Physical parameters of sdB stars from spectral energy distributions

Abstract

The atmospheric parameters of 34 hot subdwarf B stars have been obtained using a combination of 61 short and long-wave IUE spectra, together with new and existing optical and infrared photometric data. Using a grid of high-gravity helium-deficient model atmospheres and a chi ²-minimization procedure, 15 single sdB stars and 19 composite systems, containing a hot subdwarf B star and a cool main-sequence companion, were analyzed. From the (log L-log T_eff) diagram of our results, we conclude that the majority of the cool companions to our sample of binary sdB stars are main sequence stars, in the range of 4000 < T_eff / K < 6000 and mass 0.8 < M/M_sun < 1.3. The lower limit on detectability of cool stars in composite sdB of our sample is ~ 3600 K, corresponding to a spectral type of M1 or later (Lang \cite{Lang-92}). Based on observations made with the Jacobus Kapteyn Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and on INES data obtained with the IUE satellite.