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

4.2.2 Molecular Physics
The theoretical advances made by our group during the past four years have been mainly focused on low-energy
excitation processes involving small polyatomic molecules in collisions with H2 and electrons. In addition to their
astrophysical relevance, our works have provided significant advances of broad interest in molecular physics.
The next challenges we whish to address are relevant to higher temperature processes and larger molecules. This
evolution of our theoretical activity naturally follows the current trend of observing more and more complex
molecules in more and more “harsh” astronomical environments.
1. Ro-vibrational excitation A particular challenging issue of current collisional studies is the computation of
rovibrational rates for polyatomic species. These rates become astrophysically relevant at relatively high
temperature (typically above 300 K) where a large number of rovibrational quantum channels open up.
Thus, even for a light molecule like water, the calculation of rovibrational cross sections at the relevant
energies is currently computationally impossible at the close-coupling “exact” level of theory. Furthermore,
approximate quantum methods such as the widely used vibrational close-coupling rotational infiniteorder-sudden
(VCC-IOS) approximation are questionable for molecules with relatively large rotational
constants, as we have shown for water in a recent paper (Faure et al., JCP 2005). The reliability of
standard quantum/classical approximations must therefore be assessed and controlled in a number of
representative cases. In this context, we note that H2O and HC3N are two very interesting candidates as
they present both very different rotational constants and vibrational frequencies. As a result, our next
short-term theoretical efforts will be put on the treatment of the rovibrational dynamics of these and other
larger systems.
In addition to these developments, the relevance of rovibrational processes in actual astronomical sources
must also be investigated. Our preliminary results for H2O−H2 (Faure et al. 2005) have thus shown that
the collisional excitation of the bending mode of water should be efficient only for densities larger than
about 10 10 cm 3 s −1 . Such high densities are found for example in the envelopes of late-type stars. For other
molecules with low vibrational frequencies, however, critical densities should be significantly lower. As a
first step, qualitative investigations based on the expected orders of magnitude should thus help to clarify
the relevance of the various collisional processes in environments of moderate to high temperatures.
2. Heterogeneous reactivity In spite of numerous recent experimental and theoretical studies, grain surface
chemistry is still far more poorly understood than gas-phase chemistry, notably because of our lack of a detailed
knowledge of the physical nature of the surface. The two major problems faced by astrochemists are
currently: (i) the detailed mechanisms for the sticking/reaction/desorption processes on low-temperature
interstellar surfaces and (ii) the dependence of these processes on the morphology and structure of the
grains. Given the high dimensionality of these dynamical problems, classical dynamics simulations are
particularly suited. Quantum effects such as tunneling under diffusive barriers might however prove crucial
for some reactive processes at very low temperature. Inclusion of “local” quantum effects in large-scale
classical simulations is thus a major issue we wish to address in the upcoming years. The specific astrophysical
problems we wish to tackle are directly related to recent observational results of our group: (i)
how hydrogenated molecules (H2O, H2CO, CH3OH, etc.) form on grain surfaces? (ii) how different is
it for deuterated species? (iii) what processes govern the depletion of molecules like CO and N2 in cold
pre-stellar cores?
In addition to these theoretical developments, the possibility of carrying out original experiments on
interstellar ice analogs has been recently discussed with Bernard Schmitt and Eric Quirico, specialists of the
synthesis, characterisation and properties of ices of planetary interest at the Laboratoire de Planétologie
de Grenoble. A collaborative project might be structured around the sticking and desorption processes at
work in the cold interstellar medium.
3. Towards larger molecules : far infra-red or micro-wave ? While the observation of small molecules, 2
to 6–10 atoms is now routine (from OH to (CH3)2O or HC7N) there is a natural tendency to look for
larger molecules, comprising cycles or several functionalities. The obvious goal is to get nearer and nearer
to molecules of great chemical importance (like 5-membered or 6-membered heterocycle) or molecules
pertain to biochemistry, like sugars, amino-acids, . . . Observing these molecules is rather difficult today,
for several, not mutually exclusive, reasons: (i) Evidently their abundances are small (ii) Being heavy
molecules, their rotational constants are small and they are usually asymmetric tops; consequently their
rotational lines for moderate J lie very low in frequency in domains that may be not so well explored
(iii) The oscillator strength is divided into many low frequency lines, rendering the observations all the
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