Session 25 - Stars in the Ultraviolet.
Display session, Tuesday, June 10
South Main Hall,

## [25.05] The Properties and Evolution of Very Massive Stars

S. R. Heap (NASA/GSFC), A. de Koter (ACC/GSFC), I. Hubeny (NOAO/GSFC), E. Malumuth (STX/GSFC)

We present HST/FOS and GHRS spectra of 15 stars in the dense star cluster, R136a, in the Large Magellanic Cloud. These stars define an empirical isochrone for very massive stars (M > 40 M_ødot). We find that the cluster age is about 2.0 Myr, a time when the isochrone is very nearly vertical on the HR diagram. We have therefore used these stars: \beginitemize \item to refine the luminosity classification of early O-type stars \item to calibrate spectra of early O stars in terms of their fundamental parameters; \item to derive a new relation between the rate of mass-loss and the fundamental stellar parameters. \enditemize The new mass-loss law is significantly different from previous prescriptions used by stellar evolutionary models. We therefore expect that it will have a significant impact on our knowledge of massive-star evolution.