PROPERTIES OF THE WO WOLF-RAYET STARS

We present optical spectrophotometry for five WO Wolf-Rayet stars, thr ee of them in our own Galaxy and one in each of the SMC and LMC. IUE u ltraviolet spectrophotometry has also been obtained for the two Magell anic Cloud WO stars, including a high resolution spectrum for one of t hem. Quantitative spectral typing criteria are defined for WO subtypes spanning WO1 to WO5 and, for the case of the two WO stars in spectros copic binaries, spectral types for the O-type primaries are derived. F rom our spectrophotometry we derive reddenings and magnitudes for each star. Absolute visual magnitudes of -2.5 and -1.8 are derived for a W O4 star and for a WO2 star, respectively, each star lying at a known d istance. Wind terminal velocities ranging from 4200 km s(-1) to 5500 k m s(-1) are derived from the black absorption edge of an ultraviolet P Cygni profile and from the FWZI of strong optical emission lines. The relative abundances of helium, carbon and oxygen in the winds of each of the WO stars are derived using a recombination theory analysis of selected ultraviolet and optical emission lines to determine the ionic abundances of He2+, C4+, O4+, O5+ and O6+. The derived abundance rati os show relatively narrow ranges. C/He number ratios of 0.51-0.52 are derived for two Galactic WO stars and one LMC WO star, with their C/O ratios ranging between 4.6 and 5.2, and their (C+O)/He ratios equal to 0.62. The one SMC WO star has a C/He ratio of 0.81, a C/O ratio of 2. 7 and a (C+O)/He number ratio of 1.10. These abundance ratios are broa dly consistent with evolutionary models for the advanced stages of mas sive stars, and promising agreement as a function of initial metallici ty is found with the most recent evolutionary models.