A. J. Willis

The Observations Of The X-Ray Source Hz Herculis-Hercules X-1

The results of a campaign of monitoring HZ Her with the IUE satellite are presented. The spectrum and light variation outside of eclipse are consistent with the Milgrom and Salpeter model for emission from the X-ray heated photosphere of the primary. Near eclipse there is UV flux in excess of the prediction of the heated photosphere model, which shows significant 35 day variations and is consistent with an origin in a precessing disk, as described by Gerend and Boynton. The spectrum shows the strong emission lines of N V and C IV, which generally vary in the sense of the continuum. The N V/C IV ratio is greater than 2 near orbital phase 0.5 and approaches unity near eclipse. It is suggested that these emission lines are optically thick and arise from both the heated photosphere and the accretion disk.

Rosat X-Ray Observations of the Wr Stars HD 50896 (WN5) and γ Velorum (WC8+O9I)

We present a preliminary analysis of multiple X-ray (0.1-2.5 keV) observations of HD 50896 and γ Velorum obtained with the ROSAT satellite. For HD 50896, our 8 observations show variability at the 30% level on timescales of ~ 1 day, together with larger (× 1.7) epoch-changes, but no evidence for rapid variability. No phase-dependent modulation is apparent on the 3d.766 optical period. The mean PSPC spectrum gives kT = 0.28 keV, log N(H) = 20.6, and Lx = 3.8 × 1032 erg s-1, and implies that the observed X-rays have undergone little absorption in the WN5 wind.

ISO'S VIEW ON MASSIVE STAR EVOLUTION: NEON ABUNDANCES IN WOLF-RAYET STARS

Wolf-Rayet (WR) stars exhibit an extensive array of atomic emission lines of impressive strength and width in their spectra, indicating a strong stellar wind and sizable source of radiative energy from the envelope surrounding the core. Three spectral classes of WR stars are defined by the elements dominating the optical region: the WN class, characterized by He and N; the WC class, rich in lines of C, He, and O; and the very rare WO class identified by strong O lines. Hydrogen is sometimes present but weak in the WN stars, and is completely absent in the WC and WO stars. Nitrogen is also absent in WC and WO except in rare WN/WC hybrid cases.

Mass Loss Bates for Twenty One Wolf-Rayet Stars

Mass loss rates have been derived for twenty one WR stars encompassing most subtypes in the WN and WC sequences, from measurements of their infrared free-free fluxes. The resultant mass loss rates show a range of only a factor of four. WC stars generally have larger mas loss rates than WN stars, the mean rates being (dot m)(WC) = 4.1×10-5 M⊙y-1 and (dot m)(WN) = 2.7×10-5 M⊙y-1. Optical and ultraviolet data have been used to estimate bolometric luminosities for a range of WR spectral types, and it is shown that the derived mass loss rates are too large to be powered by radiation pressure. The total kinetic energy ejected into the interstellar medium through mass loss during the WR phase of a massive star is estimated to be 7×1050 ergs, comparable to that of a supernova event.