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Accurate quantum chemical studies at the CCSD(T)/CBS//CCSD/cc-pVTZ level predicted the depletion reaction of C3O by both singlet and triplet O-atoms to be barrierless, leading to the astrophysically very abundant CO plus triplet CCO. The barrierless nature of the reaction fully complied with the long conjecture, whereas the product differed significantly. Our kinetic calculations indicated that the reaction possesses significant negative temperature effect below 30 K. The calculations should be useful for understanding the astrophysical recycling for both the carbon and oxygen species. [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]

The adsorption of H atoms and their recombination to form an H2 molecule on slab models of the crystalline Mg2SiO4 forsterite (001) and (110) surfaces was studied by means of quantum mechanical calculations based on periodic density functional theory (DFT). Present results are compared with those previously reported for the most stable (010) surface, showing the relevance of the surface morphology and their stability on the H2 formation. Different H chemisorption states were identified, mostly on the outermost O atoms of the surfaces. In agreement with the higher instability of the (001) and (110) surfaces, the calculated adsorption energies are larger than those for the (010) surface. Computed energy barriers for the H hopping on these surfaces are exceedingly high to occur at the very low temperatures of deep space. For the adsorption of two H atoms, the most stable complexes are those in which the H atoms form Mg-H and SiOH surface groups. From these complexes, we did not identify energetically feasible paths for H2 formation through a Langmuir-Hinshelwood mechanism on the (001) surface because the initial states are more stable than the final products. However, on the (110) surface one path was found to be exoergic with very low energy barriers. This differs to that observed for the (010) surface, for which two feasible Langmuir-Hinshelwoodbased channels were identified. H2 formation through the Eley-Rideal mechanism was also simulated, in which an incoming H atom impinges on a pre-adsorbed H atom at the (001) and (110) surfaces in a barrierless way. [ABSTRACT FROM AUTHOR]

We report nearly exact quantum time-independent calculations of rate coefficients for the collisional (de-)excitation of H2 by H, from low to high temperatures. Our calculations are based on a highly accurate global potential energy surface. The reactive hydrogen exchange channels are taken into account rigorously. New collisional data are obtained for the rovibrational relaxation of highly excited H2 (with internal excitation up to ≃22 000 K) for temperatures ranging from 100 to 5000 K. We also provide a comparison with the available experimental rate coefficients at room temperature. The good agreement between theory and experiment is an illustration of the accuracy of the present calculations. The new results significantly differ from previous data presently used in astrophysical models, especially at low temperatures, the difference being essentially due to the inclusion of the reactive channels. The impact of these new data in astrophysics is discussed. In particular, the coolingmechanism will have to be reviewed for several astrophysical media. [ABSTRACT FROM AUTHOR]

Recent quantum mechanical calculations of rate coefficients for collisional transfer of population between the hyperfine states of 13CN enable their population densities to be determined. We have computed the relative populations of the hyperfine states of the N = 0, 1, 2 rotational states for kinetic temperatures 5 ≤ T ≤ 20 K and molecular hydrogen densities 1 ≤ n(H2) ≤1010 cm-3. Spontaneous and induced radiative transitions were taken into account. Our calculations show that, if the lines are optically thin, the populations of the hyperfine states, F, within a given rotational manifold are proportional to their statistical weights, (2F + 1) - i.e. in local thermodynamic equilibrium - over the entire range of densities. We have re-analysed IRAM 30 m telescope observations of 13CN hyperfine transitions (N = 1 → 0) in four starless cores. A comparison of these observations with our calculations confirms that the hyperfine states are statistically populated in these sources. [ABSTRACT FROM AUTHOR]

Irradiation of samples of pure solid methane at 3Kwith far-ultraviolet light from a synchrotron yielded products CH3, C2H2, C2H4 and C2H6 that were quantitatively identified through their infrared absorption spectra. The greatest wavelengths at which products CH3, C2H4, C2H6 and C2H2 were generated were 140, 140, 175 and 190 nm, respectively. Cassini spacecraft has observed the condensed methane on Titan. The observation demonstrates the existence of pure condensed methane in astro-environments. Information about the dissociation of CH4 at a low temperature with photons of varied energy has implications for astrophysical environments. [ABSTRACT FROM AUTHOR]

We report laboratory measurements of the absorption coefficient of solid para-H2, within the wavelength range from 1 to 16.7 μm, at high spectral resolution. In addition to the narrow rovibrational lines of H2 which are familiar from gas-phase spectroscopy, the data manifest double transitions and broad phonon branches that are characteristic specifically of hydrogen in the solid phase. These transitions are of interest because they provide a spectral signature which is independent of the impurity content of the matrix. We have used our data, in combination with a model of the ultraviolet absorptions of the H2 molecule, to construct the dielectric function of solid para-H2 over a broad range of frequencies. Our results will be useful in determining the electromagnetic response of small particles of solid hydrogen. The dielectric function makes it clear that pure H2 dust would contribute to infrared (IR) extinction predominantly by scattering starlight, rather than absorbing it, and the characteristic IR absorption spectrum of the hydrogen matrix itself will be difficult to observe. [ABSTRACT FROM AUTHOR]

ABSTRACT It is commonly believed that the earliest stages of star formation in the Universe were self-regulated by global radiation backgrounds - either by the ultraviolet (UV) Lyman-Werner (LW) photons emitted by the first stars (directly photodissociating H2), or by the X-rays produced by accretion on to the black hole (BH) remnants of these stars (heating the gas but catalysing H2 formation). Recent studies have suggested that a significant fraction of the first stars may have had low masses (a few M⊙). Such stars do not leave BH remnants and they have softer spectra, with copious infrared (IR) radiation at photon energies ∼1 eV. Similar to LW and X-ray photons, these photons have a mean-free path comparable to the Hubble distance, building up an early IR background. Here we show that if soft-spectrum stars, with masses of a few M⊙, contributed ≳0.3 per cent of the UV background (or their mass fraction exceeded ∼80 per cent), then their IR radiation dominated radiative feedback in the early Universe. The feedback is different from the UV feedback from high-mass stars, and occurs through the photodetachment of H− ions, necessary for efficient H2 formation. Nevertheless, we find that the baryon fraction which must be incorporated into low-mass stars in order to suppress H2 cooling is only a factor of a few higher than for high-mass stars. [ABSTRACT FROM AUTHOR]

H2 formation is an important process in post-shock regions, since H2 is an active participant in the cooling and shielding of the environment. The onset of H2 formation therefore has a strong effect on the temperature and chemical evolution in the post-shock regions. We recently developed a model for H2 formation on a graphite surface in warm conditions. The graphite surface acts as a model system for grains containing large areas of polycyclic aromatic hydrocarbon structures. Here, this model is used to obtain a new description of the H2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H2 formation rate is substantially higher at high gas temperatures as compared to the original implementation of this rate in shock models, because of the introduction of H atoms which are chemically bonded to the grain (chemisorption). Since H2 plays such a key role in the cooling, the increased rate is found to have a substantial effect on the predicted line fluxes of an important coolant in dissociative shocks [O i] at 63.2 and 145.5 μm. With the new model, a better agreement between the model and observations is obtained. Since one of the goals of Herschel/PACS will be to observe these lines with higher spatial resolution and sensitivity than the former observations by Infrared Space Observatory-LWS, this more accurate model is very timely to help with the interpretation of these future results. [ABSTRACT FROM AUTHOR]

We report the first results of Time-dependent Density Functional Theory calculations of electronic transitions of protonated stable neutral polycyclic aromatic hydrocarbon (PAH) molecules. Unlike most neutral closed-shell PAHs which generally absorb strongly in the ultraviolet (UV) or near-UV, the corresponding isoelectronic protonated forms of PAH molecules such as coronene and ovalene are predicted to have absorptions in the visible spectral region. Combining this with other astrophysical, chemical and physical aspects, it is concluded that the protonated forms of PAHs represent a strong class of candidate carriers of diffuse interstellar bands. [ABSTRACT FROM AUTHOR]

We report new calculations of interstellar 15N-fractionation. Previously, we have shown that large enhancements of 15N/14N can occur in cold, dense gas where CO is frozen out, but that the existence of an NH + N channel in the dissociative recombination of N2H+ severely curtails the fractionation. In the light of recent experimental evidence that this channel is in fact negligible, we have reassessed the 15N chemistry in dense cloud cores. We consider the effects of temperatures below 10 K, and of the presence of large amounts of atomic nitrogen. We also show how the temporal evolution of gas-phase isotope ratios is preserved as spatial heterogeneity in ammonia ice mantles, as monolayers deposited at different times have different isotopic compositions. We demonstrate that the upper layers of this ice may have 15N/14N ratios an order of magnitude larger than the underlying elemental value. Converting our ratios to δ-values, we obtain δ15N > 3000 ‰ in the uppermost layer, with values as high as 10 000 ‰ in some models. We suggest that this material is the precursor to the 15N ‘hotspots’ recently discovered in meteorites and IDPs. [ABSTRACT FROM AUTHOR]

Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of cold dark molecular clouds, and especially prestellar cores. Modelling of CN emission spectra from these dense molecular clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. We calculate fine- and hyperfine-structure-resolved excitation rate coefficients of CN(X2Σ+) by para- and ortho-H2. The calculations are based on a new potential energy surface obtained recently from highly correlated ab initio calculations. State-to-state rate coefficients between fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100 K. The new results are compared to available CN rate coefficients. Significant differences are found between the different sets of rate coefficients. This comparison shows that the new CN–H2 rate coefficients have to be used for observations interpretations. We expect that their use will help significantly to have a new insight into the physical conditions of prestellar cores. [ABSTRACT FROM AUTHOR]

The early evolution of dense star clusters is possibly dominated by close interactions between stars, and physical collisions between stars may occur quite frequently. Simulating a stellar collision event can be an intensive numerical task, as detailed calculations of this process require hydrodynamic simulations in three dimensions. We present a computationally inexpensive method in which we approximate the merger process, including shock heating, hydrodynamic mixing and mass loss, with a simple algorithm based on conservation laws and a basic qualitative understanding of the hydrodynamics of stellar mergers. The algorithm relies on Archimedes' principle to dictate the distribution of the fluid in the stable equilibrium situation. We calibrate and apply the method to mergers of massive stars, as these are expected to occur in young and dense star clusters. We find that without the effects of microscopic mixing, the temperature and chemical composition profiles in a collision product can become double-valued functions of enclosed mass. Such an unphysical situation is mended by simulating microscopic mixing as a post-collision effect. In this way we find that head-on collisions between stars of the same spectral type result in substantial mixing, while mergers between stars of different spectral type, such as type B and O stars (∼10 and respectively), are subject to relatively little hydrodynamic mixing. Our algorithm has been implemented in an easy-to-use software package, which we have made publicly available for download. 1 [ABSTRACT FROM AUTHOR]