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

Abstract

Cuntz (1997) suggested that apparent velocity shifts in Fe II emission lines observed in Betelgeuse ( Orionis: M2 Iab) indicate that non-magnetic wave modes are relevant for the heating and dynamics of 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 pro le ts to Fe II emission lines (Carpenter & Robinson 1997), which is now identi ed as coincidental. While acoustic waves may indeed be important for the heating and dynamics of Ori's chromosphere, the interpretation of the Fe II emission line pro les 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 ndings, we discuss directions of future research.