AAS 203rd Meeting, January 2004
Session 43 Nearby Stars: Observations
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[43.05] Spatially Resolved STIS Spectra of Betelgeuse's Outer Atmosphere and Circumstellar Dust Envelope

A. Lobel (Harvard-Smithsonian CfA)

We present spatially resolved spectra observed with the Space Telescope Imaging Spectrograph of the upper chromosphere and dust envelope of \alpha Orionis (M2 Iab). In the fall of 2002 and the spring of 2003 a set of seven high-resolution near-UV spectra was obtained by scanning at intensity peak-up position and six off-limb target positions up to three arcseconds, using a small aperture, to investigate the thermal conditions and flow dynamics in the outer atmosphere of this important nearby cool supergiant star.

Based on the Mg~{\sc ii} h & k emission lines we provide the first evidence for the presence of warm chromospheric plasma up to three arcseconds away from the star at ~120 \rm R* (1 \rm R*~q700 \rm R\odot). Other strong emission lines as Fe~{\sc ii} \lambda2716, C~{\sc ii} \lambda2327, and Al~{\sc ii} ] \lambda2669 are detected out to a full arcsecond. The STIS spectra reveal that Betelgeuse's upper chromosphere extends far beyond the circumstellar H\alpha envelope of ~5 \rm R*, determined from previous ground-based imaging. The flux in the broad and self-absorbed resonance lines of Mg~{\sc ii} decreases by a factor of 104 compared to the flux at chromospheric disk center. We observe strong asymmetry changes in the Mg~{\sc ii} h and Si~{\sc i} resonance emission line profiles when scanning off-limb, signaling outward acceleration of gas outflow in the upper chromosphere. We compare the changes of the detailed Mg~{\sc ii} line profiles in Betelgeuse's upper atmosphere with Mg~{\sc ii} line profiles observed above the limb of the quiet Sun.

From the radial intensity distributions of Fe~{\sc i} and Fe~{\sc ii} emission lines we determine the radial non-LTE iron ionization balance. We compute that the local kinetic gas temperatures of the warm chromospheric gas component in the outer atmosphere exceed 2600~K, when assuming local gas densities of the cool gas component we determine from radiative transfer models that fit the 9.7 \mum silicate dust emission feature. The spatially resolved STIS spectra directly demonstrate that warm chromospheric plasma co-exists with cool gas in Betelgeuse's circumstellar dust envelope.

This research is based on data obtained with the NASA/ESA Hubble Space Telescope, collected at the STScI, operated by AURA Inc., under contract NAS5-26555. Financial support is provided by STScI grant HST-GO-09369.01 to the Smithsonian Astrophysical Observatory.

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The author(s) of this abstract have provided an email address for comments about the abstract: alobel@cfa.harvard.edu

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.