Hot horizontal-branch stars in the galactic bulge. I.

We present the first results of a survey of blue horizontal-branch (BHB) stars in the Galactic bulge. In this exploratory study, candidates with 15 less than or equal toV less than or equal to 17.5 covering a wide range in B-V color were selected from Cerro Tololo Inter-American Observatory (CTIO) Schmidt UBV photometry. Blue spectra were recorded at 2.4 Angstrom FWHM resolution for 164 stars in a 1.3 deg(2) field, similar to7.degrees5 from the Galactic center. Radial velocities were measured for all stars. For stars with strong Balmer lines, we devised and applied a spectroscopic technique to determine stellar temperature T-eff, gravity log g, and metallicity [Fe/H] independent of reddening. The reddening and distance to each star were then found from UBV photometry. Reddening proved highly variable, with E(B-V) ranging from 0.0 to 0.55, around a mean of 0.28. The B-V colors of cool HB stars of solar metallicity reddened by E(B-V) greater than or equal to0.3 overlap those of foreground main-sequence stars, but the U-B versus B-V diagram distinguishes these groups until E(B-V) >0.5. Forty-seven BHB candidates were identified with T-eff greater than or equal to 7250 K. Seven have the gravities of Population I stars, three are ambiguous, and 37 are HB stars, including perhaps a dozen RR Lyrae stars. The unambiguous BHB stars are all cooler than 9000 K. They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB stars are more metal-poor and loosely concentrated toward the Galactic center, while the cooler ones are of somewhat higher metallicity and are situated closer to the center. We detect two cool solar-metallicity HB stars in the bulge of our own Galaxy, the first such stars known. Still elusive are their fainter hot counterparts, the metal-rich sdB/O stars strong in ultraviolet light.