Spatially Resolved Hubble Space Telescope Spectra of the Chromosphere of α Orionis^*

Spatially resolved UV spectra of the supergiant α Orionis (Betelgeuse) obtained in 1995 March with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope (HST) show that the chromospheric emission in the Mg II h and k (around 280 nm) lines reaches a diameter of at least ∼270 milliarcseconds, about twice the size of ultraviolet continuum images obtained at the same time with the HST Faint Object Camera (FOC), and about 5 times the star's optical diameter. The signature of the unresolved bright spot observed in the 1995 March FOC images occurs in the spectrum as an asymmetry in the intensity measured across the disk at constant wavelength. Because the spectra were obtained scanning across the stellar image, it is possible to measure a differential line shift across the disk of the star that can be interpreted as being due to rotation. If this interpretation is correct, we can determine the axis of rotation of Betelgeuse and estimate its rotational speed to be 1.2 × 10^-8 rad s^-1, corresponding to a rotation period of 17 yr. In addition, it is plausible that the 1995 March bright spot is congruent with the pole of the star, suggesting that star's angle of inclination is ∼20° to the line of sight. Considering the small number of bright spots that are present at any one time on the surface of Betelgeuse, and the signature of the 1995 March spot in the Mg II resonance lines as observed with the GHRS, it appears that such spots are not the consequence of convective flows. Differences in the spatial distribution of the flux emerging from different layers of the atmosphere and the line asymmetries of the Mg II h and k lines suggest that the spot is the signature of an outwardly propagating shock wave in an atmosphere modified by rotation.