Interstellar medium structure

Interstellar matter

Gravity is driving the evolution of the Universe and is at the origin of the formation of structures at all scales from superclusters of galaxies to stars. But the visible matter involved is an extremely diluted magnetized fluid, highly compressible and turbulent. Its evolution is regulated both at the microphysics scale by a variety of collisional processes (chemical reactions, dissipative processes, collisional excitation) and at galactic scales by star formation.

Stars produce most of the elements in the Universe. They have also a major influence on the interstellar gas, producing the heating radiation and being at the origin of the strongest kinematical perturbations which feed the turbulence in the gas, a negative feed-back to further development of gravitational instability.

Many of the theoretical and observational activities in our laboratory therefore focus on the physics of interstellar matter and star formation.

Small scale structure observations

We have found, either by direct imaging or by inferring their presence from spectroscopic data, structures in the cold interstellar gas at scales too small (close to the size of a planetary system) and of densities too low for these structures to be held by self-gravity. Unexpectedly, these structures belong by their mass, size and internal kinetic energy, to the whole hierarchy of structures observed in the cold molecular gas, which spans five orders of magnitude in size. Work is in progress on the determination of the threshold of the hierarchy and the origin of these structures (E. Falgarone, J.-F. Panis, M. Pérault and collaboration with Köln University).

Analysis of the visible extinction toward stars and galaxies located behind interstellar clouds have shown that the dust is smoothly distributed at small scales, with no structure corresponding to the AU-sized fragments inferred for the gas. This result is consistent with the inertia of dust particles which limits the coupling between the gas and dust grains (P. Boissé & S. Thoraval in collaboration with Observatoire de Grenoble). Observations (CFHT, FUSE and OHP) are currently performed and analysed in order to study the small scale distribution of H2 molecules in relation to that of dust grains and other tracers such as CH, CH⁺, NaI and CaII.