Physics
Scientific paper
Jul 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002hst..prop.9369l&link_type=abstract
HST Proposal ID #9369
Physics
Hst Proposal Id #9369
Scientific paper
We propose to perform with STIS a critical test of the physical mechanism of wind acceleration by dust drag in cool stars. Spatially resolved spectra of the circumstellar environment of Alpha Ori {M2 Iab} will directly test if radiation pressure onto dust grains provides the momentum that causes the high mass-loss rates {up to 10^-6 M_odot yr^-1} observed in asymptotic giant branch and red supergiant stars. Terminal gas outflow velocities of 13-14 km, s^-1 are observed in Betelgeuse's circumstellar dust shell. However, the smaller chromospheric outflow velocities {below 6 km, s^- 1}, point to an extended region in the circumstellar environment where the wind accelerates. Stellar wind theory suggests radiation pressure onto dust grains as the driving mechanism that drags the gas outflow to these high terminal velocities. Dynamic radiative transport calculations that fit the star's 9.7 Mum silicate dust emission indicate that this wind accelerating region is located between 0.78" and 3". We propose to use STIS to obtain a high-resolution spatial and spectral raster scan across this region. These data can only be obtained for this unique nearby supergiant with the exceptional capabilities of the STIS. An increase of the observed asymmetry of the self-absorbed Mg ii h line {which forms in an expanding gas shell} with distance from the star, will directly confirm {or reject} the theory of dust-driven wind acceleration in cool stars.
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