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

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 nonequilibrium 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 ( 12 CO, 13 CO, SiS, 28 SiO, 29 SiO, 30 SiO, HCN, CN, CS, SO, SO 2 ) 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 SO 2 lines is cumbersome, possibly related to line misidentifications or problems with the collisional rates.

Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen byHerschel/HIFI

“…The latter most likely reflects the very difficult calibration of this frequency setting (for which standing waves heavily perturbed the baseline, see Bujarrabal et al 2010). This result is consistent with the temperature structure in the region between 20 and 100 R derived by Decin et al (2010).…”

“…The Gaussian HCN line profiles indeed point toward line formation partially in the inner wind where the stellar wind has not yet reached its full terminal velocity (Bujarrabal & Alcolea 1991). The results of Decin et al (2010) infer a wind acceleration that is lower than derived from solving the momentum equation.…”

“…Different transitions can be used to investigate different regions of the envelope, probing the density, the temperature, and the velocity of the CSE. Decin et al (2010) used the 12 CO J = 1-0 to J = 7-6 lines to determine the thermophysical structure of the CSE of IK Tau beyond ∼100 R , based on a non-local thermodynamic equilibrium (non-LTE) radiative transfer analysis of the available transitions. In the first instance, the kinetic temperature and velocity structure of the envelope were calculated by solving the equations of motion of gas and dust and the energy balance simultaneously.…”

“…Adopting the thermodynamic structure derived in Decin et al (2010) (and reproduced in Fig. B.1 of the Appendix), the theoretical line profiles for the 12 CO J = 10-9 and J = 16-15 are calculated (see Fig.…”

“…Currently, a dozen different molecules and some of their isotopologs have been discovered in IK Tau, including CO, HCN, SiO, SiS, SO, SO 2 , and NaCl (e.g. Milam et al 2007;Decin et al 2010).…”