Excess helium as an explanation for metal deficiency in coronal plasma?

We suggest an alternative explanation for the apparently low metal abundances observed in some very active late-type stellar coronae: that the He abundance in these coronae is enhanced, causing lower line-to-continuum ratios. The dependence of the total free-free Gaunt factor on the square of the nuclear charge means that He can begin to dominate the EUV and X-ray thermal continua of a hot plasma with only a doubling of the abundance of He2+ nuclei relative to HI nuclei. Using model continua, we calculate the relation between He abundance and apparent metal deficiency. We suggest that He might be selectively enhanced by preferential mechanisms for retention of He nuclei in the corona or by diffusion processes, as recent sophisticated calculations indicated might be the case under certain coronal conditions. While it might seem unlikely that He nuclei could be enhanced in the hot gas phase of elliptical galaxies, until the possible presence of small-scale structure and spatial inhomogeneitiess in their interstellar media are better understood, diffusion and other element fractionation processes should perhaps not be ruled out entirely in the context of trying to understand the puzzlingly low metal abundances found from X-ray studies of elliptical galaxies.