EDP Sciences
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Volume 376, Number 1, September II 2001
Page(s) 154 - 157
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010957

A&A 376, 154-157 (2001)
DOI: 10.1051/0004-6361:20010957

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

M. Cuntz1, G. M. Harper2 and P. D. Bennett2, 3

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

(Received 29 January 2001 / Accepted 21 May 2001)

Cuntz (1997) 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 1997), 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.

Key words: hydrodynamics -- waves -- stars: chromospheres -- stars: individual: $\alpha$ Ori -- stars: supergiants

Offprint request: M. Cuntz, cuntz@uta.edu

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© ESO 2001