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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

An accurate determination of the physical conditions in astrophysical environments relies on the modeling of molecular spectra. In such environments, densities can be so low (n ≪ 1010 cm-3) that local thermodynamical equilibrium conditions cannot be maintained. Hence, radiative and collisional properties of molecules are needed to correctly model molecular spectra. For comets at large heliocentric distances, the production of carbon monoxide (CO) gas is found to be larger than the production of water, so that molecular excitation will be induced by collisions with CO molecules. This paper presents new scattering calculations for the collisional energy transfer in CO-CO collisions. Using the quantum coupled states approach, cross sections and rate coefficients are provided between the first 37 rotational states of the CO-CO system. Cross sections were calculated for energies up to 800 cm-1, and excitation rate coefficients were derived for temperatures up to 100 K. In comparison with data available in the literature, significant differences were found, especially for the dominant transitions. Due to the high cost of the calculations, we also investigated the possibility of using an alternative statistical approach to extend our calculations both in terms of rotational states and temperatures considered. The use of these new collisional data should help in accurately deriving the physical conditions in CO-dominated comets. ispartof: PHYSICAL CHEMISTRY CHEMICAL PHYSICS vol:24 issue:19 pages:11910-11918 ispartof: location:England status: published

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

N2H+ is one of the first molecular ions observed in the interstellar medium and it is of particular interest to probe the physical conditions of cold molecular clouds. Accurate modelling of the observed lines requires the knowledge of collisional excitation rate coefficients. Thus, we have calculated rate coefficients for the excitation of N2H+ by H2, the most abundant collisional partner. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. This 4D-interaction surface exhibits a very deep well of ≈2530 cm−1 making fully converged scattering calculations very difficult to carry out, when one takes into account the rotational structure of H2. To overcome this difficulty, two approximate approaches, the adiabatic hindered rotor approach (AHR) and the statistical adiabatic channel model, were tested by comparing the results with those obtained from full 4D close-coupling calculations. The AHR treatment, which reduces the scattering calculations to a 2D problem was found to give the best results at all temperatures and even for transitions involving high N2H+ rotational levels. State-to-state rate coefficients between the 26 first N2H+ rotational levels were calculated for temperatures ranging from 5 K up to 500 K. Using a recoupling technique, rate coefficients are obtained among hyperfine transitions.

Several phosphorus-bearing molecules, such as the phosphine of interest here, have been detected in astrophysical media. With the aim of satisfying the precision required by the astrophysical community, we report the rate coefficients of PH3 in collision with helium from low to moderate temperature. To this end, we constructed the first three-dimensional potential energy surface (3D-PES) of the PH3–He van der Waals complex, which governs the nuclear motions. The 3D-PES was worked out by means of the standard coupled cluster with single, double and non-iterative triple excitation approach, in conjunction with the aug-cc-pVQZ basis set and complemented by mid-bond functions. This 3D-PES presents a well of 34.92 cm−1 at {R, θ, Φ} = {5.76 a0, 90°, 60°}. Afterwards, we incorporated this 3D-PES into time-independent close-coupling quantum dynamical computations to derive the inelastic cross-sections of rotational excitation of (ortho-) para-PH3 after collision with He up to (1000) 500 cm−1. Subsequently, we evaluated the rate coefficients for temperatures up to (100 K) 50 K populating the (41) 42 low-lying rotational levels of (ortho-) para-PH3. These data were derived by averaging the cross-sections thermally over the Maxwell–Boltzmann velocity distribution. No general propensity rules are found. We also performed a comparison with the rates for NH3–He. Differences are observed that invalidate the use of NH3 rates for deducing accurate abundances of phosphine in cold astrophysical media. Our results should be of great help in determining accurate PH3 abundances and, more generally, constraining the interstellar PH3 chemistry better.

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