Volume 480, Number 2, March III 2008
|Page(s)||431 - 438|
|Section||Interstellar and circumstellar matter|
|Published online||17 January 2008|
Detection of “parent” molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry
Department of Physics and Astronomy, Institute for Astronomy, K.U.Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: firstname.lastname@example.org
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: email@example.com
4 Laboratoire AIM, CEA/DSM – CNRS – University Paris Diderot, DAPNIA/SAp, 91191 Gif-sur-Yvette, France e-mail: firstname.lastname@example.org
5 European Southern Observatory, Karl-Schwarschild Strasse, 85748 Garching bei München, Germany e-mail: email@example.com
6 MPI für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: firstname.lastname@example.org
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.