SCATTERING OF X-RAY-EMISSION LINES BY ATOMIC HELIUM

We calculate the differential cross section for the scattering of astr ophysically interesting X-ray emission lines by atomic helium. Compare d to the scattering by atomic hydrogen, a factor of 2 wider energy gap (due to the structure of discrete energy levels) and a considerably f latter Compton profile (due to the broader distribution of the electro n momentum in the helium atom) open up a fundamental possibility for d iscriminating between the contributions of hydrogen and helium by obse rving the scattering spectra of X-ray emission lines. With the advent of a new generation of X-ray telescopes which combine a large effectiv e area and a high energy resolution, it will be possible to measure th e helium abundance in molecular clouds in the Galactic-center region, in the vicinity of active galactic nuclei, and on the surface of cool flare stars.