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Spatially-resolved high-resolution long-slit spectra of Betelgeuse'scircumstellar shell are described for a spectral window centered on the 7699\AAresonance line of neutral potassium. The K I emission from resonancefluorescent scattering of photospheric photons which is mapped out to 50 arcsecfrom the star is approximately spherically symmetric with a brightnessdecreasing as r^{-2.36 \pm 0.03}, where $r$ is the radial distance from thestar. Our measurements together with the earlier theoretical interpretation byRodgers & Glassgold suggest that the mass loss rate is about 2 . 10^{-6} solarmass/year. The K I emission is far from homogeneous: intensity inhomogeneitiesare seen down to the seeing limit of about 1 arcsec and the velocity resolutionof 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 asS2. Estimates are made of the density of K atoms in this shell (approx. 6 .10^{-5} cm^{-3}).Comment: 10 pages, 7 figure

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