CO and CS in the Magellanic Clouds: ax(2)-analysis of multitransitional data based on the MEP radiative transfer model
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Aims. 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. Methods. We present observations of the (CO)-C-12(1-0), (2-1), (3-2), (CO)-C-13(1-0), (2-1), and CS (2-1), (3-2) line emission toward six molecular clouds in the SMC: N66, N88, Lirs 36, Lirs 49, Hodge 15, and SMC- B1# 1. These data, as well as published data on three clouds of the LMC: 30Dor- 10, N 159- 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 chi(2) approach. Results. Assuming that the [(CO)-C-12/(CO)-C-13] 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 (CO)-C-12/ (CO)-C-13 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 chi(2), 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 20 K gas with high optical depth and a less dense gas with temperatures of 100 K reproduces well the main features of the CO data.