Tidskriftsartikel (refereegranskat)abstract
- Context. S-type AGB stars (C/O approximate to 1) are thought to be transition objects from M-type (O-rich) AGB stars to carbon stars and as such are interesting objects in themselves. Of particular interest is to determine accurate circumstellar properties and molecular abundances, due to their predicted sensitivity to the photospheric C/O-ratio. Aims. Presented here are new sensitive sub-millimetre line observations of molecules towards the S-type AGB star chi Cyg, using the HIFI instrument on-board the Herschel Space Observatory. The observed lines predominantly probe warm gas relatively close to the central star. Methods. Detailed, non-LTE, radiative transfer modelling has been used in order to interpret the circumstellar molecular line observations performed using HIFI, assuming a spherically symmetric, smooth, accelerating wind. Results. Lines from common molecules such as H2O, CO and SiO, which are expected to be abundant in an S-type AGB star, are clearly detected (as well as some of their isotopologues) in the HIFI spectra. In addition, we detect lines from carbon-bearing molecules such as HCN and CN. The CO line modelling indicates that the mass-loss rate has not undergone any significant modulations during the past approximate to 1000 yr. The derived o-H2O fractional abundance is approximate to 7x 10(-6), i.e., lower than those obtained for a small sample of M-type AGB stars but higher than what has been derived for a few carbon stars. We further obtain a p-H2O fractional abundance of approximate to 5 x 10(-6) giving an o/p-ratio of approximate to 1.4. Molecular line cooling is dominated by H2O only in a region close to the star (approximate to 6x 10(14) cm). The SiO abundance is estimated to be approximate to 1 x 10(-5). The (CO)-C-12/(CO)-C-13 ratio is 43 +/- 6. The high-excitation rotational lines clearly probe the acceleration region of the stellar wind (less than or similar to 2 x 10(15) cm) and put constraints on dynamical wind models. We are unable to fit consistently the combined ground-based and HIFI data for HCN and CN. Conclusions. The derived H2O abundance is reasonably consistent with recent chemical model predictions and so is the SiO abundance. The o/p-ratio of approximate to 1.4 supports a chemical formation under non-LTE conditions for the H2O molecules, and the presence of carbon-bearing molecules at relatively large abundances is also indicative of the importance of non-LTE chemical processes in regulating the circumstellar chemistry. The velocity field derived from the molecular line modelling is consistent with that obtained from solving for the wind dynamics through the coupled momentum equations of the dust and gas particles.
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| 38. |
- Schöier, Fredrik, 1969, et al.
(författare)
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AGB Mass-Loss Characteristics: Rates, Temporal Variation, and Geometries
- 2011
-
Ingår i: Proceedings from conference Why Galaxies Care about AGB stars II, in Vienna 16 - 20 August 2010; published in Astronomical Society of the Pacific conf series, vol 445. - 9781583817704 ; 445, s. 253-262
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 39. |
- Schöier, Fredrik, 1969, et al.
(författare)
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An atomic and molecular database for analysis of submillimetre line observations
- 2005
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 432, s. 369-379
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Tidskriftsartikel (refereegranskat)abstract
- Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to $E/k \sim 1000$ K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.
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| 40. |
- Schöier, Fredrik, 1969, et al.
(författare)
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Probing chemical processes in AGB stars
- 2008
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Ingår i: in Proceedings of "Organic matter in space", Hong Kong Feb. 18-22, 2008, IAU symposium 251, eds. S. Kwok, S. Sandfjord, Cambridge University Press. - 9780521889827 ; , s. 201-205
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 41. |
- Schöier, Fredrik, 1969, et al.
(författare)
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The abundance of HCN in the circumstellar envelopes of AGB stars different chemical type
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 550
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. Methods. In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. Results. The median values for the derived abundances of HCN (with respect to H-2) are 3 x 10(-5), 7 x 10(-7) and 10(-7) for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. Conclusions. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.
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| 42. |
- Schöier, Fredrik, et al.
(författare)
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The abundance of SiS in circumstellar envelopes around AGB stars
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 473:3, s. 871-882
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Tidskriftsartikel (refereegranskat)abstract
- Aims.Given their photospheric origin and refractive nature, SiS molecules can provide major constraints on the relative roles of dust condensation and non-equilibrium processes in regulating the chemistry in circumstellar envelopes around evolved stars.Methods: New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including assessment of the effect of thermal dust grains in the excitation analysis.Results: We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O > 1) have a mean fractional abundance of 3.1 × 10-6, about one order of magnitude higher than that found for the M-type AGB stars (C/O < 1) where the mean value is 2.7 × 10-7. These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similarly to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.
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| 43. |
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| 44. |
- Schöier, Fredrik, et al.
(författare)
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The Distribution of H13CN in the Circumstellar Envelope around IRC+10216
- 2007
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Ingår i: The Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 670:1, s. 766-773
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Tidskriftsartikel (refereegranskat)abstract
- H13CN J=8-->7 submillimeter line emission produced in the circumstellar envelope around the extreme carbon star IRC+10216 has been imaged at subarcsecond angular resolution using the SMA. Supplemented by a detailed excitation analysis, the average fractional abundance of H13CN in the inner wind (<~5×1015 cm) is estimated to be about 4×10-7, translating into a total HCN fractional abundance of 2×10-5 using the isotopic ratio 12C/13C = 50. Multitransitional single-dish observations further require the H13CN fractional abundance to remain more or less constant in the envelope out to a radius of ~4×1016 cm, where the HCN molecules are effectively destroyed, most probably, by photodissociation. The large amount of HCN present in the inner wind provides effective line cooling that can dominate over that generated from CO line emission. It is also shown that great care needs to be taken in the radiative transfer modeling where nonlocal, and non-LTE, effects are important and where the radiation field from thermal dust grains plays a major role in exciting the HCN molecules. The amount of HCN present in the circumstellar envelope around IRC+10216 is consistent with predicted photospheric values based on equilibrium chemical models and indicates that any nonequilibrium chemistry occurring in the extended pulsating atmosphere has no drastic net effect on the fractional abundance of HCN molecules that enters the outer envelope. It further suggests that few HCN molecules are incorporated into dust grains.
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| 45. |
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| 46. |
- Schöier, Fredrik, et al.
(författare)
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The Distribution of SiO in the Circumstellar Envelope around IRC +10216
- 2006
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Ingår i: The Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 649:2, s. 965-972
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Tidskriftsartikel (refereegranskat)abstract
- New interferometric observations of SiO J=5-->4 circumstellar line emission around the carbon star IRC +10216 using the Submillimeter Array are presented. Complemented by multitransition single-dish observations, including infrared observations of rovibrational transitions, detailed radiative transfer modeling suggests that the fractional abundance of SiO in the inner part of the envelope, between ~3 and 8 stellar radii, is as high as ~1.5×10-6. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (re~2.4×1016 cm) low fractional abundance (f0~1.7×10-7) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind, while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2-5 on an angular scale corresponding to a timescale of about 200 yr. In addition, constraints on the velocity structure of the wind are obtained.
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| 47. |
- Tenenbaum, E.D., et al.
(författare)
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The 1 mm spectrum of VY Canis Majoris: Chemistry in an O-rich envelope
- 2008
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Ingår i: in Proceedings of "Organic Matter in Space", Hong Kong Feb. 18-22, 2008, IAU symposium 251, eds. S. Kwok, S. Sandfjord, Cambridge University Press. - 9780521889827 ; , s. 171-172
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present preliminary results of an unbiased spectral survey at 1 mm of the oxygen-rich supergiant, VY CMa. A number of exotic molecules have been detected, including NaC1 and PO, and a relatively rich organic chemistry is observed. Results of the survey will be compared with carbon-rich stars.
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| 48. |
- Teyssier, D., et al.
(författare)
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Herschel/HIFI observations of red supergiants and yellow hypergiants I. Molecular inventory
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545, s. A99 (article no.)-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Red supergiant stars (RSGs) and yellow hypergiant stars (YHGs) are believed to be the high-mass counterparts of stars in the asymptotic giant branch (AGB) and early post-AGB phases. As such, they are scarcer and the properties and evolution of their envelopes are still poorly understood. Aims. We study the mass-loss in the post main-sequence evolution of massive stars, through the properties of their envelopes in the intermediate and warm gas layers. These are the regions where the acceleration of the gas takes place and the most recent mass-loss episodes can be seen. Methods. We used the HIFI instrument on-board the Herschel Space Observatory to observe sub-millimetre and far-infrared (FIR) transitions of CO, water, and their isotopologues in a sample of two RSGs (NML Cyg and Betelgeuse) and two YHGs (IRC + 10420 and AFGL 2343) stars. We present an inventory of the detected lines and analyse the information revealed by their spectral profiles. A comparison of the line intensity and shape in various transitions is used to qualitatively derive a picture of the envelope physical structure. On the basis of the results presented in an earlier study, we model the CO and (CO)-C-13 emission in IRC + 10420 and compare it to a set of lines ranging from the millimetre to the FIR. Results. Red supergiants have stronger high-excitation lines than the YHGs, indicating that they harbour dense and hot inner shells contributing to these transitions. Consequently, these high-J lines in RSGs originate from acceleration layers that have not yet reached the circumstellar terminal velocity and have narrower profiles than their flat-topped lower-J counterparts. The YHGs tend to lack this inner component, in line with the picture of detached, hollow envelopes derived from studies at longer wavelengths. NH3 is only detected in two sources (NML Cyg and IRC + 10420), which are also observed to be the strongest water-line emitters of the studied sample. In contrast, OH is detected in all sources and does not seem to correlate with the water line intensities. We show that the IRC + 10420 model derived solely from millimetre low-J CO transitions is capable of reproducing the high-J transitions when the temperature in the inner shell is simply lowered by about 30%.
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