Chromospheric Dynamics of Betelgeuse from STIS Spectra
Alex Lobel

Institute: Harvard-Smithsonian Center for Astrophysics
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Abstract: We present a high-resolution spectral analysis of Betelgeuse (M2 Iab). Between 1998 January and 1999 March four spatially resolved raster scans have been obtained with the STIS spectrograph on the Hubble Space Telescope. The near-UV echelle spectra reveal double-peaked permitted emission lines of neutral and singly ionized metals, with self-absorbed line 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 2516 A 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 toward the eastern hemisphere in 1999 March. The spatial scans exhibit simultaneous up- and down-flows across the lower chromosphere with mean velocities of ~ 2 km/s. We infer non-radial (or non-coherent) mass movements during certain phases of the stellar variability cycle from these subsonic flows. We present a discussion of constructing semi-empiric models for the chromosphere of this cool supergiant, and of its temporal variability.

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Index Keywords: Alpha Ori ; Betelgeuse

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Manuscript submitted: 2001-Sep-5
"The Future of Cool-Star Astrophysics", 2003, Eds. A. Brown, G. M. Harper, & T. R. Ayres. Proceedings of 12th Cambridge Workshop on Cool Stars, Stellar Systems, & The Sun,
© 2003 University of Colorado.