Volume 655, November 2021
|Number of page(s)||28|
|Section||Interstellar and circumstellar matter|
|Published online||24 November 2021|
ATOMIUM: halide molecules around the S-type AGB star W Aquilae
Department of Physics and Astronomy, Institute of Astronomy,
KU Leuven, Celestijnenlaan 200D,
2 University of Leeds, School of Chemistry, Leeds LS2 9JT, UK
3 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
4 LSAMA, Department of Physics, Faculty of Sciences, University Tunis El Manar, Campus Universitaire, 1060 Tunis, Tunisia
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6 JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
7 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
8 Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33615 Pessac, France
9 University of Amsterdam, Anton Pannekoek Institute for Astronomy, 1090 GE Amsterdam, The Netherlands
10 National Astronomical Research Institute of Thailand, Chiangmai 50180, Thailand
11 Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB), CP 226, 1060 Brussels, Belgium
12 Max-Planck-Institut für Radioastronomie, 53121 Bonn, Germany
13 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
14 Université Côte d’Azur, Laboratoire Lagrange, Observatoire de la Côte d’Azur, 06304 Nice Cedex 4, France
15 Universität zu Köln, I. Physikalisches Institut, 50937 Köln, Germany
16 California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA 91109, USA
17 SRON Netherlands Institute for Space Research, 3584 CA Utrecht, The Netherlands
18 Radboud University, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Nijmegen, The Netherlands
19 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
20 University College London, Department of Physics and Astronomy, London WC1E 6BT, UK
21 University of Hong Kong, Laboratory for Space Research, Pokfulam, Hong Kong
22 School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Accepted: 9 September 2021
Context. S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. W Aql is a nearby S-type star, with well-known circumstellar parameters, making it an ideal object for in-depth study of less common molecules.
Aims. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results.
Methods. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of Al35Cl, Al37Cl, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species.
Results. AlCl is found in clumps very close to the star, with emission confined within 0′′.1 of the star. AlF emission is more extended, with faint emission extending 0′′.2 to 0′′.6 from the continuum peak. We find peak abundances, relative to H2, of 1.7 × 10−7 for Al35Cl, 7 × 10−8 for Al37Cl, and 1 × 10−7 for AlF. From the PACS spectra, we find abundances of 9.7 × 10−8 and ≤10−8, relative to H2, for HCl and HF, respectively.
Conclusions. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen inthe wind.
Key words: stars: AGB and post-AGB / circumstellar matter / submillimeter: stars / stars: individual: W Aql / stars: individual: χ Cyg
© ESO 2021
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