Studies of hot B subdwarfs. II. Energy distributions of three bright sdB/sdOB stars in the 950-5500 A range.

Abstract

Voyager ultraviolet spectrometer observations of the subdwarf B or OB stars HD 205805, UV 1758+36 and Feige 66 are presented. All three objects display the H I Layman series in absorption. These observations are combined with low dispersion IUE spectrophotometry and with Stromgren photometry to construct virtually complete energy distributions, which extend over the range 950-5500 angstroms. Effective temperatures based on model atmosphere calculations for high gravity, hygrogen rich stars are determined. Our analyses yield T_e 28,200 + or - 1300 K for HD 205805, T_e 31, 800 + or - 1100 K for UV 1758+36, and T_e 35,700 + or 1500 K for Feige 66. The importance of far ultraviolet observations below L_alpha in reducing the uncertainties associated with the interstellar extinction and the degradation of the IUE sensitivity is emphasized.