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A solar-type system starts from an initial molecular core that acquires organic complexity as it evolves. The so-called prestellar cores that can be studied are rare, which has hampered our understanding of how organic chemistry sets in and grows. Aims. We selected the best prestellar core targets from the cold core catalogue that represent a diversity in terms of their environment to explore their chemical complexity: 1390 (in the compressed shell of Lambda Ori), 869 (in the MBM12 cloud), and 4149 (in the California nebula). We obtained a spectral survey with the IRAM 30 m telescope in order to explore the molecular complexity of the cores. We carried out a radiative transfer analysis of the detected transitions in order to place some constraints on the physical conditions of the cores and on the molecular column densities. We also used the molecular ions in the survey to estimate the cosmic-ray ionisation rate and the S/H initial elemental abundance using a gas-phase chemical model to reproduce their abundances. We found large differences in the molecular complexity (deuteration, complex organic molecules, sulphur, carbon chains, and ions) and compared their chemical properties with a cold core and two prestellar cores. The chemical diversity we found in the three cores seems to be correlated with their chemical evolution: two of them are prestellar (1390 and 4149), and one is in an earlier stage (869). The influence of the environment is likely limited because cold cores are strongly shielded from their surroundings. The high extinction prevents interstellar UV radiation from penetrating deeply into the cores. Higher spatial resolution observations of the cores are therefore needed to constrain the physical structure of the cores, as well as a larger-scale distribution of molecular ions to understand the influence of the environment on their molecular complexity., 26 pages, 13 figures, accepted in A&A in January 2022

AGB stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ISM). Dust grains formed by AGB stars are thought to be large. However, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (GSD) is unknown. This is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as AGB dust forms $\sim$ 70% of the starting point of its evolution. We expand on our chemical kinetics model, which uniquely includes a comprehensive dust-gas chemistry. The GSD is now allowed to deviate from the commonly assumed canonical Mathis et al. (1977) distribution. We find that the specific GSD can significantly influence the dust-gas chemistry within the outflow. Our results show that the level of depletion of gas-phase species depends on the average grain surface area of the GSD. Gas-phase abundance profiles and their possible depletions can be retrieved from observations of molecular emission lines when using a range of transitions. Due to degeneracies within the prescription of GSD, specific parameters cannot be retrieved, only (a lower limit to) the average grain surface area. Nonetheless, this can discriminate between dust composed of predominantly large or small grains. We show that when combined with other observables such as the spectral energy distribution and polarised light, depletion levels from molecular gas-phase abundance profiles can constrain the elusive GSD of the dust delivered to the ISM by AGB outflows., Comment: 17 pages, 10 figures, 6 tables. Accepted for publication in MNRAS

Phosphorus nitride (PN) is the P-bearing species with the highest number of detections in star-forming regions. Multi-line studies of the molecule have shown that the excitation temperature of PN is usually lower than the gas kinetic temperature, suggesting that PN is likely in conditions of sub-thermal excitation. We present an analysis of PN which takes the possible sub-thermal excitation conditions into account in a sample of 24 massive star-forming regions. We observed PN (2-1), (3-2), (4-3), and (6-5) with the IRAM-30m and APEX telescopes and detected PN lines in 15 of them. Together with 9 similar sources detected in PN in previous works, we have analysed the largest sample of star-forming regions to date, made of 33 sources with 24 detections in total (among which 13 are new detections). Hence, we have increased the number of star-forming regions detected in PN by more than a factor 2. Our analysis indicates that the PN lines are indeed sub-thermally excited, but well described by a single excitation temperature. We have compared line profiles and fractional abundances of PN and SiO, a typical shock tracer, and found that almost all objects detected in PN have high-velocity SiO wings. Moreover, the SiO and PN abundances with respect to H2 are correlated over several orders of magnitude, and uncorrelated with gas temperature. This clearly shows that the production of PN is strongly linked to the presence of shocked gas, and rules out alternative scenarios based on thermal evaporation from iced grain mantles., Comment: 14 pages, 10 figures, accepted for publication in MNRAS

Molecules with hyperfine splitting of their rotational line spectra are useful probes of optical depth, via the relative line strengths of their hyperfine components.The hyperfine splitting is particularly advantageous in interpreting the physical conditions of the emitting gas because with a second rotational transition, both gas density and temperature can be derived. For HCN however, the relative strengths of the hyperfine lines are anomalous. They appear in ratios which can vary significantly from source to source, and are inconsistent with local thermodynamic equilibrium. This is the HCN hyperfine anomaly, and it prevents the use of simple LTE models of HCN emission to derive reliable optical depths. In this paper we demonstrate how to model HCN hyperfine line emission, and derive accurate line ratios, spectral line shapes and optical depths. We show that by carrying out radiative transfer calculations over each hyperfine level individually, as opposed to summing them over each rotational level, the anomalous hyperfine emission emerges naturally. To do this requires not only accurate radiative rates between hyperfine states, but also accurate collisional rates. We investigate the effects of different sets of hyperfine collisional rates, derived via the 'proportional method' and through direct recoupling calculations. Through an extensive parameter sweep over typical low mass star forming conditions, we show the HCN line ratios to be highly variable to optical depth. We also reproduce an observed effect whereby the red-blue asymmetry of the hyperfine lines (an infall signature) switches sense within a single rotational transition., Comment: 12 pages, 7 figures. Accepted for publication in MNRAS

The hydrogen fluoride (HF) molecule is seen in absorption in the interstellar medium (ISM) along many lines of sight. Surprisingly, it is observed in emission toward the Orion Bar, which is an interface between the ionized region around the Orion Trapezium stars and the Orion molecular cloud. We aim to understand the origin of HF emission in the Orion Bar by comparing its spatial distribution with other tracers. We examine three mechanisms to explain the HF emission: thermal excitation, radiative dust pumping, and chemical pumping. We used a Herschel/HIFI strip map of the HF J = 1-0 line, covering 0.5' by 1.5' that is oriented perpendicular to the Orion Bar. We used the RADEX non-local thermodynamic equilibrium (non-LTE) code to construct the HF column density map. We use the Meudon PDR code to explain the morphology of HF. The bulk of the HF emission at 10 km s^-1 emerges from the CO-dark molecular gas that separates the ionization front from the molecular gas that is deeper in the Orion Bar. The excitation of HF is caused mainly by collisions with H2 at a density of 10^5 cm^-3 together with a small contribution of electrons in the interclump gas of the Orion Bar. Infrared pumping and chemical pumping are not important. We conclude that the HF J = 1-0 line traces CO-dark molecular gas. Similarly, bright photodissociation regions associated with massive star formation may be responsible for the HF emission observed toward active galactic nuclei., 12 pages, 12 figures, Accepted by A&A on September 23, 2019

It is not well known what drives the chemistry of a protostellar envelope, in particular the role of the stellar mass and the outflows on its chemical enrichment. We study the chemical structure of NGC 1333 IRAS 4A in order to (i) investigate the influence of the outflows on the chemistry, (ii) constrain the age of our object, (iii) compare it with a typical high-mass protostellar envelope. In our analysis we use JCMT line mapping and HIFI pointed spectra. To study the influence of the outflow on the degree of deuteration, we compare JCMT maps of HCO+ and DCO+ with non-LTE (RADEX) models in a region that spatially covers the outflow activity of IRAS 4A. To study the envelope chemistry, we derive empirical molecular abundance profiles for the observed species using the radiative transfer code (RATRAN) and adopting a 1D dust density/temperature profile from the literature. We compare our best-fit observed abundance profiles with the predictions from the time dependent gas grain chemical code (ALCHEMIC). The CO, HCN, HNC and CN abundance require an enhanced UV field which points towards an outflow cavity. The abundances (wrt H2) are 1 to 2 orders of magnitude lower than those observed in the high mass protostellar envelope (AFGL 2591), while they are found to be similar within factors of a few with respect to CO. Differences in UV radiation may be responsible for such chemical differentiation, but temperature differences seem a more plausible explanation. The CH3OH modeled abundance profile points towards an age of > 4x10^4 yrs for IRAS 4A. The spatial distribution of H2D+ differs from that of other deuterated species, indicating an origin from a foreground colder layer (, 26 pages, 23 Figures, 6 tables, accepted for publication in A&A

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. 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. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument onboard the Herschel Space Observatory. 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 case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H2 volume densities, and column densities. 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 13CO, 6 transitions of C18O, 10 transitions of H2CO, and 6 transitions of H2O. Most species trace kinetic temperatures in the range between 100 and 150 K and H2 volume densities in the range between 10^5 and 10^6 cm^-3. The species with temperatures and / or densities outside of this range include the H2CO transitions tracing a very high temperature (315 K) and density (1.4x10^6 cm^-3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. The observed lines/species reveal a range of physical conditions (gas density /temperature) involving structures at high density / high pressure, obsoleting the traditional 'clump / interclump' picture of the Orion Bar., Comment: Accepted for publication in A&A, abstract abridged

We aim to reveal the gas energetics in the circumstellar environment of the prototypical high-mass protostellar object AFGL2591 using space-based far-infrared observations of linear rotor molecules. Rotational spectral line signatures of CO, HCO+, CS, HCN and HNC from a 490-1240 GHz survey with Herschel/HIFI, complemented by ground-based JCMT and IRAM 30m spectra, cover transitions with E(up)/k between 5 and ~300 K (750K for 12C16O, using selected frequency settings up to 1850 GHz). The resolved spectral line profiles are used to separate and study various kinematic components. The line profiles show two emission components, the widest and bluest of which is attributed to an approaching outflow and the other to the envelope. We find evidence for progressively more redshifted and wider line profiles from the envelope gas with increasing energy level, qualitatively explained by residual outflow contribution picked up in the systematically decreasing beam size. Integrated line intensities for each species decrease as E(up)/k increases from 150 K for all species. This indicates that warm gas should be added to the model and that the model's geometry should provide low optical depth pathways for line emission from this warm gas to escape, for example in the form of UV heated outflow cavity walls viewed at a favorable inclination angle. Physical and chemical conditions derived for the outflow gas are similar to those in the protostellar envelope, possibly indicating that the modest velocity (, Accepted for publication in A&A; 16 pages, 8 figures, 4 tables

The abundances of interstellar CH+ and SH+ are not well understood as their most likely formation channels are highly endothermic. Using data from Herschel, we study the formation of CH+ and SH+ in a typical high UV-illumination photon-dominated region (PDR), the Orion Bar. Herschel/HIFI provides velocity-resolved data of CH+ 1-0 and 2-1 and three hyperfine transitions of SH+. Herschel/PACS provides information on the excitation and spatial distribution of CH+ (up to J=6-5). The widths of the CH+ 2-1 and 1-0 transitions are of ~5 km/s, significantly broader than the typical width of dense gas tracers in the Orion Bar (2-3 km/s) and are comparable to the width of tracers of the interclump medium such as C+ and HF. The detected SH+ transitions are narrower compared to CH+ and have line widths of 3 km/s, indicating that SH+ emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH+ and SH+, and that reactive collisions need to be taken into account to calculate the excitation of CH+. Comparison to PDR models shows that CH+ and SH+ are tracers of the warm surface region (AV, Accepted for publication in A&A, 12 pages, 11 figures, abstract shortened

International audience; We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5 × 1016 cm-2, and assuming that the source has an equal beam-filling factor for all transitions (beam widths 44, 28, and 19''), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is (0.3-7.3) × 10-6. The unusual velocity suggests an association with a ~5'' diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 M sun and the dust temperature is >=150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T >= 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature >=180 K. The inferred O2 column density sime5 × 1018 cm-2 can be produced in Peak A, having N(H2) ~= 4 × 1024 cm-2. An alternative mechanism is a low-velocity (10-15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Astronomy and Astrophysics 530 (2011): L2 Reproduced with permission from Astronomy & Astrophysics, Despite its importance in the thermal balance of the gas and in the determination of primeval planetary atmospheres, the chemistry in protoplanetary discs remains poorly constrained with only a handful of detected species. We observed the emission from the disc around the Herbig Be star HD 100546 with the PACS instrument in the spectroscopic mode on board the Herschel Space Telescope as part of the GAS in Protoplanetary Systems (GASPS) programme and used archival data from the DIGIT programme to search for the rotational emission of CH+. We detected in both datasets an emission line centred at 72.16 μm that most likely corresponds to the J = 5−4 rotational emission of CH+. The J = 3−2 and 6−5 transitions are also detected albeit with lower confidence. Other CH+ rotational lines in the PACS observations are blended with water lines. A rotational diagram analysis shows that the CH+ gas is warm at 323+2320 −151 K with a mass of ∼3×10−14−5×10−12 M_.We modelled the CH+ chemistry with the chemo-physical code ProDiMo using a disc density structure and grain parameters that match continuum observations and near- and mid-infrared interferometric data. The model suggests that CH+ is most abundant at the location of the disc rim at 10−13 AU from the star where the gas is warm, which is consistent with previous observations of hot CO gas emission, This work is partly supported by the Spanish grant AYA 2008-01727. Also is funding from the EC 7th Framework Programme (PIEF-GA-2008-220891, PERG06- GA-2009-256513)

The HCN, HCO+, and HNC molecules are commonly used as tracers of dense star-forming gas in external galaxies, but such observations are spatially unresolved. Reliably inferring the properties of galactic nuclei and disks requires detailed studies of sources whose structure is spatially resolved. We compare the spatial distributions and abundance ratios of HCN, HCO+, and HNC in W49A, the most massive and luminous star-forming region in the Galactic disk, based on maps of a 2' (6.6 pc) field at 14" (0.83 pc) resolution of the J=4-3 transitions of HCN, H13CN, HC15N, HCO+, H13CO+, HC18O+ and HNC. The kinematics of the molecular gas in W49A appears complex, with a mixture of infall and outflow motions. Both the line profiles and comparison of the main and rarer species show that the main species are optically thick. Two 'clumps' of infalling gas appear to be at ~40 K, compared to ~100 K at the source centre, and may be ~10x denser than the rest of the outer cloud. Chemical modelling suggests that the HCN/HNC ratio probes the current gas temperature, while the HCN/HCO+ ratio and the deuterium fractionation were set during an earlier, colder phase of evolution. The data suggest that W49A is an appropriate analogue of an extragalactic star forming region. Our data show that the use of HCN/HNC/HCO+ line ratios as proxies for the abundance ratios is incorrect for W49A, suggesting the same for galactic nuclei. Our observed isotopic line ratios such as H13CN/H13CO+ approach our modeled abundance ratios quite well in W49A. The 4-3 lines of HCN and HCO+ are much better tracers of the dense star-forming gas in W49A than the 1-0 lines. Our observed HCN/HNC and HCN/HCO+ ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating that irradiation hardly affects the gas chemistry in W49A. Overall, the W49A region appears to be a useful template for starburst galaxies., Comment: Accepted by A&A; 17 pages, 15 figures

Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than Helium are expected to freeze-out onto dust grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be maintained by Far-UV photons locally produced by the impact of galactic cosmic rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating water wapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system., Comment: The Astrophysical Journal Letters, in press

The clumpy density structure of photon-dominated regions is well established, but the physical properties of the clumps and of the surrounding interclump medium are only approximately known. The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photon-dominated region. We present observations of the HF J=1-0 line, which appears in emission toward the Orion Bar, and compare the brightness of the line to non-LTE radiative transfer calculations. The large width of the HF line suggests an origin of the emission in the interclump gas, but collisional excitation by H2 in the interclump gas underpredicts the observed line intensity by factors of 3-5. In contrast, an origin of the line in the dense clumps requires a density of ~10^9 cm^-3, 10-100 times higher than previous estimates, which is unlikely. However, electron impact excitation reproduces our observations for T = 100 K and n(e) = 10 cm^-3, as expected for the interclump gas. We conclude that HF emission is a signpost of molecular gas with a high electron density. Similar conditions may apply to active galactic nuclei where HF also appears in emission., Comment: Accepted for publication in A&A; 4 pages, 1 figure

The understanding of the formation process of massive stars (>8 Msun) is limited, due to theoretical complications and observational challenges. We investigate the physical structure of the large-scale (~10^4-10^5 AU) molecular envelope of the high-mass protostar AFGL2591 using spectral imaging in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160 spectral features, this paper uses the 35 that are spatially resolved. The observed spatial distributions of a selection of six species are compared with radiative transfer models based on a static spherically symmetric structure, a dynamic spherical structure, and a static flattened structure. The maps of CO and its isotopic variations exhibit elongated geometries on scales of ~100", and smaller scale substructure is found in maps of N2H+, o-H2CO, CS, SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all molecular lines presented here, except SO, SO2, and H2CO. We find two emission peaks in warm (Eup~200K) methanol separated by 12", indicative of a secondary heating source in the envelope. The spherical models are able to explain the distribution of emission for the optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and CS, nor for the optically thin C17O. The introduction of velocity structure mitigates the optical depth effects, but does not fully explain the observations, especially in the spectral dimension. A static flattened envelope viewed at a small inclination angle does slightly better. We conclude that a geometry of the envelope other than an isotropic static sphere is needed to circumvent line optical depth effects. We propose that this could be achieved in envelope models with an outflow cavity and/or inhomogeneous structure at scales smaller than ~10^4 AU. The picture of inhomogeneity is supported by observed substructure in at least six species., 17 pages; accepted for publication in A&A