Context. The fractal structure of the interstellar medium suggests that the interaction of UV radiation with the ISM as described in the context of photon-dominated regions (PDR) dominates most of the physical and chemical conditions, and hence the far-infrared and submm emission from the ISM in the Milky Way. Aims. We investigate to what extent the Galactic FIR line emission of the important species CO, C, C + , and O, as observed by the Cosmic Background Explorer (COBE) satellite can be modeled in the framework of a clumpy, UV-penetrated cloud scenario. Methods. The far-infrared line emission of the Milky Way is modeled as the emission from an ensemble of clumps with a power law clump mass spectrum and mass-size relation with power-law indices consistent with the observed ISM structure. The individual clump line intensities are calculated using the KOSMA-τ PDR-model for spherical clumps. The model parameters for the cylindrically symmetric Galactic distribution of the mass density and volume filling factor are determined by the observed radial distributions. A constant FUV intensity, in which the clumps are embedded, is assumed. Results. We show that this scenario can explain, without any further assumptions and within a factor of about 2, the absolute FIR-line intensities and their distribution with Galactic longitude as observed by COBE.

Context. The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. Aims. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas.Methods. The NANTEN2-4 m submillimeter telescope was used to map the [C i] 3 P 1 − 3 P 0 , 3 P 2 − 3 P 1 and CO 4-3, 7-6 lines in two 4 × 4 regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr 14, and the second region is in the molecular cloud south of η Car and Tr 16. These data were combined with 13 CO SEST spectra, HIRES/IRAS 60 µm and 100 µm maps of the FIR continuum, and maps of 8 µm IRAC/Spitzer and MSX emission. Results. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 10 3 in Draine units while the field of the southern region is about a factor 10 weaker. While the IRAC 8 µm emission lights up at the edges of the molecular clouds, CO and also [C i] appear to trace the H 2 gas column density. The northern region shows a complex velocity and spatial structure, while the southern region shows an edge-on PDR with a single Gaussian velocity component. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38 beam (0.43 pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2 × 10 5 cm −3 is needed to reproduce the observed line emission at two selected interface positions. Conclusions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

Context. Star formation at earlier cosmological times took place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, C i and C ii, which originate in the surface layers of molecular clouds illuminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12 CO J = 4 → 3, J = 7 → 6, and 13 CO J = 4 → 3 rotational and [C i]3 P 1 − 3 P 0 and 3 P 2 − 3 P 1 finestructure transitions. The 13 CO J = 4 → 3 and [C i] 3 P 2 − 3 P 1 transitions are detected for the first time in the LMC. We derive the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in the N159W region has temperatures of about 80 K and densities of about 10 4 cm −3 . The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [C ii] line intensity reported in the literature, in the context of a clumpy cloud PDR model constrains the UV intensity to about χ ≈ 220 and an average density of the clump ensemble of about 10 5 cm −3 , thus confirming the presence of high density material in the LMC N159W region.

Context. Star formation at earlier cosmological times takes place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, C i and C ii, which originate in the surface layers of molecular clouds illuminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12 CO J = 4 → 3, J = 7 → 6, and 13 CO J = 4 → 3 rotational and [C i] 3 P 1 − 3 P 0 and 3 P 2 − 3 P 1 fine-structure transitions. The 13 CO J = 4 → 3 and [C i] 3 P 2 − 3 P 1 transitions are detected for the first time in the LMC. We derive the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in the N159W region has temperatures of about 80 K and densities of about 10 4 cm −3 . The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [C ii] line intensity reported in the literature, in the context of a clumpy cloud PDR model constrains the UV intensity to about χ ≈ 220 and an average density of the clump ensemble of about 10 5 cm −3 , thus confirming the presence of high density material in the LMC N159W region.

Aims. In this paper we discuss the physical conditions of clumpy nature in the IC 348 molecular cloud. We show that the millimeter and sub-millimeter line emission from the IC 348 molecular cloud can be modelled as originating from a photon dominated region (PDR).Methods. We combine new observations of fully sampled maps in [C i] at 492 GHz and 12 CO 4-3, taken with the KOSMA 3 m telescope at about 1 resolution, with FCRAO data of 12 CO 1-0, 13 CO 1-0 and far-infrared continuum data observed by HIRES/IRAS.To derive the physical parameters of the region we analyze the line ratios of [C i]3 P 1 -3 P 0 / 13 CO 1-0, and 12 CO 4-3/ 12 CO 1-0. A first rough estimate of abundance is obtained from an LTE analysis. To understand the [C i] and CO emission from the PDRs in IC 348, we use a clumpy PDR model. With an ensemble of identical clumps, we constrain the total mass from the observed absolute intensities. Then we apply a more realistic clump distribution model with a power law index of 1.8 for clump-mass spectrum and a power law index of 2.3 for mass-size relation. Results. We provide detailed fits to observations at seven representative positions in the cloud, revealing clump densities between 4 × 10 4 cm −3 and 4 × 10 5 cm −3 and C/CO column density ratios between 0.02 and 0.26. The derived FUV flux from the model fit is consistent with the field calculated from FIR continuum data, varying between 2 and 100 Draine units across the cloud. We find that both an ensemble of identical clumps and an ensemble with a power law clump mass distribution produce line intensities which are in good agreement (within a factor ∼2) with the observed intensities. The models confirm the anti-correlation between the C/CO abundance ratio and the hydrogen column density found in many regions.