Opacity modelling of heavy-metal hot subdwarfs. Photoionization of Sr⁰, Y⁺ and Zr²⁺.

Heavy-metal hot subdwarfs (sdB and sdO) represent a small group of stars with unusually high concentrations of trans-iron elements in their atmospheres, having abundances ∼10000 times solar. One example is LS IV-14º116, where a number of heavy-metal absorption lines of SrII, YIII and ZrIV have been observed in the optical band 4000 - 5000 Å. We use a fully relativistic Dirac Atomic R-Matrix (DARC) to calculate photoionization cross sections of Sr⁰, Y⁺ and Zr²⁺ from their ground state to the twentieth excited level. We use the cross sections and the oscillator strengths to simulate the spectrum of a hot subdwarf. We obtain complete sets of photoionization cross sections for the three ions under study. We use these data to calculate the opacity of the stellar atmospheres of hot subdwarf stars, and show that for overabundances observed in some heavy-metal subdwarves, photo-excitation from zirconium, in particular, does contribute some back warming in the model.