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We present a model of the photodissociation regions of NGC 4038, which is part of the Antennae galaxies. We have considered one-dimensional slabs of uniform density, all having a maximum AV = 10 mag, interacting with plane-parallel radiation. The density range in our simulations spans four orders of magnitude (100 ≤ n ≤ 106 cm−3) and the UV field strength spans more than three orders of magnitude (10 ≤ χ ≤ 104.5 multiples of the Draine field), from which we generated a grid of about 1400 simulations. We compare our results with Herschel SPIRE-FTS, CSO and ISO-LWS observations of eight CO transition lines (J = 1-0 to 8-7) and the [C i] 609 μm and [O i] 146 μm fine-structure lines. We find that the molecular and atomic emission lines trace different gas components of NGC 4038; thus, single emission models are insufficient to reproduce the observed values. In general, low-J CO transition lines correspond to either low-density regions interacting with low UV field strengths, or high-density regions interacting with high UV field strengths. Higher J CO transition lines are less dependent on the UV field strength and are fitted by gas with density n ∼ 104.5-105.2 cm− 3. We find that the observed fine-structure line ratio of [C i] 609 μm/[O i] 146 μm is reproduced by clouds subject to weaker UV fields compared to the CO lines. We make estimates of the XCO factor which relates the CO emission with the column density of molecular hydrogen, and find that it is less than the canonical Milky Way value. [ABSTRACT FROM PUBLISHER]

In order to study the temperature distribution of the extended gas within the Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide (CH3CN) in its J = 6K-5K, J = 12K-11K, J = 13K-12K, and J = 14K-13K transitions at an average angular resolution of ∼10 arcsec (22 arcsec for the 6K-5K lines), as part of a new 2D line survey of this region using the IRAM 30m telescope. These fully sampled maps show extended emission from warm gas to the northeast of IRc2 and the distinct kinematic signatures of the hot core and compact ridge source components. We have constructed population diagrams for the four sets of K-ladder emission lines at each position in the maps and have derived rotational excitation temperatures and total beam-averaged column densities from the fitted slopes. In addition, we have fitted LVG model spectra to the observations to determine best-fit physical parameters at each map position, yielding the distribution of kinetic temperatures across the region. The resulting temperature maps reveal a region of hot (T > 350 K) material surrounding the north-eastern edge of the hot core, whereas the column density distribution is more uniform and peaks near the position of IRc2. We attribute this region of hot gas to shock heating caused by the impact of outflowing material from active star formation in the region, as indicated by the presence of broad CH3CN lines. This scenario is consistent with predictions from C-shock chemical models that suggest that gas-phase methyl cyanide survives in the post-shock gas and can be somewhat enhanced due to sputtering of grain mantles in the passing shock front. [ABSTRACT FROM AUTHOR]

Context. We present a study of the sulfur-bearing species detected in a line confusion-limited survey towards Orion KL performed with the IRAM 30-m telescope in the frequency range 80-281 GHz. Aims. This study is part of an analysis of the line survey divided into families of molecules. Our aim is to derive accurate physical conditions, as well as molecular abundances, in the different components of Orion KL from observed SO and SO2 lines. Methods. As a starting point, we assumed local thermodynamic equilibrium (LTE) conditions obtain rotational temperatures. We then used a radiative transfer model, assuming either LVG or LTE excitation to derive column densities of these molecules in the different components of Orion KL. Results. We have detected 68 lines of SO, 34SO, 33SO, and S18O and 653 lines of SO2, 34SO2, 33SO2, SO18O, and SO2 ?2 = 1. We provide column densities for all of them and also upper limits for the column densities of S17O, 36SO, 34S18O, SO17O, and 34SO2 ?2 = 1 and for several undetected sulfur-bearing species. In addition, we present 2′ × 2′ maps around Orion IRc2 of SO2 transitions with energies from 19 to 131 K and also maps with four transitions of SO, 34SO, and 34SO2. We observe an elongation of the gas along the NE-SWdirection. An unexpected emission peak appears at 20.5 kms-1 in most lines of SO and SO2. A study of the spatial distribution of this emission feature shows that it is a new component of a few arcseconds (∼5″) in diameter, which lies ∼4″ west of IRc2. We suggest the emission from this feature is related to shocks associated to the BN object. Conclusions. The highest column densities for SO and SO2 are found in the high-velocity plateau (a region dominated by shocks) and in the hot core. These values are up to three orders of magnitude higher than the results for the ridge components. We also find high column densities for their isotopologues in both components. Therefore, we conclude that SO and SO2 are good tracers, not only of regions affected by shocks, but also of regions with warm dense gas (hot cores). [ABSTRACT FROM AUTHOR]

ABSTRACT Photodissociation regions (PDRs) define the transition zone between an ionized and a dark molecular region. They consist of neutral gas which interacts with far-ultraviolet radiation and are characterized by strong infrared line emission. Various numerical codes treating one-dimensional PDRs have been developed in the past, simulating the complexity of chemical reactions occurring and providing a better understanding of the structure of a PDR. In this paper we present the three-dimensional code, 3d-pdr, which can treat PDRs of arbitrary density distribution. The code solves the chemistry and the thermal balance self-consistently within a given three-dimensional cloud. It calculates the total heating and cooling functions at any point in a given PDR by adopting an escape probability method. It uses a healpix-based ray tracing scheme to evaluate the attenuation of the far-ultraviolet radiation in the PDR and the propagation of the far-infrared/submm line emission out of the PDR. We present benchmarking results and apply 3d-pdr to (i) a uniform-density spherical cloud interacting with a plane-parallel external radiation field, (ii) a uniform-density spherical cloud interacting with a two-component external radiation field and (iii) a cometary globule interacting with a plane-parallel external radiation field. We find that the code is able to reproduce the benchmarking results of various other one-dimensional numerical codes treating PDRs. We also find that the accurate treatment of the radiation field in the fully three-dimensional treatment of PDRs can in some cases leads to different results when compared to a standard one-dimensional treatment. [ABSTRACT FROM AUTHOR]

ABSTRACT We present for the first time a detailed study of the properties of molecular gas in metal-rich environments such as early-type galaxies (ETGs). We have explored photon-dominated region chemistry for a wide range of physical conditions likely to be appropriate for these sources. We derive fractional abundances of the 20 most chemically reactive species as a function of the metallicity, as a function of the optical depth and for various volume number gas densities, far-ultraviolet (FUV) radiation fields and cosmic ray ionization rates. We also investigate the response of the chemistry to the changes in α-element enhancement as seen in ETGs. We find that the fractional abundances of CS, H2S, H2CS, H2O, H3O+, HCO+ and H2CN seem invariant to an increase of metallicity whereas C+, CO, C2H, CN, HCN, HNC and OCS appear to be the species most sensitive to this change. The most sensitive species to the change in the fractional abundance of α-elements are C+, C, CN, HCN, HNC, SO, SO2, H2O and CS. Finally, we provide line brightness ratios for the most abundant species, especially in the range observable with Atacama Large Millimeter Array (ALMA). Discussion of favourable line ratios to be used for the estimation of supersolar metallicities and α-elements are also provided. [ABSTRACT FROM AUTHOR]

As a part of the Herschel key programme PRISMAS, we have used the Herschel/HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards eight high-mass star-forming regions in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. Here, we report observations towards W49N of the NH N = 1-0, J = 2-1, and J = 1-0, ortho-NH2 NKa,KcJ = 11,13/2-00,01/2, ortho-NH3 JK = 10-00 and 20-10, para-NH3 JK = 21-11 transitions, and unsuccessful searches for NH+. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH, which is found to correlate with H2 in the solar neighbourhood diffuse gas. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2 × 10-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities ≤2-14% of N(NH), which is in contrast to the behaviour of the abundances of CH+ and OH+ relative to the values determined for the corresponding neutrals CH and OH. Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ≈ 0.5-0.7 ± 0.1, in the strongest absorption components. This result cannot be explained by current models as we had expected to find a value of unity or higher. [ABSTRACT FROM AUTHOR]

Far-infrared [C ii] 158-μm, [O i] 145-μm and [O i] 63-μm fine structure emission-line fluxes were measured from archival Infrared Space Observatory Long Wavelength Spectrometer spectra of 46 extragalactic sources, with 28 sources providing detections in all three lines. For 12 of the sources, the contribution to the [C ii] 158-μm line flux from H ii regions could be estimated from their detected [N ii] 122-μm line fluxes. The measured [C ii]/[O i] and [O i] 63/145-μm line flux ratios were compared with those from a grid of photodissociation region (PDR) models previously computed using theucl_pdr code. Persistent offsets between the observed and modelled line ratios could be partly attributed to the effects of [O i] 63-μm self-absorption. Using the Spherical Multi-Mol (smmol) code, we calculated model [O i] line profiles and found that the strength of the [O i] 63-μm line was reduced by 20–80 per cent, depending on the PDR parameters. We conclude that high PDR densities and radiation field strengths, coupled with the effects of [O i] 63-μm self-absorption, are likely to provide the best match to the observed line flux ratios. [ABSTRACT FROM AUTHOR]

The synthesis of organic molecular anions in TMC-1 and IRC+10216 is investigated. Modelled C2H−, CN−, C3N−, C5N− and C7N− column densities are sufficiently great that these species might be observable in IRC+10216. Density-enhanced shells in the outer envelope of IRC+10216 are found to enhance the C2H− and CN− column densities by shielding these anions from destruction by UV radiation. From a newly-derived upper column density limit of 6.6 × 1010 cm−2 for C2H− in IRC+10216 we deduce the primary production mechanism for this anion to be C2H2 + H− ⇒ C2H− + H2. In TMC-1, due to the low radiative electron attachment rates calculated for C2H−, CN− and CH2CN−, these species have modelled column densities below the detection threshold. They could, however, be produced in reactions we have not yet considered. [ABSTRACT FROM PUBLISHER]