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Elemental abundances in normal late-B and HgMn stars from co-added IUE spectra *
Received 22 March 1996 / Accepted 21 August 1996
Atmospheric mercury abundances are derived for a sample of 40 main-sequence, late-B stars, of which 14 are classified normal, and 26 are known chemically peculiar stars of HgMn or related He-weak types. The observational material for this study comprises co-added, short-wavelength IUE spectra encompassing the Hg II 1942 resonance line, coupled with a selection of new and published measurements (equivalent widths and centroid wavelengths) of the optical Hg II 3984 and Hg I 4358 lines. The analysis includes an explicit treatment of the isotopic and hyperfine structure of these lines, and allows for star-to-star variations in the isotopic composition of mercury within the framework of an assumed, mass-dependent fractionation model.
The relative isotopic abundances of mercury (as defined by a dimensionless mix parameter, q) are determined using the graphical method pioneered by White et al. In agreement with previous studies, q is found to be strongly anti-correlated with effective temperature, in the sense that the coolest stars are dominated by the heaviest isotopes (e.g., 202 Hg and 204 Hg). New isotopic-mix parameters for three programme stars - 87 Psc (), 28 Her (), and HR 7775 () - reinforce that anti-correlation. Synthetic reconstruction of the 3984 line in those programme stars for which high-resolution spectra are available tends to confirm the isotopic mixtures derived using the graphical method, and lends validity to the mass-dependent fractionation model in general. However, the 3984 feature observed in the cool HgMn star HR 7775 can only be satisfactorily reproduced by using a tailored isotopic mixture, which departs significantly from that predicted by the q -formalism.
The 1942 resonance line is detected in 10 normal B stars, for which the mean isotope-summed mercury abundance of dex (on the scale where ) exceeds the meteoritic value by nearly . The mercury abundances derived for the HgMn stars vary between and 7 dex, except for two objects (53 Tau and HR 2676) in which the abundances are consistent with those observed in the normal stars. The He-weak stars in the programme (33 Gem, HR 6000, 36 Lyn, and 46 Aql) appear to be mildly enriched in mercury, but to an extent rendered uncertain by unknown isotopic-mix parameters.
The abundances obtained from the optical and ultraviolet lines agree to within their estimated errors, which lends weight to the view that the metastable lower level of 3984 (5d9 6s2 2 D ) is not overpopulated with respect to its LTE value. The isotope-summed mercury abundances are not correlated with the effective temperatures, surface gravities, or degree of isotopic fractionation of the programme stars; nor is there evidence for systematic changes in the surface mercury abundances of HgMn stars along evolutionary tracks in the H-R diagram.
Key words: stars: abundances stars: chemically peculiar stars: early-type ultraviolet: stars
Online publication: July 3, 1998