UV-bright stars in galactic globular clusters, their far-UV spectra and their contribution to the globular cluster luminosity

UV-bright stars were observed between 1150 and 3200 A in seven galactic globular clusters: BS in 47 Tuc, vZ 1128 in M3, UVB in M5, B29 and IV-52 in M13, I-33 in M10, ROB 162 in NGC6397, and K559, K648, and K996 in M15. The contribution of the UV-bright stars to the total cluster intensity is found to range from 50 percent at 1,200 A to a few percent at 3,000 A. The stars have effective temperatures between 19,000 and 35,000 K and luminosities of about 10 to the power of 3.3 of solar luminosity, and constitute about 0.3 percent of the total cluster luminosity. It was also found that the ionizing photons produced by the UV-bright stars may be an essential factor for the ionization structure of gas in the Milky Way halo.