Spatially Resolved Stis Spectra of Betelgeuse’s Upper Chromosphere and Circumstellar Dust Envelope

The Hubble Space Telescope observed red supergiant Betelgeuse (αOri) with the Space Telescope Imaging Spectrograph to investigate the outer atmosphere from spatially resolved spectra. We present a new set of seven highresolution near-UV spectra observed with HST-STIS in fall 2002 and spring 2003, by scanning at chromospheric intensity peak-up position and six off-limb target positions up to three arcseconds away from the star. A small aperture is used to study and determine the thermal conditions and flow dynamics in the upper chromosphere and inner circumstellar dust envelope of this important nearby cool supergiant (M2 Iab). We provide the first evidence for the presence of warm chromospheric plasma at least 3 away from Betelgeuse at ∼120 R∗ (1 R∗≃700 R⊙) based on detailed spectroscopic observations of the Mg ii h & k emission lines. Many other weak chromospheric emission lines as Fe ii λ2716, C ii λ2327, Al ii ] λ2669, and Fe i λ2823, are detected out to at least 1 in the spatially resolved STIS observations. The recent spectra reveal that α Ori’s upper chromosphere extends far beyond the circumstellar Hα envelope of ∼5 R∗, determined from previous ground-based images. The changes of shape of the detailed Mg ii line profiles observed in Betelgeuse’s outer atmosphere are compared with detailed Mg ii line profiles previously observed above the limb of the average quiet Sun. The profiles of the Mg ii h & k, and the Si i resonance emission lines reveal a strong increase of asymmetry by scanning off-limb, signaling the outward acceleration of wind expansion in Betelgeuse’s upper chromosphere beyond 200 mas (∼8 R∗). We discuss detailed radiative transfer models that fit the STIS observations showing that the local kinetic gas temperature in the upper chromosphere exceeds 2600 K. Our radiation transport models for the IR silicate dust emission at 9.8 μm in the upper chromosphere show however that the ambient gas temperature remains below 600 K to sustain the presence of dust grains. Hence, the STIS spectra of Betelgeuse’s upper chromosphere directly demonstrate that warm chromospheric plasma must co-exist with cool dusty plasma in its outer atmosphere.