Laurent Wiesenfeld

Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2

Context. The interpretation of water line emission from existing observations and future HIFI/Herschel data requires a detailed knowledge of collisional rate coefficients. Among all relevant collisional mechanisms, the rotational (de)excitation of H 2 O by H 2 molecules is the process of most interest in interstellar space. Aims. To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H 2 O colliding with both para and ortho-H 2 in the temperature range 20-2000 K. Methods. Rate coefficients are calculated on a recent high-accuracy H 2 O-H 2 potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H 2 molecules are assumed to be rotationally thermalized at the kinetic temperature. Results. By comparison with quantum calculations available for low lying levels, classical rates are found to be accurate within a factor of 1-3 for the dominant transitions, that is those with rates larger than a few 10 -12 cm 3 s -1 . Large velocity gradient modelling shows that the new rates have a significant impact on emission line fluxes and that they should be adopted in any detailed population model of water in warm and hot environments.

R12-calibrated H2O–H2 interaction: Full dimensional and vibrationally averaged potential energy surfaces

The potential energy surface of H(2)O-H(2) is of great importance for quantum chemistry as a test case for H(2)O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H(2)O-H(2) by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set. The resulting surface was further calibrated using high precision explicitly correlated CCSD(T)-R12 calculations on a subset of the rigid-rotor intermolecular geometries. The vibrationally averaged potential is presented in some details and is compared with the most recent rigid-rotor calculations. We explain, in particular, as to why vibrationally averaged rigid-rotor geometries are a better choice than equilibrium geometries. Our fit of the vibrationally averaged surface provides for the first time an accuracy of approximately 3 cm(-1) in the van der Waals minimum region of the interaction. The overall accuracy of the nine-dimensional surface and fit is lower but remains of the order of 3%-4% of the anisotropy in the domain spanned by the vibrational functions.

Improved low-temperature rate constants for rotational excitation of CO by H 2

Cross sections for the rotational (de)excitation of CO by ground state para- and ortho-H 2 are obtained using quantum scattering calculations for collision energies between 1 and 520 cm -1 . A new CO-H 2 potential energy surface is employed and its quality is assessed by comparison with explicitly correlated CCSD(T)-R12 calculations. Rate constants for rotational levels of CO up to 5 and temperatures in the range 5-70 K are deduced. The new potential is found to have a strong influence on the resonance structure of the cross sections at very low collision energies. As a result, the present rates at 10 K differ by up to 50% with those obtained by Flower (2001) on a previous, less accurate, potential energy surface.

A full nine-dimensional potential-energy surface for hydrogen molecule-water collisions

The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star-forming regions. We report here a full nine-dimensional interaction potential for H2O-H2 calibrated using high-accuracy, explicitly correlated wave functions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the upsilon2 bending mode of H2O obtained from quasiclassical trajectory calculations in the temperature range of 500-4000 K. Our high-temperature (T > or = 1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough reinterpretation of vibrationally excited water emission spectra from space.

On the robustness of the ammonia thermometer

Ammonia inversion lines are often used as probes of the physical conditions in the dense interstellar medium. The excitation temperature between the first two para-metastable (rotational) levels is an excellent probe of the gas kinetic temperature. However, the calibration of this ammonia thermometer depends on the accuracy of the collisional rates with H 2 . Here, we present new collisional rates for ortho- and para-NH 3 colliding with para-H 2 (J = 0), and investigate the effects of these new rates on the excitation of ammonia. Scattering calculations employ a new, high-accuracy, potential energy surface computed at the coupled-cluster CCSD(T) level with a basis set extrapolation procedure. Rates are obtained for all transitions involving ammonia levels with J ≤ 3 and for kinetic temperatures in the range 5-100 K. We find that the calibration curve of the ammonia thermometer - which relates the observed excitation temperature between the first two para-metastable levels to the gas kinetic temperature-does not change significantly when these new rates are used. Thus, the calibration of ammonia thermometer appears to be robust. Effects of the new rates on the excitation temperature of inversion and rotation-inversion transitions are also found to be small.

Influence of a new potential energy surface on the rotational (de)excitation of H

Aims.Using a newly determined 5D potential energy surface for H2-H2O we provide an extended and revised set of rate coefficients for de-excitation of the lowest 10 para- and 10 ortho- rotational levels of H2O by collisions with para-(j=0) and ortho-H2(j=1), for kinetic temperatures from 5 K to 20 K. Methods.Our close coupling scattering calculations involve a slightly improved set of coupled channels with respect to previous calculations. In addition, we discuss the influence of several features of this new 5D interaction on the rotational excitation cross sections. Results. The new interaction potential leads to significantly different rate coefficients for collisions with para-H2 (j=0). In particular the de-excitation rate coefficient for the 110 to 101 transition is increased by up to 300% at 5 K. At 20 K this increase is 75%. Rate coefficients for collisions with ortho-H2(j=1) are modified to a lesser extent, by up to 40%. The influence of the new potential on collisions with both para-(j=0) and ortho-H2(j=1) is expected to become less pronounced at higher temperatures.

_{\mathsf 2}

Aims: Rate coefficients for the rotational excitation of the ten lowest levels of ortho-H_2CO by collisions with H2 molecules are computed for kinetic temperatures in the range 5-100 K. Methods: Cross sections are obtained from extensive, fully converged, quantum-mechanical scattering calculations using a highly accurate potential energy surface computed at the CCSD(T) level with a basis set extrapolation procedure. Scattering calculations are carried out for H2 molecules in both para and ortho rotational levels. Results: The present rates are shown to differ significantly from those available in the literature. Moreover, the strength of propensity rules is found to depend on the para/ortho form of H2. Radiative transfer modeling also shows that the new rates have a significant impact on H_2CO emission line fluxes and that they should be adopted in any detailed radiative transfer model of ortho-H_2CO in cold environments (T ⪉ 30 K). This paper is dedicated to the memory of our friend and colleague, Pierre Valiron, who died on 31 August 2008. Table of rate coefficients is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/687

O by H

Context: The ortho-to-para ratio (OPR) of molecular hydrogen is a fundamental parameter in understanding the physics and chemistry of molecular clouds. In dark and cold regions, however, H2 is not directly observable and the OPR of H2 in these sources has so far remained elusive. Aims: We show that the 6 cm absorption line of ortho-formaldehyde (H2CO) can be employed to constrain both the density and the OPR of H2 in dark clouds. Methods: Green Bank Telescope (GBT) observations of ortho-H2CO toward the molecular cloud Barnard 68 (B68) are reported. Non-LTE radiative transfer calculations combined with the well-constrained structure of B68 are then employed to derive the physical conditions in the absorption region. Results: We provide the first firm confirmation of the Townes & Cheung mechanism: propensity rules for the collisions of H2CO with H2 molecules are responsible for the sub-2.7 K cooling of the 6 cm doublet. Non-LTE calculations show that in the absorption region of B68, the kinetic temperature is ˜ 10 K, the ortho-H2CO column density amounts to ˜ 2.2× 1013 cm-2, the H2 density is in the range 1.4{-}2.4× 10 4 cm-3, and the OPR of H2 is close to zero. Our observations thus provide fresh evidence that H2 is mostly in its para form in the cold gas, as expected from theoretical considerations. Our results also suggest that formaldehyde absorption originates in the edge of B68, at visual extinctions A_V⪉ 0.5 mag. This work has been inspired by our colleague and friend Pierre Valiron, who passed away in August 2008. This paper is dedicated to his memory.

_{\mathsf 2}

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star-forming regions, and in particular the Class 0 protostar IRAS 16293-2422. Aims. The CHESS (Chemical HErschel Surveys of Star forming regions) key program aims to study the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI instrument provide a unique opportunity to observe the fundamental 1(1,1)-0(0,0) transition of the ortho-D2O molecule, which is inaccessible from the ground, and determine the ortho-to-para D2O ratio. Methods. We detected the fundamental transition of the ortho-D2O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 +/- 0.11 (1 sigma). From the previous ground-based observations of the fundamental 1(1,0)-1(0,1) transition of para-D2O seen in absorption at 316.80 GHz, we estimate a line opacity of 0.26 +/- 0.05 (1 sigma). Results. We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho-to-para D2O ratio to be lower than 2.6 at a 3 sigma level of uncertainty, which should be compared with the thermal equilibrium value of 2:1.

at low temperature

We present collisional rate coefficients for SO2 with ortho and para molecular hydrogen for the physical conditions prevailing in dark molecular clouds. Rate coefficients for thefirst 31 rotational levels of this species (energies up to 55 K) and for temperatures between 5 and 30 K are provided. We have found that these rate coefficients are about ten times more than those previously computed for SO2 with helium. We calculated the expected emission of the centimeter wavelength lines of SO2. We find that the transition connecting the metastable 202 level with the 111 one is in absorption against the cosmic background for a wide range of densities. The 404−313 line is found to be inverted for densities below a few 10 4 cm −3 . We observed the 111−202 transition with the 100 m Green Bank Telescope towards some dark clouds. The line is observed, as expected, in absorption and provides an abundance of SO2 in these objects of af ew 10 −10 . The potential use of millimeter lines of SO2 as tracers of the physical conditions of dark clouds is discussed.