Wolf-Rayet stars in the giant H II region NGC 604

Abstract

Spectra obtained at various positions in NGC 604, the brightest H II region in M33, show the emission features typical of Wolf-Rayet (W-R) stars. By comparing these spectra of NGC 604 with data for the other W-R stars, it is determined from the equivalent widths of the Wolf-Rayet bands that about 50% of the observed continuum is due to W-R stars. Since most of the light in the B and V bandpasses from a Population I association is due to main sequence and supergiant O, B, and A stars, and the W-R stars have comparable absolute magnitudes at these wavelengths, a ratio of W-R to blue stars of about 1.0 is estimated. The presence of some 50 W-R stars with a mean type of WN 7 is interpreted as the result of a single burst of star formation approximately 4,000,000 years ago in the core of NGC 604.