The red supergiants and Wolf-Rayet stars of NGC 604

Abstract

We study the post-main-sequence stars in NGC 604, the most luminous H ii region in M33. A number of Wolf–Rayet (WR) stars and one red supergiant (RSG) were discovered earlier. Based on the broad-band photometry of the region, we present evidence that is consistent with the presence of this RSG and with three more RSG candidates. Using spectral energy distribution (SED) fitting based on Hubble Space Telescope UVIJHK photometry, we estimate the ages of the WR stars and RSGs and find that the two populations are from distinct formation episodes with ages of 3.2 ± 1.0 and 12.4 ± 2.1 Myr, respectively. The RSGs have greater extinctions towards their line of sight than the WR stars which is consistent with the production of a large amount of dust by the RSGs. Using the WR and RSG populations and similar SED fits to the most massive O stars, we estimate that the total stellar mass is (3.8 ± 0.6) × 105 M⊙. We find a large discrepancy between the expected Hα flux from such a massive cluster and the one observed. This suggests that 49+16−19 per cent of the ionizing photons produced by massive stars in NGC 604 are leaking from this H ii region. We also suggest that the implications of an old RSG population mean that if NGC 604 were more distant and only observed in the infrared, it would be difficult to study the youngest burst of star formation due to the contamination of RSGs.