1999ApJ...521..414W

Hg II abundances have been determined for 42 mercury-manganese (HgMn) stars by fitting synthetic spectra to observed spectra of the 3984 Å Hg II line. Twenty of the stars had lines sharp enough to allow their Hg isotopic abundance mixes to be estimated. The Hg abundance is reported for more HgMn stars here than in any other single work. No correlation was found between Hg II abundance and T_eff or the mean central wavelength of HgMn λ3984 stars. The mean central wavelength of λ3984, an indicator of the Hg isotopic mix, is loosely correlated with T_eff: stars with primarily heavy Hg isotopes tend to be cooler, although one star, 46 Aql, has almost pure ²⁰⁴Hg and T_eff in about the middle of the temperature range for HgMn stars. We find that there is no evidence that any of the HgMn stars have ¹⁹⁶Hg or ¹⁹⁸Hg. For the very sharp-lined stars, the ²⁰⁴Hg abundance decreases with increasing T_eff. No correlation is seen between the mean central wavelength and the surface gravity. No correlation was found between the projected rotational velocity and the Hg II abundance or the central wavelength of λ3984, although this result may be biased by the selection of stars with low reported vsini. Hg I λ4358 was measured at high spectral resolution for seven HgMn stars. The isotopic shifts are too small, and the hyperfine components are too weak to allow unambiguous isotopic abundance ratios to be found. Hg I abundances correlate fairly well with Hg II abundances. Some of the Hg isotopic mixtures are difficult to explain using only diffusion. HR 7245 has approximately equal abundances of ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰²Hg, and ²⁰⁴Hg but very little ²⁰¹Hg, and 11 Per has Hg that is mostly ¹⁹⁹Hg and ²⁰⁴Hg. Calculations show that hyperfine splitting of ²⁰¹Hg changes the radiative forces it feels compared with other isotopes, which may alter diffusion of that isotope enough to explain its absence in HR 7245, but we have found no possible explanation for the Hg isotopic mix found in 11 Per. These are the first very high resolution measurements of Hg II λ3984 for HR 7245 and 11 Per. Although diffusion may be acting in HgMn stars, either there are one or more other mechanisms acting to help produce the overabundances and isotopic mixtures seen or our understanding of diffusion is lacking on some important point.