What can HST-GHRS Fe II observations of α Orionis (M2 Iab) tell us about short-period heating?

Abstract

Cuntz (\cite{Cuntz97}) suggested that apparent velocity shifts in Fe II emission lines observed in Betelgeuse (alpha Orionis: M2 Iab) indicate that non-magnetic wave modes are relevant for the heating and dynamics of alpha Ori's chromosphere. This claim was based on the similarity of computed stochastic velocities in 1-D short-period acoustic wave models and velocity shifts in profile fits to Fe II emission lines (Carpenter & Robinson \cite{Carpenter97}), which is now identified as coincidental. While acoustic waves may indeed be important for the heating and dynamics of alpha Ori's chromosphere, the interpretation of the Fe II emission line profiles does not provide evidence for this possibility. The line formation of optically thick scattering lines in an extended outflow makes Fe II emission lines poorly suited as a diagnostic for small-scale structure in hydrodynamical models. Better diagnostics include electron density sensitive, low opacity lines such as C II]. In the view of these findings, we discuss directions of future research. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.