HVS 7: a chemically peculiar hyper-velocity star

Abstract

Context. Hyper-velocity stars are suggested to originate from the dynamical interaction of binary stars with the supermassive black hole in the Galactic centre (GC), which accelerates one component of the binary to beyond the Galactic escape velocity.Aims. The evolutionary status and GC origin of the hyper-velocity star SDSS J113312.12+010824.9 (aka HVS 7) is constrained from a detailed study of its stellar parameters and chemical composition.Methods. High-resolution spectra of HVS 7 obtained with UVES on the ESO VLT were analysed using state-of-the-art NLTE/LTE modelling techniques that can account for a chemically-peculiar composition via opacity sampling.Results. Instead of the expected slight enrichments of $\alpha$-elements and near-solar iron, huge chemical peculiarities of all elements are apparent. The helium abundance is very low ($<$1/100 solar), C, N and O are below the detection limit, i.e they are underabundant ($<$1/100, $\la$1/3 and $<$1/10 solar). Heavier elements, however, are overabundant: the iron group by a factor of ~10, P, Co and Cl by factors ~40, 80 and 440 and rare-earth elements and mercury even by ~10 000. An additional finding, relevant also for other chemically peculiar stars are the large NLTE effects on abundances of Ti II and Fe II (~0.6-0.7 dex). The derived abundance pattern of HVS 7 is characteristic for the class of chemical peculiar magnetic B stars on the main sequence. The chemical composition and high projected rotation velocity $v \sin i$ = 55 $\pm$2 km s$^{-1}$ render a low mass nature of HVS 7 as a blue horizontal branch star unlikely.Conclusions. Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star's place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS 7.