The Outer Atmosphere of the M - type Supergiant α Orionis : K i 7699 Å Emission Bertrand

Spatially-resolved high-resolution long-slit spectra of Betelgeuse’s circumstellar shell are described for a spectral window centered on the 7699Å 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 r, 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 2 ×10 M⊙ y . 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. 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 (≃ 6× 10 cm).