The ionizing star clusters of giant H II regions in NGC 2403

We present the results of a study on the massive star population down to ab out M-V similar to -3.1, or 12-15 M-., of the most luminous giant H II regi ons in the nearby spiral galaxy NGC 2403, based on Hubble Space Telescope i mages and ground-based spectrograms. Particular emphasis is placed on the d istribution of the Wolf-Rayet and red supergiant stars and the information they provide about the recent star-forming history of these large complexes . We find direct evidence for the presence of Wolf-Rayet (WR) stars in five of the six giant H II regions investigated; 25-40 WR stars are inferred fo r the sole NGC 2403-I giant H II region. Red supergiant (RSG) stars are mai nly distributed over a more extended halo, while the young blue stars and m ost WR stars are in or close to a compact core. One appears to be seeing yo ung cores of O and WR stars surrounded by older halos containing red superg iants. We propose a scenario in which RSG stars belonging to an early phase of star formation were followed by a more recent burst corresponding to a very blue mean sequence. Delayed trigger with preheating over several 100 p c by the first generation of massive stars allowed the build-up of the requ ired confinement for the production of parsec-scale cluster cores with lumi nosity up to a few times 10(6) L-.. Finally, we present some interesting ob jects found in the field of NGC 2403 outside the giant H II regions, such a s field WR stars, globular clusters and background galaxies.