HUBBLE SPACE TELESCOPE OBSERVATIONS OF THREE VERY YOUNG STAR CLUSTERS IN THE SMALL MAGELLANIC CLOUD

摘要

We present Space Telescope Imaging Spectre-graph (STIS) broadband imagery and optical slitless spec-troscopy of three young star clusters in the Small Magellanic Cloud (SMC). MA 1796 and MG 2 were previously known as planetary nebulae and were observed as such in 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 since it falls in the same frame of MA 1796. A 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, denning 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 ultracompact 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 BO class. These very compact, extremely reddened, and probably very dense H Ⅱ regions in the SMC offer insight into the most recent star formation episodes in a very low metallicity galaxy.