Issue |
A&A
Volume 480, Number 2, March III 2008
|
|
---|---|---|
Page(s) | 431 - 438 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20078892 | |
Published online | 17 January 2008 |
Detection of “parent” molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry
1
Department of Physics and Astronomy, Institute for Astronomy, K.U.Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: leen.decin@ster.kuleuven.be
2
Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403 1098 Amsterdam, The Netherlands
3
Institut für Astronomie, ETH Hönggerberg, Wolfgang-Pauli-Strasse 16, 8093 Zürich, Switzerland e-mail: isabelle.chechnneff@phys.ethz.ch
4
Laboratoire AIM, CEA/DSM – CNRS – University Paris Diderot, DAPNIA/SAp, 91191 Gif-sur-Yvette, France e-mail: sacha.hony@cea.fr
5
European Southern Observatory, Karl-Schwarschild Strasse, 85748 Garching bei München, Germany e-mail: cdebreuck@eso.org
6
MPI für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kmenten@mpifr-bonn.mpg.de
Received:
22
October
2007
Accepted:
13
December
2007
Context.Asymptotic Giant Branch (AGB) stars are typified by strong dust-driven, molecular outflows. For long, it was believed that the molecular content of the circumstellar envelope of AGB stars is primarily determined by the atmospheric C/O ratio. However, recent observations of molecules such as HCN, SiO, and SO reveal gas-phase abundances higher than predicted by thermodynamic equilibrium (TE) models. UV-photon initiated dissociation in the outer envelope or non-equilibrium formation by the effect of shocks in the inner envelope may be the origin of the anomolous abundances.
Aims.We aim to detect (i) a group of “parent” molecules (CO, SiO, HCN, CS), predicted by non-equilibrium studies to form with almost constant abundances independent of the C/O ratio and the stellar evolutionary stage on the Asymptotic Giant Branch (AGB), and (ii) the few molecules, such as SiS and SO, that are sensitive to the O- or C-rich nature of the star.
Methods.Several low and high excitation rotational transitions of key molecules are observed at mm and sub-mm wavelengths with JCMT and APEX in four AGB stars: the oxygen-rich Mira WX Psc, the S star W Aql, and the two carbon stars V Cyg and II Lup. A critical density analysis is performed to determine the formation region of the high-excitation molecular lines.
Results.We detect the four “parent” molecules in all four objects, implying that, indeed, these chemical species form whatever the stage of evolution on the AGB. High-excitation lines of SiS are also detected in three stars with APEX, whereas SO is only detected in the oxygen-rich star WX Psc.
Conclusions.This is the first multi-molecular observational proof that periodically shocked layers above the photosphere of AGB stars show some chemical homogeneity, whatever the photospheric C/O ratio and stage of evolution of the star.
Key words: astrochemistry / molecular processes / stars: AGB and post-AGB / stars: circumstellar matter / stars: mass-loss / submillimeter
© ESO, 2008
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