  We present spatially resolved spectra observed with HST-STIS of the upper
chromosphere and dust envelope of Alpha Orionis (M2 Iab). In the fall of 2002 a
set of five high-resolution near-UV spectra was obtained by scanning at
intensity peak-up position and four off-limb target positions up to one
arcsecond, 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 Mg II h & k, Fe II 2716 A, C II 2327 A, and Al II ] 2669 A
emission lines we provide the first evidence for the presence of warm
chromospheric plasma at least 1 arcsecond away from the star at ~40 R* (1
R*~700 Rsun). The STIS spectra reveal that Betelgeuse's upper chromosphere
extends far beyond the circumstellar H alpha 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 ~700 compared to the flux at
chromospheric disk center. We observe strong asymmetry changes in the Mg II h
and Si I resonance line profiles when scanning off-limb, signaling the outward
acceleration of gas outflow in the upper chromosphere. 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 um silicate dust
emission feature. The spatially resolved STIS spectra directly demonstrate that
warm chromospheric plasma co-exisists with cool gas in Betelgeuse's
circumstellar dust envelope.
