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

• One of us (PV) is strongly involved in the UJF project “CIMENT” (Calcul Intensif, Modélisation,
Expérimentation Numérique et Technologique) funded by the state and the region, which permitted the
creation and upgrade of several computational platforms and associated expertise centers. The originality
of CIMENT is to federate user communities with computer science ones, and was a “cement” for the emergence
of the Grenoble computer grid. CIMENT has funded in 2005 the upgrade of the computer resource
center of the Observatory which is shared between astrophysicists, planetologists, geophysicists, etc. The
new machine is a high performance cluster (30 quadriprocessors, 256 GB of core memory, 4 TB disk, over
0.6 Tflop of peak performance) and will boost all high performance initiatives in the Observatory.
• In the future, we plan to develop tools for harnessing computer grids into the Virtual Observatory, in
particular to trigger multi-parametric modelisations of a set of objets selected by a VO request. Such a
coupling needs a proper specification and scheduling of the related data flow. A pluri-disciplinary project
involving in particular the Observatory and the laboratory ID (Informatique et Distribution) has been
submitted to the ANR in september 2005. Other developments may be carried on within the GRID’5000
national initiative in collaboration also with colleagues from ID.
3.5 Proto-planetary Disks: where Astromol meets FOST
The study of the proto-planetary disks deserves an ad hoc section, because of both of its scientific interest
and the fact that it is also an important field of research for the FOST team. While the FOST Team focuses
mostly on the dust component of the disks, the Astromol team focus is rather the gaseous component. The
two approaches are evidently strictly linked and complementary, and, for this reason, Astromol and FOST
have started a tight collaboration on this theme. This is testified by the beginning of publications in common
(Duchene et al. 2005), the writing of new observing proposals at IRAM (30m and PdB) and Keck telescopes,
the project PAI vanGogh project “Proto-planetary disks” (§2.7) which involves both members of Astromol (CC
and BL) and FOST (F.Menard) and their students.
A notable example of the importance of the dust-gas link in proto-planetary disks is represented by the
interplay between the small dust grains and the gas. Indeed, the gas in the disk governs the dynamics of small
dust grains. Dust grains try to settle towards the midplane, and the gas is critical in slowing down this motion
and countering it by turbulent mixing. Without gas, the dust would collapse (basically within a single orbital
timescale) entirely to the midplane until large (Moon-sized) bodies are able to stirr the disk gravitationally.
With gas present, but without efficient turbulent mixing, the dust would settle within about 10 4 –10 6 years down
to about one gas-pressure scale height (Dullemond and Dominik, 2004 A&A 421, 1075). However, observations
show that disks as old as 10 Myrs contain significant amounts of dust at large heights above the midplane,
demonstrating that the interaction between the grains and the gas is not only taking place, but it is indeed a
crucial aspect of the disk evolution.
The reciprocal is also true: the presence of dust grains has a dramatic effect on the gas structure, thermal
balance, chemistry and dynamics (where all these aspects are strictly inter-related). In the disk upper layers
exposed to the protostar radiation, the radiation is almost totally absorbed by the grains, and the energy
transmitted indirectly to the gas by the collisions with the grains and/or by electrons emitted from grains by
the photoelectric effect (Bakes and Tielens, 1994 ApJ 427, 822). The formation of molecules, which can very
effectively cool the gas, occurs only in the regions where the photoionizing photons are absorbed by the dust.
Further down, in the disk midplane, the molecules freeze out onto the grains and disappear from the gas phase,
giving rise to physical conditions completely different and where different physical processes take over.
In summary, the interplay of gas and grains plays a crucial role in the evolution of protostellar disks, and the
collaboration between Astromol and FOST on this theme aims to address some specific aspects of this interplay.
We take advantage of the complementary expertise of FOST group, mainly on the grains, and the Astromol
group, mainly on the gas, to study the following four issues:
1- The gas in the disk midplane: ionization degree and the gas-to-dust ratio;
2- The grain settling and role of small grains in heating the gas above the midplane;
3- The gas and dust mixing processes;
4- One specific aspect of the grain processing due to the X-rays emitted by the central source.
Note that this collaboration extends to the group of C.Dominik in Amsterdam too, and has received financial
support from the PAI “van Gogh” for the 2005 to 2007 (§2.7). In addition, we have started a collaboration with
the people of the European Synchrotron Radiation Facility in Grenoble to study the chemistry on the grain
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