The red supergiants & Wolf-Rayet stars of NGC 604

Abstract

We study the post-main sequence stars in NGC 604, the most luminous HII region in M33. Previously, a number of Wolf-Rayet (WR) stars and one red supergiant (RSG) have been discovered. Based on broadband photometry of the region, we present evidence that is consistent with the presence of this RSG and with three more RSG candidates. Using SED fitting based on HST UVIJHK photometry we estimate the ages of the WR stars and RSGs finding that the two populations are from distinct formation episodes with ages 3.2$\pm$\pm$2.1Myrs,respectively.TheRSGshavegreaterextinctionstowardstheirlineofsightthantheWRstarsconsistentwiththeRSGsproducinglargeamountofdust.UsingtheWRandRSGpopulationsandsimilarSEDfitstothemostmassiveOstarsweestimatethatthetotalstellarmassis(3.8$\pm$ 0.6) x 10^5Msun. We find a large discrepancy between the expected Hα flux from such a massive cluster and that one observed. This suggests that 49 (+16,-19) percent of the ionizing photons produced by massive stars in NGC 604 is leaking from this HII region. We also suggest that the implications of an old RSG population mean that if NGC 604 was more distant and only observed in the infrared (IR) it would be difficult to study the youngest burst of star formation due to the contamination of RSGs.

Accepted for publication in MNRAS, 13 pages, 10 figures, 3 tables

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