Research Note: What can HST-GHRS Fe II observations of Orionis (M2 Iab) tell us about short-period heating? ^*

¹ Department of Physics, University of Texas at Arlington, Arlington, TX 76019-0059, USA
² Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA e-mail: gmh@casa.colorado.edu; pbennett@casa.colorado.edu
³ Department of Astronomy and Physics, Saint Mary's University, Halifax, N.S. B3H 3C3, Canada e-mail: pbennett@ap.stmarys.ca

Corresponding author: M. Cuntz, cuntz@uta.edu

Received: 29 January 2001
Accepted: 21 May 2001

Abstract

Cuntz ([CITE]) suggested that apparent velocity shifts in Fe II emission lines observed in Betelgeuse (α Orionis: M2 Iab) indicate that non-magnetic wave modes are relevant for the heating and dynamics of α 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]), which is now identified as coincidental. While acoustic waves may indeed be important for the heating and dynamics of α 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.