SPECTROSCOPY OF HOT STARS IN THE GALACTIC HALO

Spectroscopy is reported for 769 hot stars selected from the HK objective-prism/interference-filter survey of Beers et al. [AJ, 90, 2089 (1985)]. This sample spans a wide range of objective-prism classifications, from stars with extremely blue continuous spectra to stars with broad Balmer H-epsilon typical of AB- or A-type stars. Many of the stars included were originally classified as candidate metal-deficient stars. All of these stars exhibit medium-resolution spectra consistent with temperatures hotter than the main-sequence turnoff of metal-poor halo stars, or are composites of late-type and early type stars. Previously published broadband UBV photometry is available for 193 stars in this sample, and is used to confirm or refine the stellar classifications obtained from the medium-resolution spectroscopy. When available, photometric information is used to obtain estimates of the surface temperature for degenerates and O- and B-type subdwarfs based on previously derived calibrations. Several extremely hot (T(eff) > 50 000 K) He-rich sdO stars are identified. Four stars exhibit very narrow Balmer profiles typical of supergiant B-type stars or post-AGB objects. The apparent magnitudes of these stars (14 less-than-or-equal-to V less-than-or-equal-to 15) require that they must be less luminous than normal supergiants if they are members of the Galaxy. The great majority of stars in our sample (551) are clearly recognized as field horizontal-branch (FHB) or mid- to late-type A stars on the basis of their Balmer line profiles. We lack the necessary intermediate-band photometry required to unambiguously distinguish between these stars on the basis of surface gravity alone. However, the location of the subset of these stars with available broadband photometry in the (U - B)0 vs (B - V)0 two-color diagram (which is affected by both metal abundance and surface gravity) suggests that our sample contains a mixture of FHB and A stars with a wide range of abundances. The A stars exhibit rotation and line-of-sight dispersion consistent with membership in the galactic thick disk. The FHB stars (as well as the several hundred FHB/A stars which we are presently unable to uniquely assign to either FHB or A classifications) exhibit kinematic properties which suggest a transition from thick disk to halo.