[Paper Review] Circumstellar molecular composition of the oxygen-rich AGB star IK~Tau: II. In-depth non-LTE chemical abundance analysis

L. Decin, E. De Beck+7 more|arXiv (Cornell University)|Apr 12, 2010

Astrophysics and Star Formation Studies1 references68 citations

TL;DR

This study performs a non-LTE radiative transfer analysis of 11 molecules and isotopologs in the circumstellar envelope of the oxygen-rich AGB star IK~Tau, using high-resolution line profiles and velocity structure to derive chemical abundance stratifications. It reveals slower wind acceleration than classical models predict, SiO and SiS depletion likely due to dust adsorption, and anomalous SiO isotopic ratios suggesting enhanced 28SiO, while HCN and CS show discrepancies from non-equilibrium models, possibly implicating dust formation processes.

ABSTRACT

Aims: Little information exists on the circumstellar molecular abundance stratifications of many molecules. The aim is 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. The SiO isotopic ratios ($^{29}$SiO/$^{28}$SiO and $^{30}$SiO/$^{28}$SiO) point toward an enhancement in $^{28}$SiO compared to results of classical stellar evolution codes. Predictions for H$_2$O lines in the spectral range of the Herschel/HIFI mission are performed. [abbreviated]

Motivation & Objective

• To investigate the circumstellar chemical abundance stratification of 11 key molecules and isotopologs in the oxygen-rich AGB star IK~Tau.
• To address the lack of observational constraints on molecular abundances in oxygen-rich envelopes compared to their carbon-rich counterparts.
• To test theoretical models of non-equilibrium chemistry, dust-gas interactions, and photodissociation in the inner and outer wind zones.
• To assess the reliability of non-LTE radiative transfer modeling using line profiles and intensity data.
• To predict H2O emission features for future Herschel/HIFI observations.

Proposed method

• A non-LTE radiative transfer model is used to simultaneously solve for temperature, velocity structure, and molecular excitation in the circumstellar envelope.
• The model incorporates detailed molecular data including energy levels, Einstein A coefficients, and collisional rate coefficients from the LAMDA and CDMS databases.
• Line profiles and integrated intensities from observed spectra are used as diagnostics to constrain the abundance structure and wind dynamics.
• Chemical abundance patterns are compared with theoretical predictions including photodissociation, cosmic ray ionization, and dust-gas reactions.
• Vibrational and rotational transitions for CO, SiO, HCN, CN, CS, SO, SO2, and H2O are modeled with attention to hyperfine splitting, l-type doubling, and vibrational excitation.
• The analysis includes extrapolation of collisional rates to higher temperatures where necessary and uses high-accuracy line lists (e.g., Barber et al. 2006) for H2O.

Experimental results

Research questions

• RQ1 What is the true chemical abundance distribution of CO, SiO, SiS, HCN, CS, CN, SO, SO2, and H2O in the circumstellar envelope of IK~Tau?
• RQ2 How do non-LTE effects and wind acceleration profiles compare with classical AGB outflow theories?
• RQ3 To what extent are SiO and SiS depleted due to adsorption onto dust grains?
• RQ4 Why do observed HCN and CS line profiles deviate from predictions of inner wind non-equilibrium chemistry?
• RQ5 What are the implications of the SiO isotopic ratios (29SiO/28SiO and 30SiO/28SiO) for stellar nucleosynthesis and evolution?

Key findings

• The wind acceleration in IK~Tau is significantly slower than predicted by classical AGB outflow theories, indicating a need to revise dynamical models.
• SiO and SiS show strong depletion in the circumstellar envelope, likely due to adsorption onto dust grains formed in the inner wind.
• The 29SiO/28SiO and 30SiO/28SiO isotopic ratios indicate an enhancement of 28SiO compared to classical stellar evolution models, suggesting possible nucleosynthetic anomalies or non-standard mixing.
• HCN and CS exhibit line profiles inconsistent with inner wind non-equilibrium chemistry models, possibly indicating their involvement in dust formation or uncertainties in theoretical reaction networks.
• Low signal-to-noise profiles for SO and CN prevent accurate abundance determination, while modeling of high-excitation SO2 lines is hampered by potential line misidentifications or inaccurate collisional rate coefficients.
• Predictions for H2O emission lines in the Herschel/HIFI band are provided, enabling future observational validation of the non-LTE model.