Atmospheres, chemical compositions, and evolutionary histories of very metal-poor red horizontal-branch stars in the galactic field and in NGC 7078 (M15)

We have conducted spectrum analyses of 24 field metal-poor ([Fe/H] < -2) red horizontal-branch (RHB) stars identified in the HK objective-prism survey and 6 such stars in the globular cluster M15, based on high-quality spectra (R similar to 40,000, S/N similar to 100) obtained with the Magellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 m telescope at Las Campanas Observatory. The atmospheric parameters of the RHB stars provide interesting bridges between turnoff stars of similar temperature and red giant branch (RGB) stars of similar gravity, and they permit investigations of abundance trends [X/Fe] versus [Fe/H] in a relatively unexplored region of the temperature- gravity plane. We find that the T-eff, log g, v(t), and [Fe/H] values determined from our spectra are consistent with expectations from literature spectroscopic studies of other evolved metal-poor stellar classes. We show that the RHB stars have abundance distributions that are consistent with typical halo stars of similar metallicities. The photometric and spectroscopic gravities of the M15 stars differ by amounts that grow with declining temperature. We use a regression derived from these differences to calculate photometric gravities for the field RHB stars. Then we use the locations of the field RHB stars among the evolutionary tracks of Cassisi et al. in the log g versus log T-eff plane to estimate their masses and lifetimes as RHB stars. We use these lifetimes to estimate the size of the metal-poor HB population from which they arise. Then, using counts of HB and RGB stars in metal-poor globular clusters, we conclude that the number of metal-poor RGB stars at high latitudes (vertical bar b vertical bar > 30 degrees) brighter than V = 15 exceeds those identified in extant objective-prism surveys by more than an order of magnitude. Finally, we deduce the effective temperature of the fundamental red edge of the metal-poor RR Lyrae instability strip, log T-eff(FRE) = 3.80 +/- 0.01, from the interface between the temperature distributions of metal- poor field RHB stars and the RR Lyrae stars of similar [Fe/H] in five metal- poor globular clusters.