Cosmic Rays and Compressible MHD Turbulence in the Interstellar Medium – COSMIS

Understanding the physics of the interstellar medium (ISM) of galaxies is a necessary step for our knowledge of the galaxy evolution and the formation of stars. These two fundamental processes are coupled to each other by the wealth of physical phenomena, which takes place in the ISM as the compressible magneto-hydrodynamical (MHD) turbulence, the cosmic rays and the supernova explosions. Although significant progresses have been made over the last decades in this area, many questions remain open. For example the structure of the ISM, the nature and the sources of the interstellar turbulence or the exact abundance and influence of the cosmic rays on the ISM are still poorly understood. Several international observational projects as HERSCHEL, PLANCK, HESS, FERMI, ALMA and others are, or will soon, provide a wealth of new data that will bring new constraints on these important questions. The great diversity of these highly degenerated and complementary data, both from the point of the view of the observational techniques than from the point of view of the physical processes, is appealing to the development in parallel of an adequate and ambitious modelling making use of the most recent development of numerical schemes and computing facilities.

The goals of the COSMIS project are twofold. First it aims at clarifying some of the important issues related to the dynamics of the diffuse interstellar medium by performing a series of MHD, high resolution numerical simulations, which will include various processes as cosmic rays and supernova remnant. Second it will provide a broad and coherent theoretical framework with which observations can be carefully compared and interpreted. For this purpose, the simulation results will be made available through a database and a web interface. All the skills needed to successfully achieve the COSMIS project are present in the team, which gathers researchers from three institutes the LERMA (observatoire de Paris), the SAp (CEA) and the LPTA (université de Montpellier). The team encompasses theorists of the ISM dynamics, theorists of the cosmic rays acceleration and propagation, experts in heavy numerical simulations of fluid dynamics as well as observers experts in ISM processes including supernova remnants and involved in all the important tools that the international community is using and developing.

Two types of complementary numerical simulations will be performed. First, idealised simulations using periodic boundary conditions and forcing in the Fourier space will be carried out. Second, simulations including astrophysical processes as stratification, supernovae and cosmic rays will be performed. While the aim of the first type of simulations is to investigate in detail the fundamental properties of the compressible MHD turbulence, which is taking place in the ISM, the second type of simulations is more specific to several astrophysical situations and will permit detailed comparisons with various observational data. The propagation of supernovae remnants in the turbulent ISM and the evolution of the ISM at large galactic scales will receive particular attention. All these simulations will take advantage of the most recent developments made on the numerical schemes to handle supersonic MHD fluids as well as on the large computing power available in France. In a first step, the numerical simulation results will be utilised to study the propagation of cosmic rays in a realistic ISM, both at relatively large galactic scales and also at smaller molecular cloud scales, allowing one to infer realistic diffusion coefficient. Then in a second step, a two fluid approach will be developed coupling self-consistently the gas and the cosmic rays. The simulations will be used to evaluate the impact of low energy cosmic rays over the ionisation fraction in the warm and cold ISM phases. The results will also be used to compute synthetic observations.