The helium abundances in HgMn and normal stars

Abstract. The parameter-free model of diffusion in the atmospheres of HgMn stars (Michaud 1986; Michaud et al 1979) predicts that helium should sink below the Heii ionization zone in order that diffusion of other elements may take place, and that all HgMn stars should have deficits of helium in their photospheres, with a minimum deficit of 0.3 dex. In this study, the Smith & Dworetsky (1993) sample of HgMn stars and normal comparison stars is examined, and the helium abundances determined by spectrum synthesis using échelle spectra taken at Lick Observatory and the AAT. The prediction is confirmed; all HgMn stars are deficient in He by as much as 1.5 dex. Also, two HgMn stars, HR7361 and HR7664, show clear evidence of helium stratification. Introduction. Abundances were determined for 25 HgMn stars and 12 normal and superficially normal stars of similar Teff, using an LTE analysis. It is well-known that the effects of non-LTE can safely be ignored in the relevant temperature range. The analysis was performed for two Hei lines, λ4026.2 and λ4471.5. The line profile tables of Barnard et al (1969, 1974, 1975) and Shamey (1969) were used. The abundances and estimated errors were obtained