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V. Charmandaris (Cornell Univ., USA), M. Heydari-Malayeri (Obs. de Paris, France), L. Deharveng (Obs. de Marseille, France), M.R. Rosa (ST-ECF/ESO, Germany), D. Schaerer (Obs. de Midi-Pyrenees, France), H. Zinnecker (AIP, Germany)
We present our results on high resolution imaging and spectroscopy using the Hubble Space Telescope (HST) of six high excitation ``blobs'' (HEBs). HEBs constitute a rare class of compact HII regions in the Magellanic Clouds. Contrary to the typical HII regions of these galaxies, which are extended structures with sizes greater than 50pc, the compact HII regions are an order of magnitude smaller having diameters of less than about 3pc. HEBs are believed to be the final stages in the evolution of the ultra-compact HII regions -- whose Galactic counterparts are detected only at infrared and radio wavelengths.
Our imaging campaign enabled us to resolve these objects for the first time, uncovering their morphology, nebular features, as well as the distribution of their high excitation [OIII]\,\lambda 5007\,Å\, zones and the variation of the extinction across them. 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 MV 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.
We will discuss how these stellar properties fit into the context of the turbulent environment surrounding the HEBs, as well as how future observations will help us address several outstanding questions on the massive star formation properties in low metallicity environments.
Support for this work was provided in part by NASA through grant numbers GO-8246 and GO-8247 from the STScI, which is operated by AURA, Inc., under NASA contract 26555.
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The author(s) of this abstract have provided an email address for comments about the abstract: mohammad.heydari-malayeri@obspm.fr