Hubble Space Telescope observations of three very young star clusters in the Small Magellanic Cloud

Abstract

We present Space Telescope Imaging Spectrograph (STIS) broad band imagery and optical slitless spectroscopy of three young star clusters in the Small Magellanic Cloud (SMC). MA 1796 and MG 2 were previously known as Planetary Nebulae, and observed as such within our Hubble Space Telescope (HST) survey. With the HST spatial resolution, we show that they are instead H II regions, surrounding very young star clusters. A third compact H II region, MA 1797, was serendipitously observed by us as it falls in the same frame of MA 1796. Limited nebular analysis is presented as derived from the slitless spectra. We find that MA 1796 and MG 2 are very heavily extincted, with c>1.4, defining them as the most extincted optically-discovered star forming regions in the SMC. MA 1796 and MG 2 are extremely compact (less than 1 pc across), while MA 1797, with diameter of about 3 pc, is similar to the ultra compact H II regions already known in the SMC. Stellar analysis is presented, and approximate reddening correction for the stars is derived from the Balmer decrement. Limited analysis of their stellar content and their ionized radiation shows that these compact H II regions are ionized by small stellar clusters whose hottest stars are at most of the B0 class. These very compact, extremely reddened, and probably very dense H II regions in the SMC offer insight in the most recent star formation episodes in a very low metallicity galaxy.