Astronomy and Astrophysics, volume 305, 902-910 (1996/1-3)
Elemental abundances in normal late-B and HgMn stars from co-added IUE spectra. IV. Gallium.
Abstract (from CDS):
An analysis is presented of the ultraviolet gallium resonance lines in International Ultraviolet Explorer (IUE) spectra of 40 normal, superficially normal, and HgMn-type late-B stars. Gallium abundances are derived by fitting the GaIIλ1414, and GaIIIλλ1495, 1534 lines with synthetic spectra computed under the conventional assumptions of local thermodynamic equilibrium (LTE) and atmospheric chemical homogeneity. We find that the derived upper limits on the gallium abundances of the normal and superficially normal stars are consistent with the solar value. However, gallium is overabundant in essentially all HgMn stars, including the cooler examples such as HR 4072, χ Lup, and ι CrB, where the enhancements are relatively moderate and unambiguous identification of the resonance lines depends critically on newly available laboratory wavelengths. Furthermore, the gallium overabundances in the HgMn stars exhibit a loose correlation with effective temperature, similar in most respects to those identified for manganese and copper in the same stars except for the presence of the anomalously Ga-rich, cool HgMn star HR 7775. We show that a systematic discrepancy between the abundances derived from the Ga+ and Ga2+ ions in the hotter HgMn stars (Teff>12000K) can be explained by radiative transfer effects in the presence of a stratification of gallium at optical depths above logτ0≃0. This `stratification model' can also account for the anomalously shaped GaIII λ1495 profiles observed in the extreme cases of µ Lep, HR 2844, and HR 7143 (as demonstrated in a previous paper), and is consistent with published predictions of radiative diffusion theory for non-magnetic stars.