Issue |
A&A
Volume 471, Number 2, August IV 2007
|
|
---|---|---|
Page(s) | 561 - 571 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20067034 | |
Published online | 26 June 2007 |
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
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.
Key words: ISM: clouds / ISM: molecules / galaxies: ISM / galaxies: Magellanic Clouds
© ESO, 2007
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