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 ^ { -2.36 \pm 0.03 } , 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 \times 10 ^ { -6 } M _ { \odot } 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 ( \simeq 6 \times 10 ^ { -5 } cm ^ { -3 } ) .