The Density and Temperature of Molecular Clouds in M33

Details The Density and Temperature of Molecular Clouds in M33 The Density and Temperature of Molecular Clouds in M33

1997-01-30

arxiv.org/abs/astro-ph/9701245v1

Astronomy and Astrophysics

Astrophysics

14 pages, aastex, 4 postscript figures; accepted for publication in ApJ; also available at http://www.physics.mcmaster.ca/Wils

Scientific paper

10.1086/304216

We have observed the $^{12}$CO J=2-1, J=3-2, and $^{13}$CO J=2-1 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33. The $^{12}$CO/$^{13}$CO J=2-1 line ratio is constant across the entire sample, while the observed $^{12}$CO J=3-2/J=2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous HII region. A large velocity gradient analysis indicates that clouds without HII regions have temperatures of 10-20 K, clouds with HII regions have temperatures of 15-100 K, and the cloud in the giant HII region has a temperature of at least 100 K. Interestingly, the giant HII region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby ($< 100$ pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant HII region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant HII region.

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