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

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5. Summary

We have presented an extended map of the 158 µm [CII] fine structure line of the Rosette Molecular Complex from which we derive the following results:

  • By positionally averaging the data of each array, we show that the C+ line is detected in every array except one above the 3[FORMULA]-level.

  • The absolute intensities are weak with an average value of 1.7[FORMULA]10-4 erg s- 1 sr-1 cm-2 and a peak value of 5[FORMULA]10-4 erg s- 1 sr-1 cm-2.

  • The comparison between C+ and 12CO J=3[FORMULA]2 emission distribution at similar angular resolution reveals that the RMC is a UV penetrated clumpy molecular cloud in which the CO and C+ emitting gas is intimately mixed.

  • The flux ratio [FORMULA]/[FORMULA] across the region observed in the [CII] line is approximately 10-2 which is at the high end of values for other Galactic sources but can be explained by the low incident UV flux in the Rosette.

  • In contrast to higher density (n[FORMULA]105 cm-3) PDR regions with a strong incident UV flux ([FORMULA]) the Rosette observations for the first time give an observational verification that the emergent C+ intensity ([FORMULA]) scales logarithmically with the UV flux (F), supporting existing PDR models for lower density and UV intensity regions.

  • Due to the observed relationship [FORMULA] we conclude that the peak intensities around 5[FORMULA]10-4 erg s- 1 sr-1 cm-2, found (i) at the interface region between molecular cloud and HII region and (ii) at the position of the IR source IR06306+0437, are attributed to PDRs, illuminated (i) by the central OB cluster NGC 2244 and (ii) by a small embedded OB cluster.

  • Extended C+ emission on a low intensity level around 10-4 erg s- 1 sr-1 cm-2 is found deep in the molecular cloud core which implies a high clump/interclump density contrast (30-300), where the UV radiation can penetrate deep into the cloud and induce emission from many PDRs along the line of sight.

  • Both, the observed CO and C+ intensities, are explained by a PDR model incorporating the effects of a clumpy cloud structure. The emitting gas has a density of at least 104 cm-3 with an incident UV flux of 102-103 [FORMULA].
    This study of the [CII] 158 µm fine structure line emission in the Rosette Molecular cloud as an example for a low to medium density PDR, exposed to a low incident UV field, reveals systematic differences to high density and high UV intensity PDRs: The C+ intensity scales with the logarithm of the UV flux and the flux ratio [FORMULA]/[FORMULA] has a rather high value of 1%. Further observational studies of low density and UV intensity PDRs could show whether these findings are generally true for these kind of regions.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998