Blue horizontal branch stars in metal-rich globular clusters. II. 47 Tuc and NGC 362

Abstract

Atmospheric parameters (effective temperature, surface gravity) and radial velocities are derived for 12 candidate blue horizontal branch (HB) stars in the globular clusters 47 Tuc and NGC 362, which so far have been known to contain primarily red HB stars. The spectroscopic targets were selected from the catalog of hot stars detected in these clusters at 1600 A using the Ultraviolet Imaging Telescope (UIT). Spectroscopic analyses of these stars reveal, however, that one of the four HB candidate targets in 47 Tuc, and five out of the eight targets in NGC 362 are probably background stars belonging to the Small Magellanic Cloud. With the exception of the photometric binary MJ38529 in 47 Tuc, the parameters of those stars that are probable members of 47 Tuc and NGC 362 agree well with canonical HB evolution. The three hot stars in 47 Tuc all have 10,000 K < Teff < 15,000 K and include one photometric binary, which suggests that they might have a different physical origin than the dominant red HB population. The somewhat cooler blue HB stars in NGC 362 show more continuity with the dominant red HB population and might simply arise from red giants with unusually high mass loss.