Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

quantum VCC-IOS approximation may fail to properly describe the H2 O quenching. This stresses the need
to develop more advanced approximation schemes to describe the ro-vibrational collisional processes involving
molecules with large ∆J splittings such as water, ammonia, etc...
HC3N – H2
The “steric hindrance” problem solved, first quantum inelastic rates obtained.
Two new PES have been calculated at a CCSD(T) level for the HC3N–He and HC3N–H2 systems by P. Valiron.
The new HC3N–He agrees pretty well with the PES obtained by Akin-Ojo et al 2003. Pionnering CCSD(T)-R12
calculations with 920 basis set orbitals and 10 occupied (with frozen core) permitted to assess the accuracy of
this PES. Steric hindrance problems involving the HC3N rod limit the convergence of the angular expansion
of the PES, as anticipated by Green & Chapman (1978 ApJS 37, 169). However for the low energy regime
M. Wernli showned it is feasible to regularize the PES by smoothing out the repulsive walls and to achieve a
perfectly converged angular expansion.
Corresponding close-coupling calculations led to surprising results due to the rod-like features of the PES. Firstly
quantum interferences strongly defavour odd ∆j transitions and favour even ∆j ones. This propensity rule is
likely to favour the J=1 population for H2 densities in the 10 3 –10 4 cm −3 range. Secondly, despite the very large
HC3N dipole moment, the para-H2 and ortho-H2 rates are nearly identical. While the even ∆j propensity rule
could not be found in the quasiclassical calculations by Green & Chapman (1978 ApJS 37, 169), the new rates
remain within the same order of magnitude despite the very crude an electron gas model PES. This is not too
surprising as the rod-like features of the PES dominate the scattering.
3.3.3 Energy transfer processes
Energy exchange processes between molecules, atoms and particles are responsible for thermal balance and line
formation in a great variety of astronomical environments. Molecular line emissions are generally produced in
low-temperature (T < 1000 K) and low-density (n < 10 10 cm 3 s −1 ) conditions far from thermodynamic equilibrium
and through a complex competition between radiative and collisional processes. A detailed knowledge of
rate constants for all microscopic processes that drive the populations of the emitting levels is thus necessary
to interpret the observed spectra. Despite some recent progress in laboratory measurements of state-to-state
collision rates, astrophysical models still rely heavily on theoretical predictions owing to the vast network of
relevant states which span a wide range of excitation energies. During the past four years, our group have made
significant advances in the understanding of molecule-molecule and electron-molecule collisions.
Molecule-molecule collisions
In standard molecular environments such as dense interstellar clouds or star-forming regions (T < 300 K,
n > 10 4 cm 3 s −1 ), hydrogen molecules H2 are the dominant exciting species. We have recently revisited the
collisional excitation of three astronomically very important molecules: CO, H2O and HC3N in collisions with
H2. As described previously, the H2O−H2 and HC3N−H2 PES have been computed by our group using high
accuracy ab initio methods. For CO-H2, we employed a very recent PES obtained by Jankowski and Szalewicz
(2005). The accuracy of these three PES is similar and lies within a few cm −1 in the relevant regions of the
PES. Such a precision is necessary at low temperatures (T < 50 K) to guarantee that inaccuracies in the PES
represent only a small fraction of collision energies. Our main results for the above mentioned systems are
summarized below:
• Rotational excitation of CO by para- and ortho-H2
Quantum close-coupling calculations were performed for rotational levels of CO up to 5 and temperatures
in the range 5−70 K (Wernli et al 2005). Our results were compared with those obtained by Flower
(2001) on a previous, less accurate, PES. The new rigid-rotor CO−H2 PES of Jankowski and Szalewicz
(2005) was thus shown to strongly affect the resonance structure of the rotational cross sections at very
low collision energies (E < 60 cm −1 ). As a result, the calculated rates at 10 K were found to differ by up
to 50% with those obtained by Flower (2001). Conversely, at temperatures larger than about 70 K, the
effect of the new PES was found to be only minor.
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