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A. Lobel (SAO), A. K. Dupree (SAO), R. L. Gilliland (STScI)
We present a spectral analysis of the cool supergiant \alpha~Ori (M2~Iab) based on near-UV, optical and near-IR spectra obtained between Feb.~'93 and March~'99 with the Space Telescope Imaging Spectrograph ({\em STIS}), the Utrecht Echelle Spectrograph ({\em UES-WHT}) and the high-resolution {\em SOFIN} spectrograph (Nordic Optical Telescope). The {\em STIS} spectra are spatially resolved by scanning across the UV disk of the star, revealing complex dynamical velocity fields across the stellar chromosphere which confirms subtle and variable intensity patterns monitored simultaneously in the UV with the Faint Object Camera.
A detailed spectral non-LTE modeling of the Mg~{\sc ii} k & h and Si~{\sc i} resonance emission line profiles permits us to determine small changes in the chromospheric velocity structure, combined with kinetic temperature and density variations derived from radiative transfer fits to the H\alpha absorption line, by accounting for effects of spherical geometry in this extended atmosphere. We observe that intensity changes of the latter line core are correlated in time with weaker intensity changes seen in prominent TiO band-heads, which dominate the optical spectrum.
We present a semi-empirically constrained model of the chromosphere of {\em Betelgeuse} with a maximum kinetic temperature of 5400~K, showing variations which do not exceed ~400~K over time. A detailed profile study of strong UV emission lines of Si~{\sc i}, Mg~{\sc ii} and Fe~{\sc ii} reveals small Doppler shifts of 4 to 8~\rm km\,s-1 in these self-absorbed cores (observed with {\em STIS} between Jan.~'98 and March~'99), which we model by changes in the outer chromospheric velocity field. These Doppler shifts correlate with the strong intensity variations observed in the longwave emission component of these lines. When compared to the {\em STIS} spectra these fluxes appeared particularly intense in Sept.~'92, observed with the Goddard High Resolution Spectrograph, indicating a phase of enhanced chromospheric outflow.
This research is supported in part by STScI grant to the SAO.