The massive stellar content of the HII region NGC 604 and its evolutionarystate

This paper analyses the integrated ultraviolet spectra taken with the Inter national Ultraviolet Explorer (IUE) and optical ground-based spectra taken with the William Herschel Telescope (WHT) of the giant H ii region NGC 604. These data are complemented with ultraviolet (WFPC2 through F170W) and H a lpha (WFPC2 through F656N) images retrieved from the Hubble Space Telescope (HST) archive. The ultraviolet resonance wind stellar lines, the nebular optical emission lines and the higher order terms of the Balmer series, and He I absorption lines detected in the spectra of NGC 604 are interpreted using evolutionary models optimized for young star-forming regions. The evolutionary state an d the massive stellar content of the region are derived in a self-consisten t way. The three techniques applied suggest that the central ionizing cluster in N GC 604 is very young, similar or equal to 3 Myr old, and that the stars in the cluster were formed in an instantaneous burst following a Salpeter (alp ha=2.35) or flatter (alpha=1.5) initial mass function (IMF), having stars m ore massive that 80 M.. The stellar cluster is able to provide most of the ionizing photons needed to photoionize the whole nebula, and the wind power to form the central shell structure where the cluster core is located. The stellar cluster is affected by an extinction similar to the average extinc tion that affects the ionized gas. The estimated number of massive stars in the cluster is also in agreement with that derived from previous studies b ased on the detection of individual stars. The results that we present here support the use of these techniques for the interpretation of the integrat ed light of more distant star-forming regions.