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

1. Introduction

Molecular clouds reveal a high degree of internal structure on all size scales, evident as clumpiness and filamentary appearance (Bally et al. 1987, Loren 1989, Stutzki & Güsten 1990, Blitz 1991 and references therein). When molecular clumps are exposed to UV radiation from external or embedded OB stars, the radiation dissociates molecules and ionizes atoms within a thin layer of thickness A_v around 5^m on the surface of an individual clump, the so called `Photon Dominated Region' (PDR). One dimensional theoretical models from Tielens & Hollenbach (1985) and Sternberg & Dalgarno (1989), considering PDRs with hydrogen densities above 10³ cm^-3 and an UV flux above (with in units of 1.610^-3 erg cm^{- 2} s^-1, the strength of the Solar Neighbourhood interstellar radiation field) predict a sequence of different layers of various atomic and molecular species. The externally heated PDR gas cools via characteristic far-infrared and submm lines of these species. The fine structure lines of C⁺ at 158 µm, at 690 µm, at 63 µm and the mid- and high-J CO transitions were detected in a sample of bright Galactic sources and subsequently mapped with increasing spatial and spectral resolution and sensitivity ([CII] 158 µm line, e.g. Stutzki et al. 1988, Howe et al. 1991, Herrmann et al. 1997; [CI] 690 µm line, Phillips & Huggins 1981, Keene 1985; [OI] 63 µm line, Meixner et al. 1992).

The observed intensities of the C⁺, and lines as well as the infrared emission from radiatively and collisionally excited H₂ (Black & van Dishoeck 1987, Sternberg 1988) are in accordance with model predictions. But the semi-infinite PDR models find their limitation in not being able to account for the mid-J ¹³CO and high-J ¹²CO lines and in explaining the extended C⁺ emission found throughout molecular clouds (Stutzki et al. 1988). A more realistic case portrays a molecular cloud with a high clump/interclump density contrast so that accordingly, the penetration depth of the UV radiation is large and emission from many photodissociated clump surfaces along the line of sight is induced. This picture naturally explains the extended C⁺ emission and the brightness temperatures of the ¹³CO J=65 line (Köster et al. 1994) arising from high density clumps. In this case, the emergent C⁺ and CO intensities scale with the incident UV flux and the gas density according to the models of Tielens & Hollenbach (1985) and Hollenbach et al. (1991) and as discussed in Wolfire et al. (1989) and Howe et al. (1991). A constant C⁺/CO intensity ratio was found in higher density (10⁴ cm^-3) and UV flux () regions (Crawford et al. 1985, Jaffe et al. 1994). Lower density and lower UV intensity PDRs have not been investigated at high angular resolution so far, though large scale [CII] surveys with COBE (Wright et al. 1991) or BIRT (Shibai et al. 1991) show that these kinds of regions contribute substantially to the overall Galactic emission. Moreover, theoretical modelling began to describe the low to intermediate density/low UV flux regime (Hollenbach et al. 1991). However, observational confirmation of the calculated emergent FIR intensities are still lacking due to difficulties in observing these weak lines.

The Rosette Molecular Complex (RMC) is a prime example for such a lower density, low UV intensity region, and therefore an intriguing target for our extended mapping in the [CII] 158 µm fine structure line. Our objective was (i) to study the qualitative and quantitative correspondence between the C⁺ and CO emission with special attention to its behaviour at the transition zone between molecular cloud and HII region and (ii) to derive the physical conditions in the PDR region by applying a PDR model to the C⁺ data. We selected the Rosette nebula for this investigation since it is associated with an OB cluster, illuminating the edge of the Rosette molecular cloud. The major part of the molecular cloud complex was already mapped in isotopomeric lower-J rotational CO transitions with the KOSMA 3m and the IRAM 30m telescopes which were presented in a previous paper (Schneider et al. 1998) so that together with other observations (Blitz & Thaddeus 1980, Blitz & Stark 1986, Williams et al. 1995, Williams & Blitz 1998, Phelps & Lada 1997) a broad data set for comparison is available.

Fig. 1 gives an overview of the Rosette region: the part of the molecular cloud which is directly in contact with the HII region NGC 2237/NGC 2246 is outlined by a KOSMA ¹²CO J=21 map. The black squares indicate the regions observed in the [CII] 158 µm line. The Rosette nebula is illuminated by 17 OB stars of the NGC 2244 cluster with a total Lyman- luminosity of 3.810⁵ L (Cox et al. 1990). The HII region extends over 40 pc (Celnik 1985) and expands with a velocity of 20 km s^-1 driven by the stellar wind of the OB stars (Schneps et al. 1980) which are also responsible for the creation of the characteristic central cavity. The distance of the complex is estimated to be 1.6 kpc, derived from photometric observations of the nearby Mon OB2 association (Turner 1976). The most massive part of the molecular cloud is located in the southeast of the nebula with a linear extent of 98 pc in the Galactic plane. Blitz & Thaddeus (1980) described the RMC as a Giant Molecular Cloud with ongoing star formation, as indicated by the presence of at least 34 IR sources. The RMC has an average H₂ density of 30 cm^-3 and a total mass of 10⁵ M, derived from ¹²CO and ¹³CO J=10 observations of Blitz & Thaddeus (1980).

In the following, we will describe our [CII] observations in Sect. 2 and present the results from the [CII] mapping and the analysis in Sect. 3. In the discussion in Sect. 4, we derive the physical conditions in the PDR region by modeling the observed intensities. A summary is given in Sect. 5.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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