Low-excitation blobs in the Magellanic Clouds

Aims : We study an unknown, or very poorly known, interstellar HII component in the Magellanic Clouds. This is the first study ever devoted to this class of objects, which we call Low-excitation blobs (LEBs). Methods : We used low-dispersion spectroscopy carried out at ESO to obtain emission line intensities of Ha, Hb, and [OIII] (4959+5007) for 15 objects in the Large Magellanic Cloud and 14 objects in the Small Magellanic Cloud. Results are displayed in excitation ([oiii]/Hb ratio) versus Hb luminosity diagrams. Results : We show the presence of an LEB component in the Magellanic Clouds and study its relationship with the already known class of high-excitation blobs (HEBs). The newly found LEBs are lower excitation counterparts of HEBs and are powered by less massive exciting stars. Further study of LEBs is expected to provide new pieces of information for a better understanding the low mass end of the upper initial mass function in the Magellanic Clouds.Comment: Accepted in A&

Spitzer Mid-infrared Study of Compact HII Regions in the Magellanic Clouds

We present a study of the mid-infrared properties and dust content of a sample of 27 HII ``blobs'', a rare class of compact HII regions in the Magellanic Clouds. A unique feature of this sample is that even though these HII regions are of high and low excitation they have nearly the same physical sizes ~1.5-3 pc. We base our analysis on archival 3-8 microns infrared imagery obtained with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. We find that despite their youth, sub-solar metallicity and varied degrees of excitation, the mid-infrared colors of these regions are similar to those of typical HII regions. Higher excitation ``blobs'' (HEBs) display stronger 8 micron emission and redder colors than their low-excitation counterparts (LEBs).Comment: 8 pages, 4 figures, Accepted for publication in Astronomy & Astrophysics (higher resolution version is available in http://www.physics.uoc.gr/~vassilis/papers/blobs_spitzer.pdf

STIS spectroscopy of newborn massive stars in SMC N81

Using Hubble Space Telescope observations with STIS, we study the main exciting stars of N81, a high excitation compact Hii region in the Small Magellanic Cloud (SMC). These far UV observations are the first spectroscopic measurements of stars in such a region and reveal features characteristic of an O6-O8 stellar type. The astonishing weakness of their wind profiles and their sub-luminosity (up to ~ 2 mag fainter in Mv than the corresponding dwarfs) make these stars a unique stellar population in the Magellanic Clouds. Our analysis suggests that they 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.Comment: 9 pages, 3 figure