Other
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...203.4305l&link_type=abstract
American Astronomical Society Meeting 203, #43.05; Bulletin of the American Astronomical Society, Vol. 35, p.1273
Other
1
Scientific paper
We present spatially resolved spectra observed with the Space Telescope Imaging Spectrograph of the upper chromosphere and dust envelope of α 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 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 R* (1 R*≃700 R&sun;). Other strong emission lines as Fe ii λ 2716, C ii λ 2327, and Al ii ] λ 2669 are detected out to a full arcsecond. The STIS spectra reveal that Betelgeuse's upper chromosphere extends far beyond the circumstellar Hα envelope of ˜5 R*, determined from previous ground-based imaging. The flux in the broad and self-absorbed resonance lines of Mg ii decreases by a factor of 104 compared to the flux at chromospheric disk center. We observe strong asymmetry changes in the Mg ii h and Si 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 ii line profiles in Betelgeuse's upper atmosphere with Mg ii line profiles observed above the limb of the quiet Sun.
From the radial intensity distributions of Fe i and Fe 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 μ m 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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