Lobel, Alex

Contact Email: alobel@cfa.harvard.edu
Institute: Harvard-Smithsonian Center for Astrophysics
First Coauthor: Andrea K. Dupree
Institute: Harvard-Smithsonian Center for Astrophysics
Subject Area: Stellar Atmospheres, Circumstellar Material
Waveband: Ultraviolet
Technique: Spectroscopy
Presentation: Oral Contribution: Thursday
Title: Chromospheric Dynamics of Betelgeuse from STIS Spectra
Abstract: We present a spectral analysis of Betelgeuse (M2 Iab). Four spatially resolved raster scans have been obtained across the chromospheric disk with the STIS on the Hubble Space Telescope between 1998 January and 1999 March. These near-UV spectra display many double-peaked permitted emission lines of neutral and singly ionized metals, with self-absorbed scattering cores. We observe reversals in the intensity of both emission line components when scanning across the UV disk, for four unsaturated lines of Si  I, Fe  II, Al  II], and Fe  II. We model the Si  I $\lambda$2516 resonance line with detailed non-LTE radiative transport calculations in spherical geometry, and constrain the mean velocity structure in the projected aperture area, for each scan position on the chromospheric disk. We infer the spatial velocity structure of Betelgeuse's extended chromosphere, which reveals localized upflows in the western front hemisphere in 1998 September, that expand further towards the eastern hemisphere in 1999 March. The spatial scans exhibit simultaneous up- and downflows across the lower chromosphere with mean velocities of ~2 kms-1 These subsonic flows indicate non-radial (or non-coherent) mass movements during certain phases of the stellar variability cycle. A brief discussion of constructing semi-empiric models for the chromosphere of this cool supergiant, and its temporal variability will also be presented.