On the nature of early-type stars in the galactic halo

Effective temperatures and surface gravities of 46 intermediate and high galactic latitude OB stars have been estimated from Stromgren |C1| and Hβ photometry. In addition, helium, calcium and magnesium abundances relative to hydrogen have been derived for eleven stars. The typical OB-type stellar parameters indicate that they are normal Population I stars, several of which require ejection velocities of more than 100 km s−1 from the plane in order to reach their present z-distances during their lifetimes. The observed correlation between peculiar velocity and mass for stars with |z| ≽ 400 pc is compatible with the Blaauw runaway hypothesis as the acceleration mechanism. A plot of log N(Na I) versus E(B–V) implies that the gas-to-dust ratio in the halo is probably similar to that found for material in the galactic disc. This implies that the sputtering of calcium from grain surfaces is probably responsible for the observed velocity dependence of the N(Na I)/N(Ca II) ratio.