Spatially resolved stis spectra of WR + OB binaries with colliding winds

Abstract

We present spatially resolved spectra of the visual WR + OB massive binaries WR 86, WR 146, and WR 147, obtained with the Space Telescope Imaging Spectrograph on board the <i>Hubble Space Telescope</i>. The systems are classified as follows: WR 86 = WC 7 + BO III, WR 146 = WC 6 + O8 I-IIf, WR 147 = WN 8 + O5-7 I-II(f). Both WR 146 and WR 147 are known to have strong nonthermal radio emission arising in a wind-wind collision shock zone between the WR and OB components. We find that the spectra of their O companions show Hα profiles in emission, indicative of large mass-loss rates and consistent with the colliding-wind model. Our spectra indicate that the B component in WR 86 has a low mass-loss rate, which possibly explains the fact that WR 86, despite being a long-period WR + OB binary, was not found to be a strong nonthermal radio emitter. Because of the small mass-loss rate of the B-star component in WR 86, the wind collision region must be closer to the B star and smaller in effective area, hence generating smaller amounts of nonthermal radio emission. Absolute magnitudes for all the stars are estimated based on the spectral types of the components (based on the tables by Schmidt-Kaler for OB stars and van der Hucht for WR stars) and compared with actual, observed magnitude differences. While the derived luminosities for the WC 7 and BO III stars in WR 86 are consistent with the observed magnitude difference, we find a discrepancy of at least 1.5 mag between the observed luminosities of the components in each of WR 146 and WR 147 and the absolute magnitudes expected from their spectral types. In both cases, it looks as though either the WR components are about 2 mag too bright for their spectral types or that the O components are about 2 mag too faint. We discuss possible explanations for this apparent discrepancy.