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Astron. Astrophys. 319, 928-947 (1997)

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Elemental abundances in normal late-B and HgMn stars from co-added IUE spectra *

V. Mercury

K.C. Smith

Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK

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   [FORMULA] 1942 resonance line, coupled with a selection of new and published measurements (equivalent widths and centroid wavelengths) of the optical Hg II   [FORMULA] 3984 and Hg I   [FORMULA] 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 ([FORMULA]), 28 Her ([FORMULA]), and HR 7775 ([FORMULA]) - reinforce that anti-correlation. Synthetic reconstruction of the [FORMULA] 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 [FORMULA] 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 [FORMULA] 1942 resonance line is detected in 10 normal B stars, for which the mean isotope-summed mercury abundance of [FORMULA] dex (on the scale where [FORMULA]) exceeds the meteoritic value by nearly [FORMULA]. The mercury abundances derived for the HgMn stars vary between [FORMULA] 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 [FORMULA] 3984 (5d9 6s2 2 D [FORMULA]) 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

* Based on IUE observations procured from the World Data Centre archive at the UK Rutherford Appleton Laboratory

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998