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

tens of visual magnitudes, hence probe deeply into the densest regions of molecular clouds. When correlated
with the IR extinction in front of the same stars, which depends essentially on the dust grain size distribution,
one has in principle (with sufficient IR data) a powerful tool to measure the metallicity of molecular clouds.
This was done successfully by Vuong et al. (2003) for the nearby ρ Oph cloud (up to AV ∼ 50), resulting
in a solar cloud metallicity (Z ∼ Z⊙). Other clouds were also studied, but the available data do not probe
sufficiently dense regions to establish a good NH,X vs. AV correlation.
3.2.7 Dust around protostars
The continuum emission (spectral and spatial distributions) from the dust in the protostellar envelopes gives
important information on the structure of the envelope. The study of the dust continuum is in fact a technique
used by several groups in the world. It is complementary to that exposed in §3.2.3, which uses the rotational
transitions of molecules. Astromol has used both techniques wherever the opportunity presented (Maret et
al. 2004). However, Astromol research on the dust continuum has been more original in the analysis of the
dust features in the Far Infrared, and specifically in the range (45-200 µm) observed by the Long Wavelength
Spectrometer (LWS) on board ISO. This range of wavelengths was thought to not contain any interesting
dust feature. However, Astromol has revealed that an important feature is observed between 90 and 100 µm
(Ceccarelli et al. 2002b). A systematic study of the LWS spectra of low to intermediate protostars has found
that more than 50% of the protostars show up this feature (Chiavassa et al. 2005). The nature of the carrier of
the observed feature is not totally sure. At present, the most reasonable hypothesis is that calcite is responsible
for it. In support of this interpretation, very recently calcite has been revealed in the comet Temple 1 (Deep
Impact mission; Lisse et al. 2005, IAU Circular 8571). This is an important observation, because it shows a
clear link between the comets and the proto-stellar phase. Furthermore, and mostly important, the presence of
calcite in protostars and comets rises the question of its formation. On Earth and in the Solar System objects
(meteorites), calcite is formed by aqueous alteration. However, both in protostars and comets, evidently, no
liquid water is present. We have proposed that the interaction of the X-rays emitted from the central object
(§3.2.6) with the dusty envelope, could be at the origin of the calcite formation (Ceccarelli et al. 2002b). The
interest in this discover is that, if the calcite interpretation is confirmed, this may have some far reaching
consequences because the same mechanism forming calcite may be also form pre-biotic molecules.
3.3 Molecular Physics
3.3.1 Overview
The near-future large scale observatories HERSCHEL and ALMA will open up the Universe to high spatial and
spectral resolution studies of molecules. These new missions will make a major breakthrough in our knowledge of
the key astrochemical processes involved in the origin and evolution of planets, stars, and galaxies. HERSCHEL
and ALMA will lead to a multitude of molecular line data in a great variety of astrophysical environments.
Identification, analysis and interpretation of this data in terms of the physical and chemical characteristics of
the astronomical sources will require a concerted effort by physicists, chemists and astronomers in the areas of
molecular spectroscopy, collisional excitation processes, chemical reactions, and astronomical modeling.
In this ambitious context, our objective since 2002 is to focus our theoretical activity on the calculation
and renewal of potential energy surfaces (PES) and collisional data for astrophysics. Several recent studies
have shown that inaccuracies in the PES are indeed the largest source of error in collisional rate calculations.
Furthermore, the standard quantum close-coupling method used to compute these rates is computationally
efficient at low temperatures only. The development of reliable approximate quantum treatments or methods
based on classical mechanics are thus highly desirable. The specific objectives of our theoretical work are
therefore:
• Calculations of PES at the highest level of accuracy by employing state-of-the-art ab initio methods.
• Calculations of collisional rate constants by employing both full dimensional quantum calculations and
practical quantum/classical approximations.
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