CO and CS in the Magellanic Clouds: a -analysis of multitransitional data based on the MEP radiative transfer model^*

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile e-mail: silvana@das.uchile.cl
² Onsala Space Observatory, S-43992 Onsala, Sweden

Received: 27 December 2006
Accepted: 30 April 2007

Abstract

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 H II 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 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: N 66, N 88, Lirs 36, Lirs 49, Hodge 15, and SMC-B1#1. These data, as well as published data on three clouds of the LMC: 30 Dor-10, N 159-W, and N 159-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 approach.

Results.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 N 159-S. For all other clouds considered, this model gives large values of , 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.