UCL Astrophysics Group

Hot Stars Research Group

Hot-star winds are driven by radiation pressure via millions of metal lines. For this reason, the metallicity of the parent galaxy plays a crucial role in massive-star evolution, since it defines their internal structure, opacities and stellar wind properties. The precise relation between metallicity and mass loss, a key ingredient for the reliable population synthesis of young galaxies, is, however, poorly known. We are therefore employing a variety of observations to constrain this relationship. We have used the 2dF facility at the Anglo-Australian Telescope to obtain optical spectra of a truly representative sample of Small Magellanic Cloud stars, which has abundances of metals smaller than our own Galaxy by an order of magnitude.

The abundances of helium, carbon, nitrogen and oxygen are sensitive indicators of processed material. We are studying these elements, using elaborate non-LTE model atmospheres, and high-quality spectra, including far-UV, UV, optical and IR diagnostics. The most direct method of deriving empirical mass-loss rates for hot stars is through the analysis of UV resonance transitions from dominant metal ions. However, difficulties in deriving the wind ionisation balance using currently available (trace) ions means that mass-loss rates remain uncertain.

Archival Hubble Space Telescope FOS and GHRS data sets have been used to collect ultraviolet evidence for large- and small-scale stellar wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136, M31, M33, and NGC6822). By comparison to previous studies of Galactic OB stars, wind activity is principally diagnosed in individual spectrograms via the presence of `narrow absorption components' and saturated `black' absorption troughs in the resonance line doublets. Their observed characteristics broadly suggest that these stars share the same physical mechanisms for perturbing the winds as those that act in Galactic OB stars. Both these spectral indicators are also used to provide reliable measures of wind terminal velocities. These velocities are directly compared to previously published Galactic OB star values, without reliance on model profile fitting. Relative to Galactic OB stars, the most discrepant terminal velocities (and wind line profiles) are due to MAIN-SEQUENCE early O-type stars in the SMC (Prinja & Crowther, 1998 MNRAS 300 828 see above).

We are also attacking the mass-loss problem with the FUSE telescope. Relative to IUE/HST, additional wind lines will be observed, spanning a much wider range of species. From these data, the degree of ionization can be determined accurately, so that mass-loss rates can be measured with confidence. In addition, FUSE will observe massive stars in both the Galaxy and the Magellanic Clouds - spanning a factor of ten in metallicity - so that the variation of mass-loss properties with metal content will be measured, of importance in the study of high redshift galaxies.

Our studies have been greatly enhanced by the launch of the ESA (Infrared Space Observatory (ISO). We have used the Short Wavelength Spectrometer to obtain mid-IR (2.6-30 micron) spectroscopy of several WC stars. Dessart et al. (MNRAS 315 407) and Willis et al. (MNRAS 290 371) analyse UV/optical and ISO-SWS spectroscopy of several WC stars, including WR11, WR90, WR135 and WR146. In all cases, abundances derived from neon lines confirmed a substantial enrichment, by factors of 6 to 8, but these are lower than predictions from evolutionary models imply.

We have also used spectroscopic models to observationally determine the evolutionary paths between O and the late WN (WNL) stars in the LMC, involving so-called Ofpe/WN9 stars. Theoretical models predict that the most massive stars pass through an intermediate, unstable stage corresponding to either a Luminous Blue Variable (LBV), or a red supergiant (RSG) phase for lower mass stars. Our studies have shown that Ofpe/WN9 stars are probably dormant LBVs (Crowther & Smith 1997 A&A 320, 500) .

IR studies have revealed clusters of emission line sources at the Galactic Centre (click HERE for a mid-IR image) which show properties similar to Wolf-Rayet and Of stars (e.g. Pistol Star). We can therefore apply similar techniques (e.g. Crowther & Smith 1996, A&A 305 541, Crowther & Smith 1997, A&A 320, 500 and Bohannan & Crowther, 1999, ApJ 511, 374) to the GC sources in order to identify their nature, currently underway using UKIRT.

We are studying variability in the outflows from massive O, B and Wolf-Rayet stars using UV, optical, IR and radio observations. The denser winds of WR stars are well known to be variable on timescales of hours, based on previous ultraviolet and optical investigations.

Our ideas about WR and O star winds have changed dramatically over the last ten years; the picture that is now emerging is one in which the winds are time-dependent and permeated by shocks driven by radiative instabilities. We have continued extensive studies of stellar wind variability in WR stars, with particular attention to extensive IUE monitoring (over continuous periods of 6-7 days). We have been involved with the IUE MEGA campaign in which a Wolf-Rayet star, an O-supergiant and a B supergiant have been continuously monitored with the IUE satellite over 15 days. This will allow us to investigate the time-dependent UV profile variability over several consecutive stellar rotation periods, to probe the connection between rotation and the occurrence of wind structure/variability linked to radiatively-induced wind instabilities. An intensive optical variability study of an Of star, HD151804, has also recently been performed (Prinja, Fullerton and Crowther 1996 A&A 311 264).

From our previous UV studies of WR6 the P Cygni absorption profiles are seen to vary in the sense that extra absorption, at velocities exceeding the wind terminal velocity, increases and decays with a recurrence timescale of 1 day. The much smaller variations detected in the P Cygni emission components suggest that the material giving rise to the variability has a characteristic linear scale of the order of the core radius. The nature of these variations is most easily explained by radiative instabilities driving high speed, rarefied shocked gas through the stellar wind of WR6 (Willis & Stevens 1996 A&A; 310 577). In a parallel study we have secured Guest Observer PSPC ROSAT observations of WR6 at two epochs. Our results show no phase--dependent modulation in the X-ray flux, no evidence for variations on timescales of less than 1 hour (previously suggested from EINSTEIN data), and evidence for significant variations (at the 30% level) on a daily timescale. These results suggest a pattern of X-ray variability consistent with our IUE-based conclusions above.

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