Beyond the iron group: Heavy metals in hot subdwarfs

We report the discovery of strong photospheric resonance lines of Ga III, Ge IV, Sn IV and Ph IV in the UV spectra of more than two dozen sdB and sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Lines of other heavy elements are also detected, however in these cases more atomic data are needed. Based on these discoveries, we present a hypothesis to explain the apparent lack of silicon in sdB stars hotter than similar to32 000 K. The existence of triply ionised Ge, Sri, and Ph suggests that rather than silicon sinking deep into the photosphere, it is removed from the star in a fractionated stellar wind. This hypothesis provides a challenge to diffusion models of sdB stars.