Circumstellar molecular composition of the oxygen-rich AGB star IK Tauri

II. In-depth non-LTE chemical abundance analysis

¹ Department of Physics and Astronomy, Institute of Astronomy, K.U.Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: Leen.Decin@ster.kuleuven.ac.be
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, PO Box 9429, 1090 CE Amsterdam, The Netherlands
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Street 77, 50937 Köln, Germany
⁵ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ MPI für Gravitationsphysik, Calinstr. 38, 30167 Hannover, Germany
⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁸ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands

Received: 26 January 2010
Accepted: 30 March 2010

Abstract

Context. The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflow from these stars creates a circumstellar envelope in which a rich gas-phase chemistry takes place. Complex shock-induced non-equilibrium chemistry takes place in the inner wind envelope, dust-gas reactions and ion-molecule reactions alter the abundances in the intermediate wind zone, and the penetration of cosmic rays and ultraviolet photons dissociates the molecules in the outer wind region.

Aims. Little observational information exists on the circumstellar molecular abundance stratifications of many molecules. Furthermore, our knowledge of oxygen-rich envelopes is not as profound as for the carbon-rich counterparts. The aim of this paper is therefore to study the circumstellar chemical abundance pattern of 11 molecules and isotopologs (¹²CO, ¹³CO, SiS, ²⁸SiO, ²⁹SiO, ³⁰SiO, HCN, CN, CS, SO, SO₂) in the oxygen-rich evolved star IK Tau.

Methods. We have performed an in-depth analysis of a large number of molecular emission lines excited in the circumstellar envelope around IK Tau. The analysis is done based on a non-local thermodynamic equilibrium (non-LTE) radiative transfer analysis, which calculates the temperature and velocity structure in a self-consistent way. The chemical abundance pattern is coupled to theoretical outer wind model predictions including photodestruction and cosmic ray ionization. Not only the integrated line intensities, but also the line shapes are used as diagnostic tool to study the envelope structure.

Results. The deduced wind acceleration is much slower than predicted from classical theories. SiO and SiS are depleted in the envelope, possibly due to the adsorption onto dust grains. For HCN and CS a clear difference with respect to inner wind non-equilibrium predictions is found, either indicating uncertainties in the inner wind theoretical modeling or the possibility that HCN and CS (or the radical CN) participate in the dust formation. The low signal-to-noise profiles of SO and CN prohibit an accurate abundance determination; the modeling of high-excitation SO₂ lines is cumbersome, possibly related to line misidentifications or problems with the collisional rates. The SiO isotopic ratios (²⁹SiO/²⁸SiO and ³⁰SiO/²⁸SiO) point toward an enhancement in ²⁸SiO compared to results of classical stellar evolution codes. Predictions for H₂O emission lines in the spectral range of the Herschel/HIFI mission are performed.