Line formation in AGB atmospheres including velocity effects. Molecular line profile variations of long period variables

Abstract

The atmospheres of evolved red giants are considerably influenced by pulsations of the stellar interiors and developing stellar winds. The resulting complex velocity fields severely affect molecular line profiles observable in NIR spectra. With the help of model calculations the complex line formation process in AGB atmospheres was explored with the focus on velocity effects. Furthermore, we aimed for atmospheric models which are able to quantitatively reproduce line profile variations found in observed spectra of pulsating late-type giants. Models describing pulsation-enhanced dust-driven winds were used to compute synthetic spectra under the assumptions of chemical equilibrium and LTE and by solving the radiative transfer in spherical geometry including velocity effects. Radial velocities derived from Doppler-shifted synthetic line profiles provide information on the gas velocities in the line-forming region of the spectral features. On the basis of dynamic models we investigated in detail the finding that various molecular features in AGB spectra originate at different geometrical depths of the very extended atmospheres. We show that the models are able to quantitatively reproduce the characteristic line profile variations of lines sampling the deep photosphere. The global velocity fields of typical LPVs are also realistically reproduced. Possible reasons for discrepancies concerning other modelling results are outlined. In addition, we present a model showing variations of CO dv=3 line profiles comparable to observed spectra of SRVs and discuss that the non-occurence of line doubling in these objects may be due to a density effect. The results of our line profile modelling are another indication that the dynamic models studied here are approaching a realistic representation of the outer layers of AGB stars.

26 pages, 27 (colour) figures, accepted for publication in A&A

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