HD 65949: Rosetta stone or red herring

Abstract

HD 65949 is a late B star with exceptionally strong Hg ii λ3984, but it is not a typical HgMn star. The Re ii spectrum is of extraordinary strength. Abundances or upper limits are derived here for 58 elements based on a model with Teff= 13 100 K and log (g) = 4.0. Even-Z elements through nickel show minor deviations from solar abundances. Anomalies among the odd-Z elements through copper are mostly small. Beyond the iron peak, a huge scatter is found. Enormous enhancements are found for the elements rhenium through mercury (Z= 75–80). We note the presence of Th iii in the spectrum. The abundance pattern of the heaviest elements resembles the N= 126 r-process peak of solar material, though not in detail. An odd-Z anomaly appears at the triplet (Zr Nb Mo), and there is a large abundance jump between Xe (Z= 54) and Ba (Z= 56). These are signatures of chemical fractionation. We find a significant correlation of the abundance excesses with second ionization potentials for elements with Z > 30. If this is not a red herring (false lead), it indicates the relevance of photospheric or near-photospheric processes. Large excesses (4–6 dex) require diffusion from deeper layers with the elements passing through a number of ionization stages. That would make the correlation with second ionization potential puzzling. We explore a model with mass accretion of exotic material followed by the more commonly accepted differentiation by diffusion. That model leads to a number of predictions which challenge future work. New observations confirm the orbital elements of Gieseking and Karimie, apart from the systemic velocity, which has increased. Likely primary and secondary masses are near 3.3 and 1.6 M⊙, with a separation of ca. 0.25 au. New atomic structure calculations are presented in two appendices. These include partition functions for the first through third spectra of Ru, Re and Os, as well as oscillator strengths in the Re ii spectrum