Optical and UV spectroscopy of blue horizontal branch stars in the globular cluster NGC 6752.

Abstract

Optical and far-UV spectra have been obtained for blue HB stars on both sides of the observed luminosity gap in NGC 6752. On this basis effective temperatures and absolute bolometric magnitudes are derived for six stars above the gap and four stars below it. The authors find that stars above the gap arrange themselves along a ZAHB locus. Across the gap a large jump in gravity and effective temperature is evident. Stars below the gap fall in the sub-dwarf B domain. Their location in the HR diagram is consistent with a ZAHB origin. One very blue star, No. 2128, whose V magnitude falls within the gap, is shown to be a sub-dwarf O star of absolute luminosity higher than the HB. Greenstein & Sargent's suggestion of a connection between field sub-dwarf B stars and the blue HB is definitely confirmed by this cluster sample, which is shown to be the cluster counterpart of the field sub-dwarf population. Evolutionary implications are discussed.