Atmospheric parameters and chemical compositions of eighteen halo OB stars

Stellar equivalent widths and Hepsilon line profiles, measured from 5 and 10 A mm-1 IPCS spectra obtained at the AAT, are presented for 18 halo OB stars. Effective temperatures and gravities have been estimated using Stromgren [c1] and Hβ photometry in conjunction with these data. Also derived are the abundances relative to hydrogen of helium, carbon, nitrogen, oxygen and calcium. From the normal chemical compositions and atmospheric parameters obtained, we conclude that the stars are not subluminous and show no evidence of any other peculiarities. Using conservative and nonconservative evolutionary tracks, masses, ages and distances are deduced for the stars, implying that several of them have been ejected from the galactic plane with velocities in excess of 100 km s-1. (author)