CO and CS in the Magellanic Clouds: a χ^2-analysis of multitransitional data based on the MEP radiative transfer model

Details CO and CS in the Magellanic Clouds: a χ^2-analysis of multitransitional data based on the MEP radiative transfer model CO and CS in the Magellanic Clouds: a χ^2-analysis of multitransitional data based on the MEP radiative transfer model

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007a%26a...471..561n&link_type=abstract

Astronomy and Astrophysics, Volume 471, Issue 2, August IV 2007, pp.561-571

Astronomy and Astrophysics

Astrophysics

4

Ism: Clouds, Ism: Molecules, Galaxies: Ism, Galaxies: Magellanic Clouds

Scientific paper

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 χ2 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 χ^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.
The observations
were made with the Swedish-ESO Submillimetre Telescope, SEST, which was
operated jointly by the Swedish National Facility for Radioastronomy and
the ESO.

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