A HST study of young massive star clusters in compact H II regions of the Magellanic Clouds

Abstract

We present our results on high resolution imaging and spectroscopy using the HST of six high-excitation ‘blobs’ (HEB). HEBs constitute a rare class of compact H ii regions in the Magellanic Clouds. Contrary to the typical H ii regions of these galaxies, which are extended structures with sizes greater than 50 pc, the compact H ii regions are an order of magnitude smaller having diameters of less than about 3 pc. HEBs are believed to be the final stages in the evolution of the ultra-compact H ii regions, whose Galactic counterparts are detected only at infrared and radio wavelengths. We find that despite their small size, HEBs are in general excited by more than one newborn massive star. Color-magnitude diagrams of the exciting stellar population indicate that it is consistent with an O6-O8 type, and far-UV spectroscopy with HST-stis of several of these stars further confirms their youth. Surprisingly though, it also shows an astonishing weakness of their wind profiles and their sub-luminosity, up to ~ 2 mag fainter in M_V than the corresponding dwarfs. Our analysis suggests that these stars are probably in the Hertzsprung-Russell diagram locus of a particularly young class of massive stars, the so-called Vz luminosity class, as they are arriving on the zero age main sequence.