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

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-level.

• The absolute intensities are weak with an average value of 1.710^-4 erg s^{- 1} sr^-1 cm^-2 and a peak value of 510^-4 erg s^{- 1} sr^-1 cm^-2.

• The comparison between C⁺ and ¹²CO J=32 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 / 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 (n10⁵ cm^-3) PDR regions with a strong incident UV flux () the Rosette observations for the first time give an observational verification that the emergent C⁺ intensity () scales logarithmically with the UV flux (F), supporting existing PDR models for lower density and UV intensity regions.

• Due to the observed relationship we conclude that the peak intensities around 510^-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 .
  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 / 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.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
helpdesk.link@springer.de