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We have computed the rate of collisional cooling of a gas by H2 molecules under conditions appropriate to the primordial and interstellar media. We incorporated the results of recent calculations of the rate coefficients for collisional excitation of H2 by H and H+, which are essential to a reliable evaluation of the ortho : para H2 ratio and the cooling rate. Comparison is made with the results of previous calculations of the cooling function. The data are made available for grids of values of the kinetic temperature, density, H:H2 ratio, and the fractional abundance of H+, together with a programme to perform linear interpolation of the data sets for any given set of values of these parameters, within the ranges of the grids. [ABSTRACT FROM AUTHOR]

We investigate the application of the conventional quasi-steady state maser modelling algorithm of Menegozzi & Lamb (ML) to the high field transient regime of the one-dimensional Maxwell-Bloch (MB) equations for a velocity distribution of atoms or molecules. We quantify the performance of a first order perturbation approximation available within the ML framework when modelling regions of increasing electric field strength, and we show that the ML algorithm is unable to accurately describe the key transient features of R. H. Dicke's superradiance (SR). We extend the existing approximation to one of variable fidelity, and we derive a generalization of the ML algorithm convergent in the transient SR regime by performing an integration on the MB equations prior to their Fourier representation. We obtain a manifestly unique integral Fourier representation of the MB equations which is |$\mathcal {O}\left(N\right)$| complex in the number of velocity channels N and which is capable of simulating transient SR processes at varying degrees of fidelity. As a proof of operation, we demonstrate our algorithm's accuracy against reference time domain simulations of the MB equations for transient SR responses to the sudden inversion of a sample possessing a velocity distribution of moderate width. We investigate the performance of our algorithm at varying degrees of approximation fidelity, and we prescribe fidelity requirements for future work simulating SR processes across wider velocity distributions. [ABSTRACT FROM AUTHOR]

We have computed the rate of collisional cooling of a gas by H2 molecules under conditions appropriate to the primordial and interstellar media. We incorporated the results of recent calculations of the rate coefficients for collisional excitation of H2 by H and H+, which are essential to a reliable evaluation of the ortho : para H2 ratio and the cooling rate. Comparison is made with the results of previous calculations of the cooling function. The data are made available for grids of values of the kinetic temperature, density, H:H2 ratio, and the fractional abundance of H+, together with a programme to perform linear interpolation of the data sets for any given set of values of these parameters, within the ranges of the grids. [ABSTRACT FROM AUTHOR]

This article is a correction to a previous paper on the collisional rate coefficients for H3O+ - H2 interactions in warm interstellar clouds. The authors discovered errors in the supplementary material of their previous paper, which were caused by a software bug in the calculations of high energy cross sections. The corrections made to the data have a minor impact on the rate coefficients at lower temperatures, but some specific transitions are significantly affected at higher temperatures. However, these corrections do not change the overall conclusions of the original paper. The corrected data is available in the supplementary material and through various databases. [Extracted from the article]

Glycolaldehyde, the simplest sugar, is a complex organic molecule detected in many regions of the interstellar medium (ISM). Although its synthetic routes are fairly well known and consistent with many laboratory studies, queries still arise about its reactivity and its role in the complex chemistry of the ISM. This study shows the surface and bulk hydrogenation of glycolaldehyde at 10 K in order to confirm or invalidate the astrophysical models which suggest that CHOCH2OH would be a precursor of ethylene glycol through hydrogenation processes occurring on the surface of interstellar dust grains. By coupling IR spectroscopy and mass spectrometry, we show that the formation of HOCH2CH2OH from CHOCH2OH + H solid state reaction occurs, supporting the existence of a chemical link between these two organics in the ISM. This work suggests that while CHO + CH2OH and CH2OH + CH2OH radical recombination would lead to CHOCH2OH and HOCH2CH2OH, respectively, the presence of H-atoms in the ISM would be a secondary source to favour ethylene glycol over glycolaldehyde. These results are in good agreement with different astronomical observations which show simultaneous detections of glycolaldehyde and ethylene glycol with an abundance ratio HOCH2CH2OH/CHOCH2OH ranged between 1 and 15. [ABSTRACT FROM AUTHOR]

Radiative association for the formation of molecular nitrogen cation |${\rm{N}}_2^ + $| during the collision of an N(4Su) atom and an N+(3Pg) ion is investigated. The corresponding cross-sections and rate coefficients are computed by the quantum mechanical method based on ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration interaction method with the Davidson correction and aug-cc-pCV5Z-DK basis set. A number of low-lying doublet, quartet, and sextet states correlating to the N(4Su) + N+(3Pg) dissociation limit are considered. Hence, we investigate a number of dipole-allowed transitions and determine their contributions to the radiative association. The results show that transitions originating in the f4Πu, D2Πg, |${{\rm{B}}^2}\Sigma _{\rm{u}}^ + $|⁠ , |${{\rm{1}}^4}\Sigma _{\rm{g}}^{\rm{ + }}$|⁠ , and |${{\rm{1}}^6}\Sigma _{\rm{u}}^{\rm{ + }}$| states are the main contributors for the radiative association process. The calculated rate coefficients are valid for temperatures from 100 to 10 000 K and fitted to the analytical function suitable for astrochemical reaction applications. [ABSTRACT FROM AUTHOR]

Models for the abundances of phosphorus-bearing molecules, such as PO and PN, rely on rate coefficients guessed from similar reactions of NO. Given the importance of these molecules in pre-biotic chemistry, in this work we carry out accurate multireference configuration interaction calculations on the N+PO and O+PN reactions, unravelling their underlying mechanisms and potential energy barriers. The collisions may occur in two triplet electronic states and involve six potential wells lying under the N+PO limit. The interaction between atomic nitrogen and PO is shown to occur barrierlessly only when dynamical correlation is included in the calculations. The theoretical results confirm the assumptions used in the astrochemical models that depletion of PO by nitrogen atoms occurs fast, with a branching ratio largely favouring O+PN over the more exothermic P+NO. Among other results, we predict that PN should be stable with respect to collisions with oxygen atoms under low-temperature environments, but will be depleted to P + NO in shocks and other conditions with higher translational energies. [ABSTRACT FROM AUTHOR]

Interstellar complex organic molecules (iCOMs) have been identified in different interstellar environments including star forming regions as well as cold dense molecular clouds. Laboratory studies show that iCOMs can be formed either in gas phase or in the solid state, on icy grains, from 'non-energetic' (atom-addition/abstraction) or energetic (UV-photon, particle bombardments) processes. In this contribution, using a new experimental approach mixing matrix isolation technique, mass spectrometry, and infrared and EPR spectroscopies, we want to investigate the COM formation at 35 K from a complex mixture of ground state radicals trying to draw a general reaction scheme. We photolyse (121 nm) CH3OH diluted in Ar at low temperature (below 15 K) to generate |$\mathrm{H^.CO}$|⁠ , |$\mathrm{HO^.CO}$|⁠ , |$\mathrm{^.CH_2OH}$|⁠ , |$\mathrm{CH_3O^.}$|⁠ , |$\mathrm{^.OH}$|⁠ , and |$\mathrm{^.CH_3}$| radicals and 'free' H-atoms within the matrix. Radicals have been identified using infrared and EPR spectroscopies. With the disappearance of the Ar matrix (at 35 K), these unstable species are then free to react, forming new species in a solid film. Some recombination products have been detected using infrared spectroscopy and mass spectrometry in the solid film after Ar removal, namely methyl formate (CH3OCHO), glycolaldehyde (HOCH2CHO), ethylene glycol (HOCH2CH2OH), glyoxal (CHOCHO), ethanol (CH3CH2OH), formic acid (HCOOH), dimethyl ether (CH3OCH3), methoxymethanol (CH3OCH2OH), and CH4O2 isomers (methanediol and/or methyl hydroperoxide). The detected molecules are fully consistent with the radicals detected and strongly support the solid state scenario of iCOM formation in interstellar ices based on radical–radical recombination. We then discuss astrophysical implications of the radical pathways on the observed gas phase iCOMs. [ABSTRACT FROM AUTHOR]

In this work, we have examined the radiation-induced synthetic chemistry occurring in an astrochemically important C2H2–CO system at the molecular level using a matrix isolation approach. The 1:1 C2H2···CO intermolecular complex of linear structure was obtained in the solid low-temperature (5 K) noble gas matrices by deposition of the C2H2/CO/Ng (Ng = Ar, Kr, Xe) gaseous mixtures and characterized by Fourier-transform infrared spectroscopy. It was found that the X-ray radiolysis of the C2H2···CO complex resulted in formation of C3O (tricarbon monoxide), HCCCHO (propynal), c-H2C3O (cyclopropenone), H2CCCO (propadienone), and HC3O (oxoprorynyl radical). This means that the studied complex may be considered as the simplest building block (or minimal size of intermolecular reactor) for cold astrochemistry occurring in mixed interstellar ices. Remarkably, the discovered transformations of the complex actually represent synthetic routes leading to various C3 species, whereas the acetylenic C–H bond cleavage yielding ethynyl radical appears to be a minor process. Prolonged irradiation results in dehydrogenation, while the C3 skeleton is retained. The interpretation of the C2H2···CO radiolysis mechanism (possible reactions pathways) is provided based on the analysis of kinetic curves and matrix effect. Astrochemical implications of the results are discussed. [ABSTRACT FROM AUTHOR]

In the interstellar medium, inelastic collisions are so rare that they cannot maintain a local thermodynamical equilibrium (LTE). Atomic and molecular populations therefore do not follow a simple Boltzmann distribution and non-LTE spectra are the rule rather than the exception. In such conditions, accurate state-to-state collisional data are crucial for a quantitative interpretation of spectra. In recent years, considerable progress has been made in quantum calculations of inelastic cross sections for a variety of targets, types of transitions and projectiles. For a few benchmark species, detailed comparisons between theory and experiment were also carried out at the state-to-state level and in the quantum regime. In this article, we highlight such comparisons for three important molecules: CO, H2O and CH+. We also describe current computational efforts to extend these advances to ever larger targets, new transition types, and new environments (e.g. stellar envelopes or cometary atmospheres). [ABSTRACT FROM AUTHOR]

The photodissociation process of magnesium oxide (MgO) may be a main source of the magnesium atom in planetary exospheres due to the impact events on planets. In this work, we calculated the state-resolved cross-sections of the photodissociation of MgO for the transitions from the full range of rovibrational levels of the X1Σ+ and a3Π states, where the photon wavelength ranged from 50 to 5000 nm. The applications of the state-resolved cross-sections to the local thermodynamic equilibrium (LTE) conditions were also investigated for the temperatures ranging from 500 to 10 000 K. Based on the calculated LTE cross-sections, the photodissociation rates in the interstellar, solar, and blackbody radiation fields were obtained as well. The rates in all studied radiation fields increase with the temperature increasing and are dominated by the e3Σ− ← a3Π and F1Π ← X1Σ+ transitions. The knowledge of the photodissociation cross-sections and rates of MgO plays a key role in the investigations of the chemical behaviour of the alkaline-earth element Mg in planetary exospheres, atmospheres of cool stars, and envelopes of evolved stars. [ABSTRACT FROM AUTHOR]

Cross sections and rate coefficients for transitions between hyperfine levels associated with the 14 lowest rotational/fine-structure levels of OD induced by collisions with ortho -H2 and para -H2 are presented. These collisional parameters have been computed in time-independent close-coupling quantum-scattering calculations with a potential energy surface (PES) describing the OD–H2 interaction, which was obtained by transformation of the OH–H2 PES. Rate coefficients have been computed for temperatures from 5 to 200 K. Cross sections for OD transitions between rotational/fine-structure levels are found to be significantly larger than the corresponding transitions in OH, mainly because of the reduced energy gaps in OD. The hyperfine-resolved rate coefficients were employed in simple radiative transfer calculations for OD and compared with analogous calculations for OH. [ABSTRACT FROM AUTHOR]

Surface heterogeneity of model amorphous silica films used as a model for interstellar grain surfaces is revealed through the application of the pre-exponential optimized inversion method to previously reported sub-monolayer thermal desorption studies of carbon monoxide (CO) desorption. The impact of that surface heterogeneity, as represented by the coverage dependence of the CO activation energy for desorption from the amorphous silica surface, on the IR spectroscopy of the CO stretching vibration is explored through vibrational line profile synthesis. Comparison is then made to previous investigations of CO line profiles on this surface and on amorphous solid water as reported in Taj et al. (2017, 2019a). A tentative conclusion is drawn that CO vibrationally promoted desorption from, and diffusion on, the amorphous silica surface may be responsible for the correspondingly short vibrational excited state lifetime of CO on that surface. The contrast with CO on amorphous solid water, where direct and rapid vibrational relaxation into the solid water phonon bath occurs, is highlighted. The consequences of this from the standpoint of CO deposition on grain surfaces are discussed. [ABSTRACT FROM AUTHOR]

Although complex organic molecules are observed in a wide variety of environments, chemical reaction networks heading to their formation are higly debated. It is a major endeavour to model the rates of reactions and incorporate them into chemical networks. The vast majority of the computational investigations in astrochemistry take into consideration oversimplified molecular models where chemical reactions are simulated under vacuum conditions (gas phase) and with crudely approximated entropic contributions to the free energy. We use density functional theory-based molecular dynamics techniques coupled with state-of-the-art metadynamics methods to investigate the role of ices embedding the reactants in shaping the free-energy landscape of selected reactions. Ices are chemically defined at the same level of theory of the reactants themselves. We consider as test case the transformation of methane and isocyanic acid into molecular hydrogen and methyl isocyanate, a species bearing similarities with peptide bonds. We examine the thermodynamically unfavoured case of very stable reactants to magnify modifications in the energy configuration induced by a solid amorphous water ice, either pure or mixed with CO. The presence of an active medium modifies significantly the free-energy surface, widening the path connecting reactants and products, and decreasing substantially the energy barriers. Ices not only act as gatherers of reactants, but also create thermodynamic conditions favouring chemical evolution. [ABSTRACT FROM AUTHOR]

Organic residues are considered as part of the chemical composition of the interstellar dust grains. They are formed under the extreme conditions of the interstellar medium and play an important role in exobiology. They may contain pre-biotic organic species such as amino acids, constituents of proteins and building blocks of DNA and RNA, key elements of life. By investigating the formation of organic residues in an astrophysical context, many groups have been focusing in the UV irradiation and subsequent warm-up of astrophysical ice analogues. This aims to suggest that organic residues are mainly formed in regions of molecular clouds exposed to UV light or cosmic rays. This study shows an organic residue formation involving glyoxal ice and H atoms. While the hydrogenation of glyoxal at 10 K leads mainly to small molecules such as CO and H2CO and CH3OH, we show that the heating of the hydrogenated ice in the 10–300 K temperature range leads to solid residues whose structure is similar to that of glycolaldehyde but they remain stable in solid phase at 300 K and atmospheric pressure. The analysis of the IR data shows that the organic residues formed through the thermal processing of CHOCHO + H reaction would be a mixture of hydroxypyruvaldehyde and methyl glyoxylate, two solid organics whose formation starts with an H-abstraction from glyoxal to form CHOCO• radical which recombines to •CH2OH and •OCH3 radicals. These latter may be formed and trapped in glyoxal ice as secondary products from H2CO + H secondary reaction. [ABSTRACT FROM AUTHOR]

The recent detections of CNCN and HNCCN+ are seen as further evidence of the large abundance of NCCN in the interstellar medium. The accurate determination of the abundance of these chemically related compounds from the observational spectra requires the prior calculation of collisional rate coefficients. In this work, we aimed at computing hyperfine resolved rate coefficients for the CNCN–He collisional system. First, we determined a new potential energy surface for the CNCN–He van der Waals complex from which we computed rotationally resolved excitation cross-sections for energies up to 800 cm−1 using the quantum mechanical close-coupling approach. Then, hyperfine resolved transitions between the 30 low-lying pure rotational levels of CNCN were computed for temperatures ranging from 5 to 150 K using an improved infinite order sudden approach. The analysis of the scattering results showed a propensity rule in favour of Δ j  = Δ F 1 = Δ F for the hyperfine transitions and a slight dominance of the odd Δ j transitions. Using these data, we carried out non-LTE radiative transfer calculations to simulate the excitation of CNCN in molecular clouds and to constrain the physical conditions of cold dark clouds. Preliminary results showed that the abundance of CNCN derived from observational spectra has to be revisited using these new collisional data. [ABSTRACT FROM AUTHOR]

Radiative association (RA) of Al+ with H2 is the first step in the formation of AlH in gas phase and is here investigated theoretically. We use recent potential energy and dipole moment surfaces and a quantum approach based on the driven equations formalism for performing the dynamics for both the Al+-H2 and Al+-D2 systems. The obtained RA rate coefficients are compared with previous evaluations based on transition state theory and found to be orders of magnitude larger. They are also compared to those obtained recently for the similar systems Na+-H2/D2. The possible role played by RA of Al+ with H2 in the gas phase chemistry of dense molecular clouds is discussed. [ABSTRACT FROM AUTHOR]

A theoretical model of water ortho–para conversion induced by blackbody radiation in space is developed. The model is based on two main ingredients: the mixing of water ortho and para states by a hyperfine spin-rotation interaction in the molecule and the interruption of this mixing by surrounding blackbody radiation. The model predicts the lifetime of water spin isomers τ = 2.7 Myr for radiation with a temperature of 100 K and τ = 1.3 Gyr for microwave background radiation. The time dependence of the ortho-to-para ratio (OPR) of water molecules interacting with microwave background radiation is determined. The temperature dependence of the stationary OPR predicted by the model is found to be different from the usually accepted OPR for water spin isomers. [ABSTRACT FROM AUTHOR]

Radiative association (RA) of Al+ with H2 is the first step in the formation of AlH in gas phase and is here investigated theoretically. We use recent potential energy and dipole moment surfaces and a quantum approach based on the driven equations formalism for performing the dynamics for both the Al+-H2 and Al+-D2 systems. The obtained RA rate coefficients are compared with previous evaluations based on transition state theory and found to be orders of magnitude larger. They are also compared to those obtained recently for the similar systems Na+-H2/D2. The possible role played by RA of Al+ with H2 in the gas phase chemistry of dense molecular clouds is discussed. [ABSTRACT FROM AUTHOR]

Olive (Olea europaea L.) is an evergreen xerophytic tree characterizing vegetative landscape and historical-cultural identity of the Mediterranean Basin. More than 2600 cultivars constitute the rich genetic patrimony of the species cultivated in approximately 60 countries. As a subtropical species, the olive tree is quite sensitive to low temperatures, and air temperature is the most critical environmental factor limiting olive tree growth and production. In this present review, we explored the detrimental effects caused of low temperatures on olive cultivars, and analyzed the most frequently experimental procedures used to evaluate cold stress. Then, current findings freezing stress physiology and gene are summarized in olive tree, with an emphasis on adaptive mechanisms for cold tolerance. This review might clear the way for new research on adaptive mechanisms for cold acclimation and for improvement of olive growing management.

Water is a key molecule for interstellar chemistry. Observations with Herschel telescope show significant population of very high rotational transitions (j ≳ 8) in young stellar objects, indicating significant amounts of water in hot (T ≳ 1500 K) and dense (n ≳ 106 cm−3) gas. Non-local thermodynamic equilibrium (LTE) modelling of these observations requires the knowledge of the collisional and radiative properties of highly excited water at high temperature. The aim of this work is to calculate a new set of excitation rate coefficients for both para- and ortho- H2O induced by collisions with H2 for energy levels up to j = 17. Quantum scattering calculations were performed using a reduced dimensional approach and the coupled states approximation. Rate coefficients were obtained for 97 pure rotational energy levels of both para- and ortho- H2O and for temperatures up to 2000 K. With the forthcoming launch of the James Webb Space Telescope , these new collisional data will allow us to gain more insight into the physical conditions in star- and planet-forming regions. [ABSTRACT FROM AUTHOR]

Water is a key molecule for interstellar chemistry. Observations with Herschel telescope show significant population of very high rotational transitions (j ≳ 8) in young stellar objects, indicating significant amounts of water in hot (T ≳ 1500 K) and dense (n ≳ 106 cm−3) gas. Non-local thermodynamic equilibrium (LTE) modelling of these observations requires the knowledge of the collisional and radiative properties of highly excited water at high temperature. The aim of this work is to calculate a new set of excitation rate coefficients for both para- and ortho- H2O induced by collisions with H2 for energy levels up to j = 17. Quantum scattering calculations were performed using a reduced dimensional approach and the coupled states approximation. Rate coefficients were obtained for 97 pure rotational energy levels of both para- and ortho- H2O and for temperatures up to 2000 K. With the forthcoming launch of the James Webb Space Telescope , these new collisional data will allow us to gain more insight into the physical conditions in star- and planet-forming regions. [ABSTRACT FROM AUTHOR]

Ices are the dominant surface material of many Solar system objects, such as comets and trans-Neptunian objects. They are continuously exposed to ion bombardment by solar wind ions and cosmic rays, which trigger secondary ion emission, contributing to the exosphere formation. Laboratory studies demonstrated the effects of energetic processing of ices at low temperature, showing the production of molecules and free radicals of astrophysical interest. Nitrous oxide (N2O) is one of the molecular species observed in star-forming sites, reason why it may be present in the ices covering some minor bodies in the outer Solar system. In the current work, N2O ice at 10 K was irradiated by energetic (MeV/u) multicharged heavy ions (e.g. 105Rh and 140Ba); the sputtered species were detected and analysed by the TOF-PDMS technique (time-of-flight plasma desorption mass spectrometry). Small positive and negative secondary ions were identified: N+, N2+, NO+, O+, and O−. The bombardment also induces production of ion cluster series: (N2) n R |$_{m}^+$|⁠ , (NO) n R |$_{m}^+$|⁠ , (N2O) n R |$_{m}^+$|⁠ , where R = N+, N2+, NO+, N2O+, O m + (n up to ∼ 10, m = 1−3). Their yield distributions follow the sum of two decreasing exponentials, one fast - F and another slow - S , suggesting a two-regime formation. Most of the yield distributions have the same pair of exponential decay constants, around k F ∼ 1.4 and k S ∼ 0.15 u−1. Based on this behavior, an emission description for aggregates is proposed, useful to understand the processes by which neutral and ionized molecular species are delivery to the gas phase in space. [ABSTRACT FROM AUTHOR]

Cyanoacetylene molecules are widespread in the interstellar medium (ISM) and several of its isomers have been detected in cold molecular clouds and circumstellar gas. Accurate estimates of the abundance ratio of cyanoacetylene isomers may provide deep insight into their environment. Such knowledge requires rigorous modelling of the emission spectra based on non-local thermodynamic equilibrium (LTE) radiative transfer calculations. To this end, we computed excitation cross-sections of HC2NC and HNC3 induced by collision with para- and ortho-H2, using a quantum mechanical close-coupling method. Then, by thermally averaging these data, we derived rate coefficients for the first 31 low-lying rotational levels of each isomer for temperatures up to 80 K. For the para-H2 collider, the propensity rules are in favour of rotational transitions involving Δ j 1 = 2 for both isomers, while for the ortho-H2 collider, Δ j 1 = 2 and Δ j 1 = 1 rotational transitions are favoured for HC2NC and HNC3, respectively. A comparison of rate coefficients for the HC3N isomers shows differences up to an order of magnitude, especially at low temperatures. Finally, we performed non-LTE radiative transfer calculations to assess the impact of such variations in the analysis of observations. Our simulation suggests that the lack of collisional data specific to each isomer could lead to errors up to a factor of 2–3 in the excitation temperatures. We expect that these data could help in better understanding the cyanoacetylene chemistry and constraining the nitrogen chemistry in the ISM. [ABSTRACT FROM AUTHOR]

To explain the properties of dust in the interstellar medium (ISM), the presence of a refractory organic mantle is necessary. The outflows of AGB stars are among the main contributors of stellar dust to the ISM. We present the first study of the refractory organic contribution of AGB stars to the ISM. Based on laboratory experiments, we included a new reaction in our extended chemical kinetics model: the photoprocessing of volatile complex ices into inert refractory organic material. The refractory organic feedback of AGB outflows to the ISM is estimated using observationally motivated parent species and grids of models of C-rich and O-rich outflows. Refractory organic material is mainly inherited from the gas phase through accretion on to the dust and subsequent photoprocessing. Grain-surface chemistry, initiated by photodissociation of ices, produces only a minor part and takes place in a sub-monolayer regime in almost all outflows. The formation of refractory organic material increases with outflow density and depends on the initial gas-phase composition. While O-rich dust is negligibly covered by refractory organics, C-rich dust has an average coverage of |$3\!-\!9{{\ \rm per\ cent}}$|⁠ , but can be as high as |$8\!-\!22{{\ \rm per\ cent}}$|⁠. Although C-rich dust does not enter the ISM bare, its average coverage is too low to influence its evolution in the ISM or significantly contribute to the coverage of interstellar dust. This study opens up questions on the coverage of other dust-producing environments. It highlights the need for an improved understanding of dust formation and for models specific to density structures within the outflow. [ABSTRACT FROM AUTHOR]

The quantity of NH3 produced on grain surfaces in the pre-stellar core is thought to be one of the determining factors regarding the chemical complexity achievable at later stages of stellar birth. In order to investigate how this quantity might be influenced by the gas–grain cycling of molecular material within the cloud, we employ a modified rates gas–grain chemical code and follow the time-dependent chemistry of NH3 as the system evolves. Our models incorporate an updated version of the most recent UDfA network of reaction rate coefficients, desorption from the grains through standard thermal and non-thermal processes, and physisorbed and chemisorbed binding of atomic and molecular hydrogen to a population of carbonaceous and siliceous grains. We find that (1) observable abundances of NH3 can exist in the gas phase of our models at early times when the N atom is derived from CN via an efficient early-time hydrocarbon chemistry, (2) a time-dependent gradient exists in the observational agreement between different species classes in our models, consistent with possible physical substructures within the TMC-1 Cyanopolyyne Peak, and (3) the gaseous and solid-state abundances of NH3 are sensitive to the presence of gas–grain cycling within the system. Our results suggest that the degree of chemical complexity achievable at later stages of the cloud's chemical evolution is indeed influenced by the manner in which the gas–grain cycling occurs. [ABSTRACT FROM AUTHOR]

Complex organic molecules from extraterrestrial source are expected to have contributed to the Early Earth chemistry. Methylamine (CH3NH2)has already been observed in the interstellar medium (ISM) and is generally related to the formation of glycine, although the latter has not been identified in the ISM yet. In this work, a chemical model for CH3NH2 was investigated, comprising twenty-eight reactions and including reactions involving NH3 and HOOC, aiming to understand the main routes for formation and decomposition of methylamine and also to infer about the chemical behaviour of glycine in the ISM. Calculations were performed at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level and rate coefficients were calculated adopting the canonical variational transition state theory (CVTST), in the temperature range 100 to 4000 K, including tunnelling effects. Starting from HCN, the preferred pathway for methylamine formation is through consecutive hydrogenation steps, forming CH2N, CH2NH, and CH2NH2 intermediates. Considering the decomposition, dissociation into CH3 and NH2 is the most favourable step. NH3 and HCN are common compounds in interstellar ice analogues and react producing NH2 and CH2N through NH2NCH2 and H2NCH2N intermediates. The latter is proposed here and spectroscopic data for any future experimental investigation are given. Finally, an extension to the ISM glycine chemistry is explored and routes to its formation, from the simplest compounds found in interstellar ices, are proposed. [ABSTRACT FROM AUTHOR]

It is shown that the Born–Oppenheimer–Huang non-adiabatic coupling terms (NACTs) in a molecular Hamiltonian can be considered as equivalent to a friction force in classical mechanics and consequently responsible for the formation of triatomic species like HeH 2 + . The classical equation of motion is solved accordingly for a test case of a helium atom approaching an H 2 + molecule at its potential minimum along the collinear axis and lines parallel to it and also for the C 2 v geometry. It is shown that the singular NACTs between the first two excited states slow down the motion of the three quasi-ions (He, H, H) + while approaching each other and result in substantial loss of kinetic energy in the collision process. These results have profound implications for the formation of di- and triatomic species in the early universe, during the chemical synthesis epoch following the nucleosynthesis that followed the Big Bang. [ABSTRACT FROM AUTHOR]

It is shown that the Born–Oppenheimer–Huang non-adiabatic coupling terms (NACTs) in a molecular Hamiltonian can be considered as equivalent to a friction force in classical mechanics and consequently responsible for the formation of triatomic species like HeH 2 + . The classical equation of motion is solved accordingly for a test case of a helium atom approaching an H 2 + molecule at its potential minimum along the collinear axis and lines parallel to it and also for the C 2 v geometry. It is shown that the singular NACTs between the first two excited states slow down the motion of the three quasi-ions (He, H, H) + while approaching each other and result in substantial loss of kinetic energy in the collision process. These results have profound implications for the formation of di- and triatomic species in the early universe, during the chemical synthesis epoch following the nucleosynthesis that followed the Big Bang. [ABSTRACT FROM AUTHOR]

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn's satellites Enceladus, Dione, and Rhea. [ABSTRACT FROM AUTHOR]

Relaxation of the spin-orbit excited C+(2P3/2) ion by collisions with H2 is an important process in the interstellar medium. Previous calculations of rate coefficients for this process employed potential energies computed for only collinear and perpendicular approach of H2 to the ion. To capture the full angular dependence of the C+–H2 interaction, the angular variation of the potential has been obtained by quantum chemical calculations in this work. These data were used to compute rate coefficients for the de-excitation of the C+(2P3/2) level in collisions with H2 in its j = 0, 1, and 2 rotational levels. With the assumption that the para-H2 rotational levels are in Local Thermodynamic Equilibrium (LTE), rate coefficients were then calculated for de-excitation by para- and ortho-H2 for temperature ranging from 5 to 500 K. The rate coefficient for de-excitation by para-H2 is ca. 10 per cent higher at temperatures near 100 K but 10 per cent lower at temperatures greater than 300 K than the previous best calculation. By contrast, the de-excitation rate coefficient for ortho-H2 is 15 per cent higher at low temperatures but approximately equal as compared with the previous best calculation. The impact of these new rate coefficients is briefly tested in radiative transfer calculations. [ABSTRACT FROM AUTHOR]

We are interested in quantum calculations of polarization transfer (PT) rates due to collisions of the SiO molecule with the electrons. We determine the inelastic PT rates associated to the transitions: X 1Σ+→3Π; X 1Σ+→3Σ+; X 1Σ+→3Δ; X 1Σ+→3Σ−. In addition, we calculate the elastic PT rates due to rotational transitions inside the electronic state X 1Σ+ which are related to observed astronomical SiO MASERs. Our PT rates are obtained through linear combination of excitation rates previously calculated for SiO-electron collisions. The calculations are performed on a collision energy grid large enough to ensure converged state-to-state rates for temperatures ranging from 1000 to 10,000 K for inelastic rates and from 5 to 5000 K for elastic rates. The dependence of the inelastic rates on temperatures is obtained analytically and given in useful form.

The radiative association process for the formation of magnesium oxide (MgO) may be of great importance due to its frequent occurrence in the low-density and dust-poor astrochemical environments. In this work, the cross-sections and rate coefficients for the A1Π → X1Σ+, |${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm A}^1\Pi$|⁠ , D1Δ → A1Π, a3Π → e3Σ−, |${\rm X}^1\Sigma ^+\, \rightarrow \, {\rm X}^1\Sigma ^+$|⁠ , and A1Π → A1Π radiative association processes of forming MgO are theoretically estimated. The cross-sections for the transitions between the different states are obtained by using the semiclassical method for direct contributions and the Breit–Wigner theory as a complement for resonance contributions. For the transitions between the same states, the quantum mechanical method is used. The rate coefficients are then obtained from the cross-sections for the temperatures in the range of 10–10 000 K and the results are found to vary from 4.69 |$\times \, 10^{-16}$| to 6.27 |$\times \, 10^{-14}$| cm3 s−1. For temperatures lower than around 693 K, the rate coefficients for the A1Π → X1Σ+ process are dominant, which indicates this process is the most efficient way of producing MgO at low temperatures. However, the rate coefficients for the D1Δ → A1Π process go through a rapid increase with increasing temperature and become dominant at higher temperatures. For other processes, their rate coefficients are several orders of magnitude lower than those for the two processes mentioned above. The results can be used to further investigate the formation and evolution of MgO in low density and hot gas close to the photosphere of evolved oxygen-rich stars. [ABSTRACT FROM AUTHOR]

Methanol, which is one of the most abundant organic molecules in the interstellar medium, plays an important role in the complex grain surface chemistry that is believed to be a source of many organic compounds. Under energetic processing such as ultraviolet (UV) photons or cosmic rays, methanol may decompose into CH4, CO2, CO, HCO, H2CO, CH3O and CH2OH, which in turn lead to complex organic molecules such as CH3OCHO, CHOCH2OH and HOCH2CH2OH through radical recombination reactions. However, although molecular oxygen and its detection, abundance and role in the interstellar medium have been the subject of many debates, few experiments on the oxidation of organic compounds have been carried out under interstellar conditions. The present study shows the behaviour of solid methanol when treated by UV light and thermal processing in oxygen-rich environments. Methanol has been irradiated in the absence and presence of O2 at different concentrations in order to study how oxidized complex organic molecules may form and also to investigate the O-insertion reaction in the C–H bound to form methanediol HOCH2OH through a CH3OH + O(1D) solid-state reaction. The adding of O2 in the thermal and photochemical reaction of solid methanol leads to the formation of O3, H2O and HO2, in addition to three main organics, HCOOH, CHOCHO and HOCH2OH. We show that in an O2-rich environment, species such as CO, CH4, HCO, CH3OH and CHOCH2OH are oxidized into CO2, CH3OH, HC(O)OO, HOCH2OH and CHOCHO, respectively, while HCOOH might be formed through the H2CO + O(3P) → (OH + HCO)cage → HCOOH hydrogen-abstraction reaction. [ABSTRACT FROM AUTHOR]

We study the thermal evolution of UV-irradiated atomic cooling haloes using high-resolution three-dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Ly α  cooling radiation trapped in the optically-thick cores of three such haloes, a process that has not been included in previous simulations. Because H− is a precursor of molecular hydrogen, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2-cooling is decreased by ∼15–50 per cent in our fiducial models. Previous one-zone modelling found a larger effect, with J crit reduced by a factor of a few; we show that adopting a constant halo mass to determine the trapped Ly α  energy density, as is done in the one-zone models, yields a larger reduction in J crit, consistent with their findings. Our results nevertheless suggest that Ly α  radiation may have an important effect on the thermal evolution of UV-irradiated haloes, and therefore on the potential for massive black hole formation. [ABSTRACT FROM AUTHOR]

The absorption spectrum of solid acetylene (C2H2) at 10 K was measured in wavelength range 107–250 nm with UV light from a synchrotron source. Based on the absorption spectrum, we irradiated at 175 nm the icy acetylene at 60 K; the results yielded a new set of IR absorption lines with similar temporal profiles of production during the photolysis periods. From icy 12C2H2, these new features were recorded at 3276.8, 1598.7, 963.0, 936.3, and 671.3 cm−1, which are assigned to modes ν1, ν6, ν14, ν15, and ν11 (mixing with ν17), respectively, of vinylacetylene (C4H4) based on the results of 13C- and D-isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP/6–311++G**) predict vibrational wavenumbers and IR intensities of vinylacetylene that agree satisfactorily with our experimental results. This work enhances our understanding of the photochemistry of icy acetylene in cold astronomical environments; for instance, the present result can be applied directly to the photochemical processes on Titan's surface. [ABSTRACT FROM AUTHOR]

This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative transition rate coefficients, are required for accurate determination of the abundance of HCO in the ISM. Time-independent close-coupling quantum scattering calculations have been used to compute rate coefficients for (de-)excitation of HCO in collisions with para - and ortho -H2. These calculations utilized a potential energy surface for the interaction of HCO with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 400 K were calculated for all transitions among the fine and hyperfine levels associated with the first 22 rotational levels of HCO, whose energies are less than or equal to 144 K. [ABSTRACT FROM AUTHOR]

We re-introduce a semiclassical methodology based on theories developed for the determination of broadening coefficients. We show that this simple and extremely fast methodology provides results that are in good agreement with results obtained using the more sophisticate MQCT approach. This semiclassical methodology could be an alternative approach which allows to provide large sets of collisional data for very complex molecular systems. It saves time both on the determination of potential energy surfaces and on the collisional dynamical calculations. In addition, this paper provides more complete sets of rotational de-excitation cross-sections and rate coefficients of H2O perturbed by a thermal average of water molecules. Those data can be used in the radiative transfer modelling of cometary atmospheres. [ABSTRACT FROM AUTHOR]

Context. Nonlinear behavior in interstellar chemical models has been recognized for 25 years now. Different mechanisms account for the possibility of multiple fixed-points at steady-state, characterized by the ionization degree of the gas. Aims. Chemical oscillations are also a natural behavior of nonlinear chemical models. We study under which conditions spontaneous sustained chemical oscillations are possible, and what kind of bifurcations lead to, or quench, the occurrence of such oscillations. Methods. The well-known ordinary differential equations (ODE) integrator VODE was used to explore initial conditions and parameter space in a gas phase chemical model of a dark interstellar cloud. Results. We recall that the time evolution of the various chemical abundances under fixed temperature conditions depends on the density over cosmic ionization rate nH∕ζ ratio. We also report the occurrence of naturally sustained oscillations for a limited but well-defined range of control parameters. The period of oscillations is within the range of characteristic timescales of interstellar processes and could lead to spectacular resonances in time-dependent models. Reservoir species (C, CO, NH3,...) oscillation amplitudes are generally less than a factor two. However, these amplitudes reach a factor ten to thousand for low abundance species, e.g. HCN, ND3, that may play a key role for diagnostic purposes. The mechanism responsible for oscillations is tightly linked to the chemistry of nitrogen, and requires long chains of reactions such as found in multi-deuteration processes. [ABSTRACT FROM AUTHOR]

Context. Understanding the star-forming processes is key to understanding the evolution of galaxies. Investigating star formation requires precise knowledge of the properties of the dense molecular gas complexes where stars form and a quantification of how they are affected by the physical conditions to which they are exposed. The proximity, low metallicity, and wide range of star formation activity of the Large and Small Magellanic Clouds (LMC and SMC) make them prime laboratories to study how local physical conditions impact the dense gas reservoirs and their star formation efficiency. Aims. The aim of the Dense Gas Survey for the Magellanic Clouds (DeGaS-MC) project is to expand our knowledge of the relation between dense gas properties and star formation activity by targeting the LMC and SMC observed in the HCO+(2−1) and HCN(2−1) transitions. Methods. We carried out a pointing survey targeting two lines toward ∼30 LMC and SMC molecular clouds using the SEPIA180 instrument installed on the Atacama Pathfinder EXperiment (APEX) telescope. We performed a follow-up mapping campaign of the emission in the same transition in 13 star-forming regions. This first paper provides line characteristic catalogs and integrated line-intensity maps of the sources. Results. HCO+(2−1) is detected in 20 and HCN(2−1) in 8 of the 29 pointings observed. The dense gas velocity pattern follows the line-of-sight velocity field derived from the stellar population. The three SMC sources targeted during the mapping campaign were unfortunately not detected in our mapping campaign but both lines are detected toward the LMC 30Dor, N44, N105, N113, N159W, N159E, and N214 regions. The HCN emission is less extended than the HCO+ emission and is restricted to the densest regions. The HCO+(2−1)/HCN(2−1) brightness temperature ratios range from 1 to 7, which is consistent with the large ratios commonly observed in low-metallicity environments. A larger number of young stellar objects are found at high HCO+ intensities and lower HCO+/HCN flux ratios, and thus toward denser lines of sight. The dense gas luminosities correlate with the star formation rate traced by the total infrared luminosity over the two orders of magnitude covered by our observations, although substantial region-to-region variations are observed. [ABSTRACT FROM AUTHOR]

Context. The interaction between dust, ice, and gas during the formation of stars produces complex organic molecules. While observations indicate that several species are formed on ice-covered dust grains and are released into the gas phase, the exact chemical interplay between solid and gas phases and their relative importance remain unclear. Aims. Our goal is to study the interplay between dust, ice, and gas in regions of low-mass star formation through ice- and gas-mapping and by directly measuring gas-to-ice ratios. This provides constraints on the routes that lead to the chemical complexity that is observed in solid and gas phases. Methods. We present observations of gas-phase methanol (CH3OH) and carbon monoxide (13CO and C18O) at 1.3 mm towards ten low-mass young protostars in the Serpens SVS 4 cluster from the SubMillimeter Array (SMA) and the Atacama Pathfinder EXperiment (APEX) telescope. We used archival data from the Very Large Telescope (VLT) to derive abundances of ice H2O, CO, and CH3OH towards the same region. Finally, we constructed gas-ice maps of SVS 4 and directly measured CO and CH3OH gas-to-ice ratios. Results. The SVS 4 cluster is characterised by a global temperature of 15 ± 5 K. At this temperature, the chemical behaviours of CH3OH and CO are anti-correlated: larger variations are observed for CH3OH gas than for CH3OH ice, whereas the opposite is seen for CO. The gas-to-ice ratios (Ngas/Nice) range from 1–6 for CO and 1.4 × 10−4–3.7 × 10−3 for CH3OH. The CO gas-maps trace an extended gaseous component that is not sensitive to the effect of freeze-out. Because of temperature variations and dust heating around 20 K, the frozen CO is efficiently desorbed. The CH3OH gas-maps, in contrast, probe regions where methanol is predominantly formed and present in ices and is released into the gas phase through non-thermal desorption mechanisms. Conclusions. Combining gas- and ice-mapping techniques, we measure gas-to-ice ratios of CO and CH3OH in the SVS 4 cluster. The CH3OH gas-to-ice ratio agrees with values that were previously reported for embedded Class 0/I low-mass protostars. We find that there is no straightforward correlation between CO and CH3OH gas with their ice counterparts in the cluster. This is likely related to the complex morphology of SVS 4: the Class 0 protostar SMM 4 and its envelope lie in the vicinity, and the outflow associated with SMM 4 intersects the cluster. This study serves as a pathfinder for future observations with ALMA and the James Webb Space Telescope (JWST) that will provide high-sensitivity gas-ice maps of molecules more complex than methanol. Such comparative maps will be essential to constrain the chemical routes that regulate the chemical complexity in star-forming regions. [ABSTRACT FROM AUTHOR]

To examine the evolution processes of the nitrogen-containing polycyclic aromatic hydrocarbon (PAH) molecules occurring in interstellar environments, in this work we focus on the formation of large covalently bonded N-substituted polyaromatic species and their photochemistry behaviour in the gas phase. The experimental results show large PANH (e.g. DC/acridine and DC/phenazine) cluster cations formed in a chemical reaction between large PAH (e.g. dicoronylene, DC, C48H20) cations and small PANHs (e.g. acridine, C13H9N, or phenazine, C12H8N2) by gas-phase condensation through ion–molecule reactions. With laser irradiation, PANH cluster cations are involved in a complex photofragmentation process (e.g. dehydrogenation, HCN/CN, C2 or N2 units lost) and then form large PANH/PAH or multiple dehydrogenated molecules; in particular, the dehydrogenation of PANH clusters provides a possible way to synthesize large nitrogen-containing graphene species (e.g. C59N+ and C61N+). Also, we perform quantum-theoretical calculations on the formation and photochemistry of DC/acridine and DC/phenazine cluster cations: two types of molecular cluster are considered (C–C and C–N bond type) and the formation pathway and dissociation energy for each isomer are determined. The experimental and theoretical findings obtained give a general molecular growth pathway toward all-benzenoid aromatic species with size (> 60 C atoms) in the astrophysically relevant range, during a ground-up formation process, and offer understanding of the nitrogen element effect on their chemical-evolutionary behaviour. Also, studies of DC/acridine and DC/phenazine clusters (89–112 atoms, ∼2 nm in size) offer a feasible means of explanation for the formation of nanoscale dust grains (nitrogen element included) in space. [ABSTRACT FROM AUTHOR]

Context. The inclusion of molecular physics is an important piece that tends to be missing from the puzzle when modeling the spectra of supernovae (SNe). Molecules have both a direct impact on the spectra, particularly in the infrared, and an indirect one as a result of their influence on certain physical conditions, such as temperature. Aims. In this paper, we aim to investigate molecular formation and non-local thermodynamic equilibrium (NLTE) cooling, with a particular focus on CO, the most commonly detected molecule in supernovae. We also aim to determine the dependency of supernova chemistry on physical parameters and the relative sensitivity to rate uncertainties. Methods. We implemented a chemical kinetic description of the destruction and formation of molecules into the SN spectral synthesis code SUMO. In addition, selected molecules were coupled into the full NLTE level population framework and, thus, we incorporated molecular NLTE cooling into the temperature equation. We produced a test model of the CO formation in SN 1987A between 150 and 600 days and investigated the sensitivity of the resulting molecular masses to the input parameters. Results. We find that there is a close inter-dependency between the thermal evolution and the amount of CO formed, mainly through an important temperature-sensitive CO destruction process with O+. After a few hundred days, CO completely dominates the cooling of the oxygen-carbon zone of the supernova which, therefore, contributes little optical emission. The uncertainty of the calculated CO mass scales approximately linearly with the typical uncertainty factor for individual rates. We demonstrate how molecular masses can potentially be used to constrain various physical parameters of the supernova. [ABSTRACT FROM AUTHOR]

Context. The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time. Aims. Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions. Methods. We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance. Results. We confirm the presence in situ of predicted cations at comets, such as CH m+ $_m^+$ m + (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO 2++ $_2^{++}$ 2 + + . [ABSTRACT FROM AUTHOR]

The HCO+ and DCO+ molecules are commonly used as tracers in the interstellar medium. Therefore, accurate rotational rate coefficients of these systems with He and H2 are crucial in non-local thermal equilibrium models. We determine in this work the rotational de-excitation rate coefficients of HCO+ in collision with both para- and ortho- H2, and also analyse the isotopic effects by studying the case of DCO+. A new four-dimensional potential energy surface from ab initio calculations was developed for the HCO+–H2 system, and adapted to the DCO+–H2 case. These surfaces are then employed in close-coupling calculations to determine the rotational de-excitation cross-sections and rate coefficients for the lower rotational states of HCO+ and DCO+. The new rate coefficients for HCO+ +  para- H2 were compared with the available data, and a set of rate coefficients for HCO+ +  ortho- H2 is also reported. The difference between the collision rates with ortho- and para- H2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO+ +  para- H2 for estimating those for HCO+ +  ortho- H2 as well as for DCO+ +  para- H2 is a good approximation. [ABSTRACT FROM AUTHOR]

Based on typical physical and chemical conditions expected in massive and dense hot cores during the protostar collapse, the formation of glycolaldehyde (CH2OHCHO) and methanol (CH3OH) was investigated from H2 and CO and formaldehyde (H2CO) as an intermediate. Thermochemical properties and rate constants were obtained for gas-phase reactions using high-level electronic structure methods and chemical kinetic calculations, and the concentrations of the molecules were evolved along time. The chemical equilibrium was reached in minutes at 1500 K, a time interval much shorter than that required time for a protostar formation process. The results indicate that the formaldehyde and methanol abundances are always larger than those for glycolaldehyde, for example, at 2000 K and [H2]0 equals to 1023 molecule cm−3, the abundances of H2CO, CH3OH, and CH2OHCHO relative to H2 are equal to 3 × 10−6, 5 × 10−6, and 1 × 10−12, while for [H2]0 equals to 1020 molecule cm−3  these abundances are 3 × 10−9, 5 × 10−12, and 2 × 10−21, respectively. Considering that our results can be applied to explain the proximity of methanol and formaldehyde maser emissions, from the whole set of results, the CH3OH abundance relative to H2CO ranges from 10−3 to 102. [ABSTRACT FROM AUTHOR]

The HCO+ and DCO+ molecules are commonly used as tracers in the interstellar medium. Therefore, accurate rotational rate coefficients of these systems with He and H2 are crucial in non-local thermal equilibrium models. We determine in this work the rotational de-excitation rate coefficients of HCO+ in collision with both para- and ortho- H2, and also analyse the isotopic effects by studying the case of DCO+. A new four-dimensional potential energy surface from ab initio calculations was developed for the HCO+–H2 system, and adapted to the DCO+–H2 case. These surfaces are then employed in close-coupling calculations to determine the rotational de-excitation cross-sections and rate coefficients for the lower rotational states of HCO+ and DCO+. The new rate coefficients for HCO+ +  para- H2 were compared with the available data, and a set of rate coefficients for HCO+ +  ortho- H2 is also reported. The difference between the collision rates with ortho- and para- H2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO+ +  para- H2 for estimating those for HCO+ +  ortho- H2 as well as for DCO+ +  para- H2 is a good approximation. [ABSTRACT FROM AUTHOR]