Aims. This paper presents the richness of submillimeter spectral features in the high-mass star forming region AFGL 2591. Methods. As part of the Chemical Herschel Survey of Star Forming Regions (CHESS) key programme, AFGL 2591 was observed by the Herschel (HIFI) instrument. The spectral survey covered a frequency range from 480 to 1240 GHz as well as single lines from 1267 to 1901 GHz (i.e. CO, HCl, NH 3 , OH, and [CII]). Rotational and population diagram methods were used to calculate column densities, excitation temperatures, and the emission extents of the observed molecules associated with AFGL 2591. The analysis was supplemented with several lines from ground-based JCMT spectra. Results. From the HIFI spectral survey analysis a total of 32 species were identified (including isotopologues). Although the lines are mostly quite weak ( T mb dV∼ few K km s −1 ), 268 emission and 16 absorption lines were found (excluding blends). Molecular column densities range from 6 × 10 11 to 1 × 10 19 cm −2 and excitation temperatures from 19 to 175 K. Cold (e.g. HCN, H 2 S, and NH 3 with temperatures below 70 K) and warm species (e.g. CH 3 OH, SO 2 ) in the protostellar envelope can be distinguished.

Aims. The aim of this work is to understand the richness of chemical species observed in the isolated high-mass envelope of AFGL 2591, a prototypical object for studying massive star formation. Methods. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with a Monte Carlo radiative transfer code (Ratran), assuming a mixture of constant and 1D stepwise radial profiles for abundance distributions. The reconstructed 1D abundances were compared with the results of the time-dependent gas-grain chemical modeling, using the best-fit 1D power-law density structure. The chemical simulations were performed considering ages of 1−5 × 10 4 years, cosmic ray ionization rates of 5−500 × 10 −17 s −1 , uniformly-sized 0.1−1 μm dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O < 1 and >1. The most important model parameters varied one by one in the simulations are age, cosmic ray ionization rate, external UV intensity, and grain size. Results. Constant abundance models give good fits to the data for CO, CN, CS, HCO + , H 2 CO, N 2 H + , CCH, NO, OCS, OH, H 2 CS, O, C, C + , and CH. Models with an abundance jump at 100 K give good fits to the data for NH 3 , SO, SO 2 , H 2 S, H 2 O, HCl, and CH 3 OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO + , NH 3 , and H 2 O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of ∼10−50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of ∼5 × 10 −17 s −1 . The grain properties and the intensity of the external UV field do not strongly affect the chemical structure of the AFGL 2591 envelope, whereas its chemical age, the cosmic-ray ionization rate, and the initial abundances play an important role. Conclusions. We demonstrate that simple constant or jump-like abundance profiles constrained with 1D Ratran line radiative transfer simulations are in agreement with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C + , and CH) or with a poorly established chemical network (HCl, H 2 S) or whose chemistry is strongly affected by surface processes (CH 3 OH).

Context. Massive star-formation leads to enrichment with heavy elements of the interstellar medium. On the other hand, the abundance of heavy elements is a key parameter to study the star-formation history of galaxies. Furthermore, the total molecular hydrogen mass, usually determined by converting CO or [C ii] 158 µm luminosities, depends on the metallicity as well. The excitation of metallicitysensitive emission lines, however, depends on the gas density of H ii regions, where they arise. Aims. We used spectroscopic observations from SOFIA, Herschel, and Spitzer of the nuclear region of the starburst galaxy NGC 253, as well as photometric observations from GALEX, 2MASS, Spitzer, and Herschel in order to derive physical properties such as the optical depth to correct for extinction, as well as the gas density and metallicity of the central region. Methods. Ratios of the integrated line fluxes of several species were utilised to derive the gas density and metallicity. The [O iii] along with the [S iii] and [N ii] line flux ratios for example, are sensitive to the gas density but nearly independent of the local temperature. As these line ratios trace different gas densities and ionisation states, we examined if these lines may originate from different regions within the observing beam. The ([Ne ii] 13 µm + [Ne iii] 16 µm)/Hu α line flux ratio on the other hand, is independent of the depletion onto dust grains but sensitive to the Ne/H abundance ratio and will be used as a tracer for metallicity of the gas. Results. We derived values for gas phase abundances of the most important species, as well as estimates for the optical depth and the gas density of the ionised gas in the nuclear region of NGC 253. We obtained densities of at least two different ionised components (< 84 cm −3 and ∼ 170 − 212 cm −3 ) and a metallicity of solar value.

Context. Far-infrared (FIR) line emission provides key information about the gas cooling and heating due to shocks and UV radiation associated with the early stages of star formation. Gas cooling via FIR lines might, however, depend on metallicity. Aims. We aim to quantify the FIR line emission and determine the spatial distribution of the CO rotational temperature, ultraviolet (UV) radiation field, and H2 number density toward the embedded cluster Gy 3–7 in the CMa–l224 star-forming region, whose metallicity is expected to be intermediate between that of the Large Magellanic Cloud and the Solar neighborhood. By comparing the total luminosities of CO and [OI] toward Gy 3–7 with values found for low- and high-mass protostars extending over a broad range of metallicities, we also aim to identify the possible effects of metallicity on the FIR line cooling within our Galaxy. Methods. We studied SOFIA/FIFI-LS spectra of Gy 3–7, covering several CO transitions from J = 14–13 to 31-30, the OH doublet at 79 μm, the [OI] 63.2 and 145.5 μm, and the [CII] 158 μm lines. The field of view covers a 2′ × 1′ region with a resolution of ~7″–18″. Results. The spatial extent of CO high-J (Jup ≥14) emission resembles that of the elongated 160 μm continuum emission detected with Herschel, but its peaks are offset from the positions of the dense cores. The [OI] lines at 63.2 μm and 145.5 μm follow a similar pattern, but their peaks are found closer to the positions of the cores. The CO transitions from J = 14–13 to J = 16–15 are detected throughout the cluster and show a median rotational temperature of 170 ± 30 K on Boltzmann diagrams. Comparisons to other protostars observed with Berschel show a good agreement with intermediate-mass sources in the inner Galaxy. Assuming an origin of the [OI] and high-J CO emission in UV-irradiated C–shocks, we obtained pre-shock H2 number densities of 104–105 cm−3 and UV radiation field strengths of 0.1–10 Habing fields (G0). Conclusions. Far-IR line observations reveal ongoing star formation in Gy 3–7, dominated by intermediate-mass Class 0/I young stellar objects. The ratio of molecular-to-atomic far-IR line emission shows a decreasing trend with bolometric luminosities of the protostars. However, it does not indicate that the low-metallicity has an impact on the line cooling in Gy 3–7.