Search

We have computed cross-sections for elastic and inelastic scattering of electrons on small grains at low-collision energies. Significant differences are again found between the results obtained in the presence and the absence of a 'permanent' grain dipole moment. In addition to spherical grains, scattering on ellipsoidal grains is investigated. We conclude that the rate of electron attachment to interstellar grains may be substantially lower in regions of molecular clouds from which the radiation field is excluded, and where the grains are less likely to possess a dipole moment. [ABSTRACT FROM AUTHOR]

The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different nuclear-spin states of the hydrides of carbon, nitrogen, oxygen, and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H2O+ and H3O+) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a pre-stellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry, and cosmic ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below ∼30 K. The OPR of H2O is shown to lie between 1.5 and 2.6, depending on the spin state of H2, in good agreement with values derived in translucent clouds with relatively high extinction. In the pre-stellar core-collapse calculations, the OPR of H2O is shown to reach the statistical value of 3 in regions with severe depletion (n H > 107 cm−3). We conclude that a low water OPR (≲ 2.5) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H2) ≲ 1. Available OPR measurements in protoplanetary discs and comets are finally discussed. [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]

ABSTRACT Cross-sections and rate coefficients have been calculated for the rovibrational excitation of CO by ortho- and para-H2 and by He. We adopted the quantal coupled channels method of solving the scattering equations, including CO rotational levels in the v = 0, 1 and 2 vibrational manifolds. Comparison is made with the results of previous calculations, which employed different interaction potentials and less accurate approximations to the collision dynamics. We consider that the present calculations yield reliable values of the rate coefficients for rovibrational excitation of CO at kinetic temperatures T ≈ 103 K. [ABSTRACT FROM AUTHOR]

ABSTRACT We have developed further the technique of time-dependent modelling of magnetohydrodynamic shock waves, with a view to interpreting the molecular line emission from outflow sources. The extensively observed source L1157 B1 was chosen as an exemplar of the application of this technique. The dynamical age of the shock wave model was varied in the range 500 ≤ t≤ 5000 yr, with the best fit to the observed line intensities being obtained for t= 1000 yr; this is of the same order as the dynamical age derived by Gueth, Guilloteau & Bachiller from their observations of L1157 B1. The emission line spectra of H2, CO, SiO, ortho- and para-H2O, ortho- and para-NH3, and A- and E-type CH3OH were calculated in parallel with the dynamical and chemical parameters of the model, using the 'large velocity gradient' (LVG) approximation to the line transfer problem. We compared the predictions of the models with the observed intensities of emission lines of H2, CO, SiO, ortho-H2O, ortho-NH3 and CH3OH, which include recent Herschel satellite measurements. In the case of SiO, we show (in Appendix A) that extrapolations of the collisional rate coefficients beyond the range of kinetic temperature for which they were originally calculated lead to spurious rotational line intensities and profiles. The computed emission-line spectra of SiO, NH3 and CH3OH are shown to depend on the assumed initial composition of the grain mantles, from whence they are released, by sputtering in the shock wave, into the gas phase. The dependence of the model predictions on the adopted form of the grain-size distribution is investigated in Appendix B; the corresponding integral line intensities are given in tabular form, for a range of C-type shock speeds, in the online Supporting Information. [ABSTRACT FROM AUTHOR]

We report the first calculations of the spectrum of methanol, arising in shock waves in molecular outflows. The small grid of shock wave models that we have computed incorporates the results of very recent computations of the rate coefficients for the collisional excitation of methanol by ortho- and para-H and by He. The two strongest transitions, one of A- and the other of E-type methanol, are masers that have been observed in a Class I methanol maser source, which is believed to be related to a molecular outflow. The same collisional propensities that give rise to population inversion and maser action can, in other transitions, lead to population anti-inversion and the lines appearing in absorption against the cosmic background radiation. We attempted to model specifically the outflow source L1157 B1, in which transitions of methanol have been observed recently by means of the Herschel satellite. Comparison with the predictions of the shock wave models is complicated by uncertainty in the value of the beam filling factor that should be adopted. [ABSTRACT FROM AUTHOR]

We have computed cross-sections and rate coefficients for the rotational excitation of A- and E-type methanol by molecular hydrogen. Calculations were performed for rotational transitions within the torsional ground state, , and within the first and second excited torsional states, and . For collisions of methanol with para-H2 in its rotational ground state, , the methanol basis included rotational states , thereby extending previous calculations, which included states only. For the first time, calculations have also been performed for ortho-H2 in its rotational ground state, , although it was necessary to revert to the smaller methanol basis, , owing to the coupling to states of molecular hydrogen with . The coupled states approximation was used in the production calculations, to generate the thermal rate coefficients at temperatures , but some limited comparisons, at a few collision energies, of cross-sections obtained using the full coupled channels (CC) method have been made. The propensities of the collisions, with respect to changes in the rotational quantum number, j1, and its projection, K, on the symmetry axis of the methanol molecule, were investigated. There are qualitative differences between the K propensities for collisions with ortho- and para-H2, which relate to the fact that the inelastic cross-sections tend to be significantly larger when ortho-H2 is the perturber. [ABSTRACT FROM AUTHOR]

We have extended and improved previous calculations of cross-sections for the rotational excitation by helium of A- and E-type methanol in their torsional ground states. We increased the maximum value of the rotational quantum number, j, of the methanol molecule from , used in the previous calculations, to , thereby ensuring the completeness of the rotational energy levels and the corresponding basis of eigenfunctions up to the first threshold for exciting the internal torsional motion of the molecule. The representation of the rotational–torsional eigenfunctions of E-type methanol has been improved significantly. We compare the results of the present calculations, which were performed in both the coupled channels (CC) and the coupled states (CS) approximations, with the previous results. All the calculations exhibit the same collisional propensities, but there exist quantitative discrepancies, for individual transitions, between both the present and previous determinations and also between the present CC and CS results, notably for E-type methanol. Cross-sections have been generated on an energy grid that is adapted to the evaluation of the thermal rate coefficients at low temperatures ; these data are available online. [ABSTRACT FROM AUTHOR]

We have calculated emission by CO molecules from interstellar shock waves. Two approximations have been used to determine the population densities, , of the rotational levels, J: steady state and statistical equilibrium . A large-velocity-gradient approximation to the line-transfer problem was adopted in both cases. We find that there can be substantial differences between the values of the integrated rotational line intensities calculated in steady state and in the limit of statistical equilibrium. On the other hand, although CO can be the dominant coolant towards the rear of the cooling flow which follows the dynamical heating of the gas, the rate of cooling computed assuming statistical equilibrium is likely to be reasonably accurate, given that the limit of statistical equilibrium is approached in this region. [ABSTRACT FROM AUTHOR]

We report calculations of rate coefficients for the rovibrational excitation of H2 and HD by H. The calculations relate explicitly to non-reactive scattering, excluding the proton-exchange channels. For vibrationally inelastic transitions, the new rate coefficients at are larger than the results of earlier calculations in which the vibrational motion was treated approximately, by means of a simple harmonic oscillator model. As a result, much better agreement is obtained with the empirical estimates by Allers et al. of the rate coefficients for vibrational relaxation of the levels ( v, j) = (1, 3) and (2, 3) of H2. However, the effects of the new data on the results of illustrative astrophysical models – of a C-type shock wave and of the gravitational collapse of a condensation of the primordial gas – are less pronounced than the changes to the rate coefficients for vibrationally inelastic transitions might suggest; we explain why this is the case. [ABSTRACT FROM AUTHOR]

We report calculations of the intensities of rovibrational transitions of H[sub 2] emitted from C-type shock waves propagating in molecular gas. Attention was paid to the thermal balance of the gas and to the rates of collisional dissociation and ionization of H[sub 2]. We found that the maximum shock speeds which can be attained, prior to the collisional dissociation of H[sub 2] (which results in a sonic point in the flow and hence a J-type shock wave), can be much higher than had previously been believed. Thus, adopting the ‘standard’ scaling of the transverse magnetic induction with the gas density, B(μG)=[n[sub H](cm[sup -3])][sup 1/2] , we established that the maximum shock speed increased from 20–30 km s[sup -1] at high pre-shock densities (n[sub H]≥10[sup 6] cm[sup -3]) to 70–80 km s[sup -1] at low densities (n[sub H]≤10[sup 4] cm[sup -3]) . The critical shock speed, v[sub crit], also increases significantly with the transverse magnetic induction, B, at a given pre-shock gas density, n[sub H]. By way of an application of these results, we demonstrate that a two-component model, comprising shock waves with velocities v[sub s]=60 and 40 km s[sup -1], reproduces the column densities of H[sub 2] observed by ISO-SWS up to the highest level (possibly) detected, v=0 , J=27 , which lies 42 515 K above the ground state. We found no necessity to invoke mechanisms other than thermal collisional excitation in the gas phase; but the v=1 vibrational band remains less completely thermalized than is indicated by the observations. Fine structure transitions of atoms and ions were also considered. The intensity of the [Si i] 68.5 μm transition, observed by Gry et al. using ISO-LWS, is satisfactorily reproduced by the same model and may also originate in OMC-1, rather than Orion-KL as originally believed. The transitions of [Fe ii] and [S i] observed by Rosenthal et al. may also arise in the shock-heated gas. [ABSTRACT FROM AUTHOR]

ABSTRACTCross-sections and rate coefficients have been calculated for rotational transitions between levels J ≤ 20 of HCO+, induced by collisions with ground state (J = 0) para-H2. Results were obtained for temperatures in the range 10 ≤ T ≤ 400 K, extending previous work by Monteiro, who considered T ≤ 30 K and presented rate coefficients for J ≤ 4. Good agreement is obtained with the results of Monteiro at low temperatures. [ABSTRACT FROM AUTHOR]

ABSTRACTCross-sections and rate coefficients have been calculated for rotational transitions between levels J ≤ 20 of HCO+, induced by collisions with ground state (J = 0) para-H2. Results were obtained for temperatures in the range 10 ≤ T ≤ 400 K, extending previous work by Monteiro, who considered T ≤ 30 K and presented rate coefficients for J ≤ 4. Good agreement is obtained with the results of Monteiro at low temperatures. [ABSTRACT FROM AUTHOR]