WOLF-RAYET STARS IN THE MILKY WAY, THE LARGE MAGELLANIC CLOUD, AND EMISSION-LINE GALAXIES

Abstract

ABSTRACT Wolf-Rayet (W-R) stars are the short-lived endpoints of massive star evolution. Recent studies have identified a set of blue emission-line ''starburst'' galaxies whose integrated spectra reveal broad, resolved, emission features due to the presence of 102 - 105 such stars. We have performed a systematic study of the properties of a sample of such ''W-R galaxies''. The properties of W-R stars in the Galaxy and the Large Magellanic Cloud (LMC) provide the foundation for this analysis. Using a relation between the size of continuum discontinuities and the intrinsic (b-v)0 color found from model atmospheres of WN stars, we derive a formula for calculating the color excesses of such stars. We then calculate color excesses for nearly all WN stars in the Galaxy and the LMC. Our results are in good agreement with previous determinations of the color excesses for WN stars. Next, we derive average intrinsic He II 4686 and C IV lambda-5808 emission-line luminosities from the spectra of LMC WN and WC stars, respectively. Models of WN atmospheres are used to calculate effective temperatures, radii, luminosities, and Lyman continuum fluxes of LMC WNL stars. We have obtained a spatially integrated spectrum of the central 8' times 8' region of the giant H II region 30 Doradus in the LMC. From the observed line strengths and our average emission-line and Lyman-continuum luminosities we estimate the numbers of W-R and O stars present. Our results are in excellent agreement with direct star counts in the region. Finally, we have obtained moderate-resolution optical spectra of ten W-R galaxies and four comparison starburst galaxies. The spectra are very similar to those of giant H II regions; the various forbidden line ratios clearly indicate a stellar origin for the emission line spectrum. From the strong nebular emission lines, we determine the internal extinctions, densities, temperatures, oxygen abundances, and the Lyman continuum luminosity in these galaxies. From the flux in the broad He II lambda-4686 feature and its strength relative to H-beta, the number of W-R stars and the ratio of the number of W-R to the number of O stars present are determined. We calculate W-R/O number ratios that are an order of magnitude larger than predicted by current models of massive star evolution under conditions of continuous star formation with durations of 107 yr or longer. The observed number ratios can be reconciled with the model predictions only if the star formation in these galaxies occurs in ''bursts'' lasting </= 106 yr.