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

摘要

Context. Molecular lines in the (sub)millimeterwavelength range can provide important information about the physicaland chemical conditions in the circumstellar envelopes aroundasymptotic giant branch stars. Aims. The aim of this paper is to study themolecular composition in the circumstellar envelope around theoxygen-rich star IKTau. Methods. We observed IK Tau in several(sub)millimeter bands using the APEX telescope during three observingperiods. To determine the spatial distribution of theemission,mapping observations were performed. To constrain thephysical conditions in the circumstellar envelope, multiple rotationalCO emission lines were modeled using a nonlocal thermodynamicequilibrium radiative transfer code. The rotational temperatures andthe abundances of the other molecules were obtained assuming localthermodynamic equilibrium. Results. An oxygen-rich asymptotic giant branch starhas been surveyed in the submillimeter wavelength range. Thirty fourtransitions of twelve molecular species, including maser lines, weredetected. The kinetic temperature of the envelope was determined, andthe molecular abundance fractions of the molecules were estimated. Thededuced molecular abundances were compared with observations andmodeling from the literature and agree within a factor of 10, exceptfor SO2, which is found to be almost a factor100 stronger than predicted by chemical models. Conclusions. From this study, we found that IK Tauis a good laboratory for studying the conditions in circumstellarenvelopes around oxygen-rich stars with (sub)millimeter-wavelengthmolecular lines. We could also expect from this study that themolecules in the circumstellar envelope can be explained morefaithfully by non-LTE analysis with lower and higher transition linesthan by simple LTE analysis with only lower transition lines. Inparticular, the observed CO line profiles could be well reproduced by asimple expanding envelope model with a power-law structure. Key words: line: profiles - methods:observational - telescopes - stars: mass-loss - stars: abundances -submillimeter: stars