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Astron. Astrophys. 358, 682-688 (2000)

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4. Comparison with observations

Castets et al. (1990) and Kramer et al. (1996) have observed the 12CO and 13CO low-J lines in various star forming cloud complexes with [FORMULA] in the range [FORMULA]. Averaging the line intensities over different positions (c.f. Kramer et al.) the low-J line center brightness ratio is typically close to unity, both for the 12CO and 13CO lines. However, the 12CO/13CO ratio between the same transition is about 2-3, the 12CO/C18O ratio between the same transition is about 20. The absolute values of the line center brightness temperature is somewhat larger than 20 K for the 12CO lines, about 10 K for the 13CO lines and about 1-2 K for the C18O lines. Castets et al. demonstrated that these ratios cannot be explained by single temperature, single density models. The spherical PDR models can reproduce the observations for densities of about [FORMULA]=[FORMULA] and column densities [FORMULA]. The model results are in this parameter range relatively insensitive to the intensity of the FUV field. At single positions the 12CO/13CO line ratio can vary between 1.5-8.0 (Kramer et al. 1996). This can again be explained by the spherical PDR models by varying the clump size and the densities. PDR models thus give a very natural explanation of the observed intensities.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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