Circumstellar molecular composition of the oxygen-rich AGB star IK Tau: I. Observations and LTE chemical abundance analysis

摘要

The aim of this paper is to study the molecular composition in thecircumstellar envelope around the oxygen-rich star IK Tau. We observed IK Tauin several (sub)millimeter bands using the APEX telescope during threeobserving periods. To determine the spatial distribution of the$\mathrm{^{12}CO(3-2)}$ emission, mapping observations were performed. Toconstrain the physical conditions in the circumstellar envelope, multiplerotational CO emission lines were modeled using a non local thermodynamicequilibrium radiative transfer code. The rotational temperatures and theabundances of the other molecules were obtained assuming local thermodynamicequilibrium. An oxygen-rich Asymptotic Giant Branch star has been surveyed inthe submillimeter wavelength range. Thirty four transitions of twelve molecularspecies, including maser lines, were detected. The kinetic temperature of theenvelope was determined and the molecular abundance fractions of the moleculeswere estimated. The deduced molecular abundances were compared withobservations and modeling from the literature and agree within a factor of 10,except for SO$_2$, which is found to be almost a factor 100 stronger thanpredicted by chemical models. From this study, we found that IK Tau is a goodlaboratory to study the conditions in circumstellar envelopes aroundoxygen-rich stars with (sub)millimeter-wavelength molecular lines. We couldalso expect from this study that the molecules in the circumstellar envelopecan be explained more faithful by non-LTE analysis with lower and highertransition lines than by simple LTE analysis with only lower transition lines.In particular, the observed CO line profiles could be well reproduced by asimple expanding envelope model with a power law structure.