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

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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 68
more difficult (iv) The region where the highly excited J lines fall may be congested and these spectra are sometimes not known experimentally, at least with high precision at high J. Another path leading to the observation of large molecules comes from the far infrared spectral region (ν ∼ 0.5 · · · 2 THz, k ∼ 15 · · · 60 cm −1 ). This range of frequency corresponds to floppy bending modes or torsional modes. Some are experimentally known but most are not. The advantage of the THz modes s their higher frequency, leading to higher A Einstein coefficients, hence higher sensitivity. Also, it is possible that congestion problems would be less severe. These considerations remain however still speculative and necessitate a joint theoretical and experimental effort (the Soleil light source may be very valuable). A new generation of out of atmosphere telescopes, and principally Herschel (HIFI instrument) will open this spectral window. It is essential that our team takes part in the large molecule search that these instrument will allow. 4. Gas-phase reactivity: revisiting reaction rates Large models of gas phase chemistry have been only partially tested in their various aspects. Here we mention some major issues: • What is the relevance of present-day rates, is it worthwhile to compute some with a good precision? Many rates are guessed from analogous reactions. How important is it to know the rate evolution with temperature, in particular for neutral-neutral reactions. • What is the structural stability of the differential equation system that governs the evolution of species populations. Some work in this realm has been undertaken by V. Wakelam. • How interesting is it to simplify system and analyze some carefully chosen subsystem ? An interaction with specialists in dynamical systems theories would be particularly useful. In this context, Astromol (LW, PV, AF) received PCMI financial support for leading en effort in this field. 5. Transition state theory (TST) Many theoretical problems remain open, most of which with important applications in theoretical chemistry but also in celestial dynamics. In particular, progress must be made in TST for chemical reactions on surfaces or with many degrees of freedom. 4.3 The involvement in the large international projects: Herschel and ALMA Astromol is heavily involved in two big international projects, defined as major milestones for the French Astronomy: the European satellite Herschel Space Observatory (HSO) and the sub-millimeter interferometer ALMA. The two projects will start working in the next few years: Herschel in 2008, and ALMA soon after. Before the instruments become operative, Astromol is actively participating to and will continue to work on their preparation. More specifically, members of Astromol are official or unofficial co-Astronomers of the HIFI consortium. They are participating to the preparation of a Key Program of Herschel, entitled “Spectral Line Surveys of Star Forming Regions” (requiring about 300 hours of HIFI Guaranteed Time). The program consists in the observation of the entire (or most) spectral band of HIFI, at the highest possible spectral resolution (∼ 1 km/s), of a number of representative sources of star formation. The Key Program is leaded by a member of the Team (C.Ceccarelli). Table 4.1 summarizes the involvement of Astromol in this program. Note that C.Ceccarelli is also the leader of the “HIFI Star Formation Group”, which proposes three HIFI Key Programs: “The water in Star Forming Regions”, “Orion and SgrB2 Star Formation Regions with HIFI”, and the “‘Spectral Line Surveys of Star Forming Regions” already mentioned. After the launch of Herschel, Astromol will be evidently very active in the exploitation of the data. Indeed, we expect that this will be a major involvement for our group, and even a challenge, for the amount of expected data and information. To give an idea, based on the ground-based observations, we expect to detect around 10 lines per GHz ,in the Class 0 source IRAS16293-2422. On the ∼ 1500 GHz band of HIFI this will provide ∼ 10 4 lines from different molecules! Evidently, this is a major challenge. Two aspects make it a “major challenge”: i) the handling of such a massive amount of data; ii) the interpretation of so many lines, many of which from molecules whose physical proprieties have not yet fully studied. Our group is already preparing itself for this challenge, developing the procedures to handle all this information in an efficient way. 69
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LABORATOIRE D’ASTROPHYSIQUE DE GR
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III Team ASTROMOL 41 2 Presentation
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8.1.3 Graduate Students . . . . . .
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VI Team SHERPA 145 15 Results 147 1
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Part I Foreword: Presentation of th
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on PUEO at CFHT. The IFS experts (A
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Figure 11.5: Image of the binary st
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have taken responsibilities: A. Che
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Figure 11.10: thermal modeling of W
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Figure 12.1: All the GRIL projects
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12.2 Optical interferometry As for
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the VLTI. CHARA is equipped with an
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• ANR Alterdetect, with IMEP : si
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Mixed GRIL-other team profiles 1. A
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Name Grade Comm. Project Berger Jea
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• First fringes in the H band wit
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Table 15.1: List of the permanent S
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15.4 Transport phenomena in accreti
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states, combining a still powerful
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Excitation of turbulence by Cosmic
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Table 15.2: Former PhD students of
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Chapter 16 Perspectives The distinc
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Ray background. The second project
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Among the various areas of expertis
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-responsable du groupe ”logiciel
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Chef de l’ Equipe ASTROMOL du LAO
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Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

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