It is widely accepted that OB stars are driven by radiation pressure, whilst the situation for the denser winds of Wolf-Rayet stars is less clear. Recent evidence in support of radiation pressure as a source of driving for WR stars has been derived from studies of LMC and Galactic WC stars. These are the immediate precursors of Type Ic SN and possibly 'hypernovae' (with a connection to long duration gamma ray bursts).
We find that carbon-rich WC stars stars in the LMC (with early spectral types) have weaker winds than Galactic WC stars (later spectral types). This difference naturally explains the shift in spectral class to earlier type at lower metallicity. The situation for nitrogen-rich WN stars is less straightforward since they show a much wider range of wind properties.
CFHT and ESO have been used to quantify the properties of Wolf-Rayet stars in the Spiral Galaxies M33, NGC 300, NGC1313 and M83 to investigate how their properties are affected by metal content.
WR stars in M83, located 10 million light years away, are several billion times fainter than the brightest stars in the night sky. Nevertheless, we can use images taken in the light of emission lines and nearby continuum to identify individual massive stars in this galaxy.
We host a catalogue of Galactic Wolf-Rayet stars. I have also investigated the WR content of metal poor galaxies IC10 and especially IZw18, one of the most metal-poor galaxies known.
Fundamental properties of stars can most readily be obtained from analysis of eclipsing double-lined eclipsing binaries. We have shown that WR20a in the open cluster Westerlund 2 has the highest masses of any star known to date, with each component of the 4 day binary having a mass in excess of 80 times the solar mass, each shining a million times brighter.
Rarely, Wolf-Rayet stars are members of close binary systems with compact objects. We have identified the only two unambiguous cases of Wolf-Rayet plus black hole binaries from our surveys of IC10 and NGC300 (see figure above). These possess the highest stellar-mass black holes known to date, and are plausible progenitors of binary black-holes. See 'Farthest' star-mass black hole report from BBC Science News in January 2010.
Spiral Galaxy M33
A spectroscopic search for the non-nuclear Wolf-Rayet population of the metal-rich spiral galaxy M83, Hadfield et al. 2005.
An ultraviolet to mid-infrared study of the physical and wind properties of HD164270 (WC9) and comparison to BD+30 3639 [WC9], Crowther, Morris and Smith 2006.
Reduced Wolf-Rayet line luminosities at low metallicities, Crowther and Hadfield 2006.
The orbital period of the WR binary IC10 X-1: Dynamic evidence that the compact object is a black hole, Prestwich et al. 2007.
Physical properties of Wolf-Rayet stars, Crowther 2007.
NGC 300 X-1 is a Wolf-Rayet/black-hole binary, Crowther et al. 2010.