2007A&A...471..561N

Our goal is to determine the physical properties of molecular gas located in different environments of the SMC - from near the vicinity of hot HII regions to cold, quiescent clouds - via modelling and simulations, and compare with the properties of molecular gas found in similar environments in the LMC. We present observations of the ¹²CO(1-0), (2-1), (3-2), ¹³CO(1-0), (2-1), and CS(2-1), (3-2) line emission toward six molecular clouds in the SMC: N66, N88, Lirs36, Lirs49, Hodge15, and SMC-B1#1. These data, as well as published data on three clouds of the LMC: 30Dor-10, N159-W, and N159-S, are analysed to estimate gas kinetic temperatures, column densities, and surface filling factors using a Mean Escape Probability approximation of the radiative transfer equations. The solutions are restricted using the χ² approach. Assuming that the [¹²CO/¹³CO] abundance ratio is similar in both galaxies, we find that the CO and CS column densities of SMC clouds are a magnitude smaller than those of LMC clouds, mirroring the metallicity differences. Our analysis suggests the existence of a lower limit for the ¹²CO/¹³CO isotope ratio of 50 in both galaxies. The surface filling factors of the CO emission in the SMC clouds are a factor of a few smaller than in the LMC and seem to decrease with increasing UV radiation fields, i.e., more vigorous star formation activity. A simple model, which assumes a spherical cloud with uniform physical parameters immersed in the CMB radiation field, provides a reasonably good fit to the observed properties of the (supposedly) quiescent clouds SMC-B1#1 and N159-S. For all other clouds considered, this model gives large values of χ², strongly indicating the need for a more complex model. We present some results from 2-component modelling, e.g., for Lirs 36 a mixture of 20K gas with high optical depth and a less dense gas with temperatures of 100K reproduces well the main features of the CO data.