Unusual CO Line Ratios and Kinematics in the N83/N84 Region of the Small Magellanic Cloud

Abstract

We present new CO (1-->0) and (2-->1) observations of the N83/N84 molecular cloud complex in the southeast wing of the Small Magellanic Cloud (SMC). While the (2-->1)/(1-->0) integrated line brightness ratio (in temperature units) is uniformly 1.1 throughout most of the complex, we find two distinct regions with unusually high ratios, (2-->1)/(1-->0)>~2. These regions are associated with the N84D nebula and with the inside of the 50 pc expanding molecular shell N83. This shell is spatially coincident with the NGC 456 stellar association and the HFPK2000-448 radio continuum/X-ray source, tentatively classified as a supernova remnant. We explore possible causes for the high ratios observed and conclude that the CO emission probably arises from an ensemble of small (R~0.1 pc), warm (Tg~40 K) clumps. Analysis of the CO shell parameters suggests that it is wind driven and has an age of slightly more than 2 million years. We have also used this data set to determine the CO-to-H2 conversion factor in the SMC, an especially interesting measurement because of the low metallicity of this source (~1/9 solar). Surprisingly, after comparing the CO luminosities of clouds in N83/N84 with their virial masses, we find a CO-to-H2 conversion factor XCO only 1.9 times larger than what we obtain when applying the same algorithm to solar metallicity clouds in the Milky Way and M33. This result fits into the emerging pattern that CO observations with high linear resolution suggest nearly Galactic values of XCO in a wide range of environments.