Astron. Astrophys. 338, 262-272 (1998)

4. Dicussion

4.1. Modelling of C⁺ and CO intensities

Fig. 7a and b show C⁺ versus CO intensity, plotted on a logarithmic scale, embedded in a parameter space of density and UV flux, calculated by a model of Köster et al. (1994). In this model, a molecular cloud is exposed to isotropic UV radiation from two sides, thus giving a realistic approximation to the emission from clumps with finite size. The chemical structure and the gas temperature at one location depend on the distance to the surfaces and on the conditions within the cloud so that the line intensities are calculated iteratively, using an `Escape Probability' formalism and incorporating the ¹³CO chemistry. The free parameters of the model are the line width, the incident UV field and the extinction.

Fig. 7. In these log-log plots, the C+ intensity is plotted against the CO intensity for a visual extinction of 10m (5m) for an individual "clump" within a parameter space of hydrogen density and UV flux, calculated by the model by Köster et al. (1994). The arrows indicate how the datapoints are shifted due to different beam filling factors.

The H₂-column density of the individual clumps in the RMC reaches values up to 1010²¹ cm^-2 (obtained with IRAM CO data, Schneider 1995) so that we give model results for a visual extinction of A_v=10^m and A_v=5^m. In general, the datapoints cover a regime of low incident UV flux () and medium to high density (10³ to 10⁵ cm^-3) in both model fits (A_v=5^m and 10^m). The highest intensities of both tracers are found at the position of the IR source IR06314+0427. According to the PDR model, the densities are 10⁴-10⁵ cm^-3 and the UV flux 100 to a few . The high UV flux is provided by a small association of OB stars (see 3). The C⁺ and CO intensities from the HII region/molecular cloud interface and the cloud core indicate lower densities (10³ to 10⁴ cm^-3) and a lower UV flux (a few to a few 100 ). The CO intensity shows little positional variation whereas the C⁺ intensity decreases a factor 10 from the interface region into the cloud core.

The datapoints are shifted to higher densities and UV flux if the beam filling factors ( and ) are lower than 1, which is likely because the higher resolution CO observations with IRAM reveal substructure. The two arrows indicate filling factors (=) of 0.3 and 0.7 and demonstrate how the datapoints are shifted. In particular, a small filling factor for the CO data easily shifts the datapoints into a higher density regime whereas a small factor for the C⁺ data only slightly shifts the datapoints into a higher UV field regime.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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