Mixed Quantum/Classical Approach for Description of Molecular Collisions in Astrophysical Environments

An efficient and accurate mixed quantum/classical theory approach for computational treatment of inelastic scattering is extended to describe collision of an atom with a general asymmetric-top rotor polyatomic molecule. Quantum mechanics, employed to describe transitions between the internal states of the molecule, and classical mechanics, employed for description of scattering of the atom, are used in a self-consistent manner. Such calculations for rotational excitation of HCOOCH3 in collisions with He produce accurate results at scattering energies above 15 cm–1, although resonances near threshold, below 5 cm–1, cannot be reproduced. Importantly, the method remains computationally affordable at high scattering energies (here up to 1000 cm–1), which enables calculations for larger molecules and at higher collision energies than was possible previously with the standard full-quantum approach. Theoretical prediction of inelastic cross sections for a number of complex organic molecules observed in space becomes feasible using this new computational tool

Water and methanol in low-mass protostellar outflows: gas-phase synthesis, ice sputtering and destruction

Water in outflows from protostars originates either as a result of gas-phase synthesis from atomic oxygen at T ≳ 200 K, or from sputtered ice mantles containing water ice. We aim to quantify the contribution of the two mechanisms that lead to water in outflows, by comparing observations of gas-phase water to methanol (a grain surface product) towards three low-mass protostars in NGC 1333. In doing so, we also quantify the amount of methanol destroyed in outflows. To do this, we make use of James Clerk Maxwell Telescope and Herschel-Heterodyne Instrument for the Far-Infrared data of H2O, CH3OH and CO emission lines and compare them to RADEX non-local thermodynamic equilibrium excitation simulations. We find up to one order of magnitude decrease in the column density ratio of CH3OH over H2O as the velocity increases in the line wings up to ∼15 km s−1. An independent decrease in X(CH3OH) with respect to CO of up to one order of magnitude is also found in these objects. We conclude that gas-phase formation of H2O must be active at high velocities (above 10 km s−1 relative to the source velocity) to re-form the water destroyed during sputtering. In addition, the transition from sputtered water at low velocities to form water at high velocities must be gradual. We place an upper limit of two orders of magnitude on the destruction of methanol by sputtering effects

Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes

We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re

Linking ice and gas in the λ Orionis Barnard 35A cloud

Context. Dust grains play an important role in the synthesis of molecules in the interstellar medium, from the simplest species, such as H2, to complex organic molecules. How some of these solid-state molecules are converted into gas-phase species is still a matter of debate. Aims. Our aim is to directly compare ice and gas abundances of methanol (CH3OH) and carbon monoxide (CO) obtained from near-infrared (2.5-5 μm) and millimetre (1.3 mm) observations and to investigate the relationship between ice, dust, and gas in low-mass protostellar envelopes. Methods. We present Submillimeter Array (SMA) and Atacama Pathfinder EXperiment (APEX) observations of gas-phase CH3OH (JK = 5K-4K), 13CO, and C18O (J = 2-1) towards the multiple protostellar system IRAS 05417+0907, which is located in the B35A cloud, λ Orionis region. We use archival IRAM 30 m data and AKARI H2O, CO, and CH3OH ice observations towards the same target to compare ice and gas abundances and directly calculate CH3OH and CO gas-to-ice ratios. Results. The CO isotopologue emissions are extended, whereas the CH3OH emission is compact and traces the giant molecular outflow emanating from IRAS 05417+0907. A discrepancy between sub-millimetre dust emission and H2O ice column density is found for B35A-4 and B35A-5, similar to what has previously been reported. B35A-2 and B35A-3 are located where the sub-millimetre dust emission peaks and show H2O column densities lower than that of B35A-4. Conclusions. The difference between the sub-millimetre continuum emission and the infrared H2O ice observations suggests that the distributions of dust and H2O ice differ around the young stellar objects in this dense cloud. The reason for this may be that the four sources are located in different environments resolved by the interferometric observations: B35A-2, B35A-3, and, in particular, B35A-5 are situated in a shocked region that is plausibly affected by sputtering and heating, which in turn impacts the sub-millimetre dust emission pattern, while B35A-4 is situated in a more quiescent part of the cloud. Gas and ice maps are essential for connecting small-scale variations in the ice composition with the large-scale astrophysical phenomena probed by gas observations

The chemical structure of the Class 0 protostellar envelope NGC 1333 IRAS 4A

Context. It is not well known what drives the chemistry of a protostellar envelope, in particular the role of the stellar mass and the protostellar outflows on the chemical enrichment of such environments. Aims. We study the chemical structure of the Class 0 protostellar envelope NGC 1333 IRAS 4A in order to (i) investigate the influence of the outflows on the chemistry; (ii) constrain the age of our studied object; (iii) compare it with a typical high–mass protostellar envelope. Methods. In our analysis we use JCMT line mapping (360–373 GHz) and HIFI pointed spectra (626.01–721.48 GHz). 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 Monte Carlo radiative transfer code (RATRAN) and adopting a 1D dust density/temperature profile from the literature. We use a combination of constant abundance profiles and abundance profiles that include jumps at two radii (T ~ 100 K or T ~ 30 K) to fit our observations. We compare our best–fit observed abundance profiles with the predictions from the time dependent gas grain chemical code (ALCHEMIC). Results. We detect CO, 13CO, C18O, CS, HCN, HCO+, N2H+, H2CO, CH3OH, H2O, H2S, DCO+, HDCO, D2CO, SO, SO2, SiO, HNC, CN, C2H and OCS. We divide the detected lines in three groups based on their line profiles: a) broad emission (FWHM = 4–11 km s-1), b) narrow emission (FWHM< 4 km s-1), and c) showing absorption features. The broad component is indicative of outflow activity, the narrow component arises from dynamically quiescent gas (i.e. envelope) and the absorption is a result of infall motions or the presence of foreground material. Our maps provide information about the spatial and velocity structure of many of the molecules mentioned above, including the deuterated species, making it possible to distinguish between envelope and outflow structures also spatially. The derived abundance profiles are based only on the narrow component (envelope) of the species and are reproduced by a 1D pseudo-time-dependent gas-grain chemical model for the outer envelope, with the exceptions of HCN, HNC, CN. These species along with the CO abundance require an enhanced UV field which points towards an outflow cavity. The abundances with respect to 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 when they are estimated with respect to CO. Differences in UV radiation intensity may also be responsible for such chemical differentiation, but temperature differences seem a more plausible explanation, especially the absence of a freeze–out zone in the high mass case. The CH3OH modeled abundance profile points towards an age of ≥4 × 104 yr for IRAS 4A. The spatial distribution of H2D+ differs from that of other deuterated species (i.e. DCO+, HDCO and D2CO), indicating an origin from a colder layer (<20 K) in the foreground, which is not seen in any other tracer. Conclusions. The observed abundances can be explained by passive heating towards the high mass protostellar envelope, while the presence of UV cavity channels become more important toward the low mass protostellar envelope (e.g. CO, HCO+)

Quantum mechanical and astrophysical studies of methanol

Interstellar methanol has been observed extensively at radio frequencies from the ground, and, very recently, it has been observed at higher frequencies, by means of the Herschel satellite. Being a complex molecule, methanol has a rich spectrum exhibiting rotational and internal torsional motions. However, by the same token, the determination of the cross sections and rate coefficients for the excitation of methanol by the principal perturbers, helium and molecular hydrogen, is a far from trivial task. We have recently extended and considerably improved previous calculations of these data. In the case of molecular hydrogen, results are now available for the excitation of both A- and E-type methanol, not only by para- but also by ortho-H2. These data have been used to model the HIFI observations of the outflow source L1157 B1. The methanol emission is computed self-consistently, in parallel with the dynamics and the chemistry, allowing for the optical depths in the emission lines by means of the LVG approximation. The results of these calculations are summarized

Theoretical study of the low-lying adiabatic states of the NaLi + molecular ion

International audienceWith one active electron, the NaLi+ cation is a relatively simple system to study, and a good candidate for which results issue from different approaches can be compared to cross check the reliability of different theoretical methods for the calculation of the adiabatic potential energies. However, comparison of the ab initio results of Berriche (2003) and those of Magnier and Frécon (2001), employing model potential methods, is showing a serious disagreement concerning several molecular states. In particular, the low-lying states and obtained by Magnier and Frécon, are found to be repulsive whereas they are attractive when ab initio methods are used

The low-lying adiabatic states of the K + 2 alkali dimer

International audienceDespite the simplicity of the K + 2 alkali dimer with a single active electron, comparison of the ab-initio results of Berriche et al. [1] with those of Magnier and Frécon [2] based on a model potential approach reveals a number of serious disagreement concerning several excited states. In particular, the 5 2 Σ + u , 6 2 Σ + g , 6 2 Σ + u , 7 2 Σ + g , 7 2 Σ + u , 3 2 Π g , 4 2 Π g and 2 2 ∆ u states which are found to be repulsive by Magnier and Frécon, but attractive when ab-initio techniques are employed. To clarify the origin of this disagreement, the adiabatic energies and spectroscopic constants are re-computed for the low-lying states of the K + 2 alkali dimer within a model potential framework. Contrary to the claims of Magnier and Frécon, the new results based on a model potential approach agree well with the ab-initio ones

Extension de la méthode du potentiel modèle pour traiter la dynamique des systèmes moléculaires à couches ouvertes (applications : au transfert de charge dans les collisions entre Si3+ et He et entre He2+ et He métastable, à la détermination des potentiels adiabatiques Li2)

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Validity of the Langevin capture model for charge exchange processes at thermal energies

International audienceA detailed coupled state quantum mechanical calculation using an adiabatic basis is analysed to investigate the validity of the Langevin model for very low energy charge transfer processes in ion-atom collisions. Taking as an example the N3+/H system, it is shown that the success of the Langevin model in describing the energy variation of the cross section is due to the accuracy of the phase averaging procedure in the double passage through the avoided crossing. On the other hand, the model does not yield the correct isotopic dependence of the cross sections. These can only be accurately determined by a coupled state calculation. At low thermal and sub-thermal energies less than 1 meV, the phase averaging procedure breaks down and quantum tunnelling effects become important. In these conditions, the energy variation of the cross section exhibits an appreciable departure from the Langevin model