The outer atmosphere of the M-type supergiant α Orionis: K I 7699 Å emission

¹ GRAAL Université de Montpellier II, 34095 Montpellier Cedex 5, France
² Department of Astronomy, University of Texas at Austin, Austin, TX 78712-1083, USA

Corresponding author: B. Plez, plez@graal.univ-montp2.fr

Received: 4 July 2001
Accepted: 1 March 2002

Abstract

Spatially-resolved high-resolution long-slit spectra of Betelgeuse's circumstellar shell are described for a spectral window centered on the 7699 Å resonance line of neutral potassium. The K i emission from resonance fluorescent scattering of photospheric photons which is mapped out to 50 arcsec from the star is approximately spherically symmetric with a brightness decreasing as , where r is the radial distance from the star. Our measurements together with the earlier theoretical interpretation by Rodgers & Glassgold suggest that the mass loss rate is about y^-1. The K i emission is far from homogeneous: intensity inhomogeneities are seen down to the seeing limit of about 1 arcsec and the velocity resolution of about 2 km s^-1. There is clear evidence for a thin shell of 50 arcsec radius. This is identified with the weaker circumstellar absorption component known as S2. Estimates are made of the density of K atoms in this shell ( cm^-3).