Compiling data from the literature, we found that the gas thermal pressure increases with the intensity of the UV radiation field given by , following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help rationalising the analysis of high-J CO emission in massive star formation and provides an observational constraint for models that study stellar feedback on molecular clouds.

We present the first results from a new, high resolution, 12 CO(1-0), 13 CO(1-0), and C 18 O(1-0) molecular line survey of the Orion A cloud, hereafter referred to as the CARMA-NRO Orion Survey. CARMA observations have been combined with single-dish data from the Nobeyama 45m telescope to provide extended images at about 0.01 pc resolution, with a dynamic range of approximately 1200 in spatial scale. Here we describe the practical details of the data combination in uv space, including flux scale matching, the conversion of single dish data to visibilities, and joint deconvolution of single dish and interferometric data. A ∆-variance analysis indicates that no artifacts are caused by combining data from the two instruments. Initial analysis of the data cubes, including moment maps, average spectra, channel maps, position-velocity diagrams, excitation temperature, column density, and line ratio maps provides evidence of complex and interesting structures such as filaments, bipolar outflows, shells, bubbles, and photo-eroded pillars. The implications for star formation processes are profound and follow-up scientific studies by the CARMA-NRO Orion team are now underway. We plan to make all the data products described here generally accessible; some are already available at [https://dataverse.harvard.edu/dataverse/CARMA-NRO-Orion].

Context. We present an initial overview of the filamentary structure in the Orion A molecular cloud utilizing a high angular and velocity resolution C18O(1–0) emission map that was recently produced as part of the CARMA-NRO Orion Survey. Aims. The main goal of this study is to build a credible method to study varying widths of filaments which has previously been linked to star formation in molecular clouds. Due to the diverse star forming activities taking place throughout its ~20 pc length, together with its proximity of 388 pc, the Orion A molecular cloud provides an excellent laboratory for such an experiment to be carried out with high resolution and high sensitivity. Methods. Using the widely-known structure identification algorithm, DisPerSE, on a three-dimensional (PPV) C18O cube, we identify 625 relatively short (the longest being 1.74 pc) filaments over the entire cloud. We studied the distribution of filament widths using FilChaP, a python package that we have developed and made publicly available. Results. We find that the filaments identified in a two square-degree PPV cube do not overlap spatially, except for the complex OMC-4 region that shows distinct velocity components along the line of sight. The filament widths vary between 0.02 and 0.3 pc depending on the amount of substructure that a filament possesses. The more substructure a filament has, the larger is its width. We also find that despite this variation, the filament width shows no anticorrelation with the central column density which is in agreement with previous Herschel observations.

Context. Models of photon-dominated regions (PDRs) still fail to fully reproduce some of the observed properties. In particular they do not reproduce the combination of the intensities of different PDR cooling lines together with the chemical stratification, as observed for example for the Orion Bar PDR. Aims. We aim to construct a numerical PDR model, KOSMA-τ 3D, to simulate full spectral cubes of line emission from arbitrary PDRs in three dimensions (3D). The model will reproduce the intensity of the main cooling lines from the Orion Bar PDR and the observed layered structure of the different transitions. Methods. We built up a 3D compound, made of voxels (3D pixels) that contain a discrete mass distribution of spherical "clumpy" structures, approximating the fractal ISM. To analyse each individual clump the new code was combined with the KOSMA-τ PDR model. Probabilistic algorithms were used to calculate the local FUV flux for each voxel as well as the voxel-averaged line emissivities and optical depths, based on the properties of the individual clumps. Finally, the computation of the radiative transfer through the compound provided full spectral cubes. To test the new model we tried to simulate the structure of the Orion Bar PDR and compared the results to observations from HIFI/Herschel and from the Caltech Submillimetre Observatory (CSO). In this context new Herschel data from the HEXOS guaranteed-time key program is presented. Results. Our model is able to reproduce the line-integrated intensities within a factor of 2.5 and the observed stratification pattern within 0.016 pc for the [Cii] 158 µm and different 12/13 CO and HCO + transitions, based on the representation of the Orion Bar PDR by a clumpy edge-on cavity wall. In the cavity wall, a large fraction of the total mass needs to be contained in clumps. The mass of the interclump medium is constrained by the FUV penetration. Furthermore, the stratification profile cannot be reproduced by a model that has the same amount of clump and interclump mass in each voxel; dense clumps need to be removed from the PDR surface.Article published by EDP Sciences A2, page 1 of 32 A&A 598, A2 (2017)

Context. Probability distribution functions (PDFs) of column densities are an established tool to characterize the evolutionary state of interstellar clouds. Aims. Using simulations, we show to what degree their determination is affected by noise, line-of-sight contamination, field selection, and the incomplete sampling in interferometric measurements. Methods. We solve the integrals that describe the convolution of a cloud PDF with contaminating sources such as noise and line-ofsight emission, and study the impact of missing information on the measured column density PDF. In this way we can quantify the effect of the different processes and propose ways to correct for their impact to recover the intrinsic PDF of the observed cloud. Results. The effect of observational noise can be easily estimated and corrected for if the root mean square (rms) of the noise is known. For σ noise values below 40% of the typical cloud column density, N peak , this involves almost no degradation in the accuracy of the PDF parameters. For higher noise levels and narrow cloud PDFs the width of the PDF becomes increasingly uncertain. A contamination by turbulent foreground or background clouds can be removed as a constant shield if the peak of the contamination PDF falls at a lower column or is narrower than that of the observed cloud. Uncertainties in cloud boundary definition mainly affect the low-column density part of the PDF and the mean density. As long as more than 50% of a cloud is covered, the impact on the PDF parameters is negligible. In contrast, the incomplete sampling of the uv-plane in interferometric observations leads to uncorrectable PDF distortions in the maps produced. An extension of the capabilities of the Atacama Large Millimeter Array (ALMA) would allow us to recover the high-column density tail of the PDF, but we found no way to measure the intermediate-and low-column density part of the underlying cloud PDF in interferometric observations.

We identify 45 protostellar outflows in CO maps of the Orion A giant molecular cloud from the Combined Array for Research in Millimeter-wave Astronomy–Nobeyama Radio Observatory Orion survey. Our sample includes 11 newly detected outflows. We measure the mass and energetics of the outflows, including material at low velocities, by correcting for cloud contributions. The total momentum and kinetic energy injection rates of outflows are comparable to the turbulent dissipation rate of the cloud. We also compare the outflow position angles to the orientation of C18O filaments. We find that the full sample of outflows is consistent with being randomly oriented with respect to the filaments. A subsample of the most reliable measurements shows a moderately perpendicular outflow-filament alignment that may reflect accretion of mass across filaments and onto the protostellar cores.

Context. The [C ii] 158 µm far-infrared (FIR) fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [C ii] emission. Aims. Our aim is to determine whether the complex profiles observed in [ 12 C ii] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [ 12 C ii] and isotopic [ 13 C ii] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [ 13 C ii] emission lines, along with the [ 12 C ii] emission lines, with a high signalto-noise ratio (S/N). We first derived the [ 12 C ii] optical depth and the [C ii] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multicomponent velocity fit allows us to derive the physical conditions of the [C ii] gas: column density and excitation temperature. Results. We find moderate to high [ 12 C ii] optical depths in all four sources and self-absorption of [ 12 C ii] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent A v of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [C ii] gas, with an equivalent A v ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region (PDR) surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [C ii] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins. Key words. ISM:clouds -ISM:individual objects: M43 -ISM:individual objects: M17 -photon-dominated region (PDR) -ISM:individual objects: Horsehead -ISM:individual objects: MonR2 1 At that time, the spectroscopic data were less accurate and the wavelength of the transition was assumed to fall at 157 µm instead of 158 µm.Article number, page 1 of 40 A&A proofs: manuscript no. 34380corr_2 mentum change F=2→1, F=1→0, and F=1→1. The frequencies of the fine structure transitions of both isotopes were determined by Cooksy et al. (1986). The astronomical observations are fully consistent with these frequencies, as was discussed by Ossenkopf et al. (2013), who also noted that the relative strengths of the [ 13 C ii] hyperfine satellites (s F→F , see Table 1) given by Cooksy et al. (1986) are incorrect. We summarize all the relevant [ 12 C ii] and [ 13 C ii] spectroscopic parameters in Table 1, including the velocity offsets of the [ 13 C ii] hyperfine components relative to [ 12 C ii]. The frequency separation of the hyperfi...

Context. Photon dominated regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment. Aims. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage which includes lines of many important gas coolants such as [Cii], [Ci], and CO and other key molecules such as H 2 CO, H 2 O, HCN, HCO + , and SO. Methods. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument on board the Herschel Space Observatory. We obtained physical parameters for the observed molecules. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In the case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H 2 volume densities, and column densities. Results. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of 13 CO, 6 transitions of C 18 O, 10 transitions of H 2 CO, and 6 transitions of H 2 O. The rotational temperatures are in the range between ∼22 and ∼146 K and the column densities are in the range between 1.8 × 10 12 cm −2 and 4.5 × 10 17 cm −2. For species with at least three detected transitions and available collisional excitation rates we derived a best fit kinetic temperature and H 2 volume density. Most species trace kinetic temperatures in the range between 100 and 150 K and H 2 volume densities in the range between 10 5 and 10 6 cm −3. The species with temperatures and/or densities outside this range include the H 2 CO transitions tracing a very high temperature (315 K) and density (1.4 × 10 6 cm −3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. Conclusions. The observed lines/species reveal a range of physical conditions (gas density/temperature) involving structures at high density/high pressure, making the traditional clump/interclump picture of the Orion Bar obsolete.