ON THE EVOLUTIONARY PHASE AND MASS-LOSS OF THE WOLF-RAYET-LIKE STARS IN R136A

We report on a systematic study of the most massive stars, in which we analyzed the spectra of four very luminous stars in the Large Magella nic Cloud. The stars lie in the 30 Doradus complex, three of which are located in the core of the compact cluster, R136a (R136a1, R136a3, an d R136a5), and the fourth (Melnick 42), located about 8 '' north of R1 36a. Low-resolution spectra (<200 km s(-1)) of these four stars were o btained with the GHRS and FOS spectrographs on the Hubble Space Telesc ope. The GHRS spectra cover the spectral range from 1200 to 1750 Angst rom and the FOS spectra from 3200 to 6700 Angstrom We derived the fund amental parameters of these stars by fitting the observations by model spectra calculated with the ''ISA-WIND'' code of de Koter et al. We f ind that all four stars are very hot (similar to 45 kK), luminous, and rich in hydrogen. Their positions on the HR-diagram imply that they a re stars with masses in the range 60-90 M. that are 2 million years ol d at most, and hence, they are O-type main-sequence stars still in the core H-burning phase of evolution. Nevertheless, the spectra of two o f the stars (R136a1, R136a3) mimic those of Wolf-Rayet stars in showin g very strong He II emission lines. According to our calculations, thi s emission is a natural consequence of a very high mass-loss rate.We c onjecture that the most massive stars in R136a-those with initial mass es of similar to 100 M. or more-are born as WR-like stars and that the high mass loss may perhaps be connected to the actual stellar formati on process. Because the observed mass-loss rates are up to 3 times hig her than assumed by evolutionary models, the main-sequence and post-ma in-sequence tracks of these stars will be qualitatively different from current models. The mass-loss rate is 3.5-8 times that predicted by t he analytical solutions for radiation-driven winds of Kudritzki et al. (1989). However, using sophisticated Monte Carlo calculations of radi ative driving in unified model atmospheres, we show that-while we cann ot say for sure what initiates the wind-radiation pressure is probably sufficient to accelerate the wind to its observed terminal velocity, if one accounts for the effects of multiple photon scattering in the d ense winds of the investigated stars.