On the Ga II and Ga III Resonance Lines and the Implication for Chemical Stratification in HgMn Stars

The gallium abundance in HgMn stars has been reported to be different for analyses conducted from spectral lines in the optical versus those from the ultraviolet. We pursue this ultraviolet-optical discrepancy from a line-blending perspective by investigating the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectrum of the sharp-lined HgMn binary star χ Lup. Its gallium abundance is derived to be logN_Ga=4.5 (logN_H=12) based on nonresonance Ga II lines, and we are able to demonstrate the line-blending problems associated with the abundances determined from the resonance lines of Ga II λ1414 and Ga III λ1495. The HST STIS spectrum for χ Lup is also compared with a co-added International Ultraviolet Explorer (IUE) spectrum to further illustrate the importance of accounting for the line blending in quantitative abundance analyses. We have compared an IUE co-added spectrum of μ Lep with an LTE-based synthetic spectrum and managed to fit both Ga II λ1414 and Ga III λ1495 with an abundance of logN_Ga=7.3. However, the fitting of these lines requires a dramatic change in the radiative damping constants, which may indicate a breakdown in our LTE modeling or the need to include other atmospheric effects.