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Molecular emission-line observations of isolated Galactic star-forming regions are used to model the physical properties of the molecular interstellar medium in these systems. Observed line ratios are compared with the results predicted by models that incorporate gas-phase chemistry and the heating by stellar radiation and non-radiative feedback processes. The line ratios of characteristic tracer molecules may be interpreted using the contributions of two distinct components: a cold (40–50 K) and high-density (105–105.5 cm−3) photon-dominated region (PDR) with a nominal UV flux density and a warm (∼300 K) mechanical heating-dominated region (MHDR) with a slightly lower density (104.5–105 cm−3). The relative contributions of these structural components are used to model the observed line ratios. Ionized species may be better modelled by adopting an increase of the cosmic ray flux towards the Galactic Centre and the sulphur abundance should depleted by a factor of 200–400 relative to solar values. The line ratios of the Galactic sample are found to be very similar to those of the integrated signature of prominent (ultra)luminous IR Galaxies. The PDRs and MHDRs in the isolated Galactic regions may be modelled with slightly higher mean densities than in extragalactic systems and a higher MHDR temperature resulting from non-radiative mechanical heating. Multimolecular studies are effective in determining the physical and chemical properties of star formation regions by using characteristic line ratios to diagnose their environment. The addition of more molecular species will reduce the existing modelling redundancy. [ABSTRACT FROM AUTHOR]

This paper presents emission line intensities of CO and C° from the compact circumnuclear disk in the center of NGC 5128 (Centaurus A) obtained with the Herschel Space Observatory in the 400-1000 GHz range as well as previously unpublished measurements obtained with the ground-based observatories SEST, JCMT and APEX in the 90-800 GHz range. The results show that the Cen A center has an emission ladder of CO transitions quite different from those of either star-burst galaxies or (Seyfert) AGNs. In addition, the neutral carbon ([CI]) emission lines from the Cen A center are much stronger relative to the adjacent CO lines than in any other galaxy. The CO surface brightness of the compact circumnuclear disk (CND) is significantly higher than that of the much more extended thin disk (ETD) in the same line of sight. LVG analysis of the CO line profiles decomposed into the constituent contributions show that the ETD is relatively cool and of low excitation, wheres the brighter CND is hotter and more highly excited. Our PDR/XDR models suggest that most of the CND gas is relatively cool (temperatures 25 K-80 K) and not very dense (≈300 cm-3) if it is primarily heated by UV photons. A small fraction of the gas in both the CND and the ETD has a much higher density (typically 30 000 cm-3). A more highly excited, high-density phase is present in the CND, either in the form of an extreme PDR or more likely in the form of an XDR. Such a phase does not occur in the part of the ETD sampled. We have determined, for the first time, the molecular mass parameters of the CND. The total gas mass of the CND is MCND = 8.4 × 107 M☉, uncertain by a factor of two. The CO-H2 conversion factor (XCND) is 4 × 1020 (K kms-1)-1 also within a factor of two. [ABSTRACT FROM AUTHOR]

The observed molecular properties of a sample of FIR-luminous and OH megamaser (OH-MM) galaxies have been investigated. The ratio of high and low-density tracer lines is found to be determined by the progression of the star formation in the system. The HCO+/HCN and HCO+/HNC line ratios are good proxies for the density of the gas, and PDR and XDR sources can be distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources in the sample can be explained by PDR chemistry in gas with densities higher than 105.5cm−3, confirming the classical OH-MM model of IR pumped amplification with (variable) low gains. [ABSTRACT FROM PUBLISHER]

The nuclei of ULIRGs harbor massive young stars, an accreting central black hole, or both. Results are presented for molecular gas that is exposed to X-rays (1–100 keV, XDRs) and far-ultraviolet radiation (6–13.6 eV, PDRs). Attention is paid to species like HCO+, HCN, HNC, OH, H2O and CO. Line ratios of HCN/HCO+ and HNC/HCN discriminate between PDRs and XDRs. Very high J (>10) CO lines, observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. In XDRs, it is easy to produce large abundances of warm (T > 100 k) H2O and OH. In PDRs, only OH is produced similarly well. [ABSTRACT FROM PUBLISHER]