Astronomy and Astrophysics, volume 303, 95-106 (1995/11-1)
Hot HB stars in globular clusters: physical parameters and consequences for theory. II. NGC 6397 and its short blue horizontal branch.
DE BOER K.S., SCHMIDT J.H.K. and HEBER U.
Abstract (from CDS):
New visual spectra of 14 blue horizontal-branch (BHB) stars in NGC 6397 and 2 spectra from the literature are used to derive atmospheric parameters (Teff, logg) for these stars. With the theoretical surface brightness for the derived stellar parameters and with the given distance of the cluster one then can calculate the radius, the luminosity, and the mass of the BHB stars. Low dispersion IUE spectra have been obtained of 7 BHB stars and these are used together with visual data to determine the integrated flux of the stars directly from the spectrophotometry. This allows, using the distance, to calculate the luminosity leading to an additional determination of the mass of the BHB stars. The overall observed spectral energy distribution of the stars (visual and IUE data) is compared with atmosphere models for metal-poor stars. The models are somewhat too bright at λ<1500A and too faint in the 2500A range. Our data indicate, that the BHB stars have a mass ranging from 0.3M☉ to 0.7M☉ (with a mean of 0.4M☉) over the temperature range of 8800 to 12000K. These masses are smaller than those from classical single star evolution. Implications of the results are discussed, including problems with the cluster distance modulus, the gravities, problems with the theoretical Balmer profiles, and possibly more complicated origins for the HBB stars.