Structure and Chemistry of the Inter-Stellar Medium – SCHISM

1. Scientific background and objectives The field of astrochemistry has undergone a spectacular development in the last 20 years, and is now ready to take advantage of the unprecedented spatial and spectroscopic capabilities of ALMA, Herschel and IRAM instruments. Will instrumentation and observing modes be flexible enough to adapt to evolving requirements' Will observations be matched on the theoretical side by physical insight into fundamental processes and supported by appropriate modelling tools and analysis methods' The SCHISM project addresses several facets of this vast issue, related to the coupling of gas-phase and grain-surface chemistry and to the coupling of chemistry with magneto-hydrodynamical (MHD) turbulence (transport, dissipative structures, shocks). It offers a close connexion of state-of-the-art numerical developments and cutting-edge observations from space (Herschel) and mm/sub-mm interferometers (PdBI). Molecules are by far the most versatile tracers of diffuse matter in the universe, from high-z galaxies to proto-planetary disks, because their internal and external degrees of freedom bear the signature of the physical conditions in their environment. To fully benefit from the diagnostic power of the molecular lines, the formation and destruction paths of the parent molecules must be quantitatively understood. While many progresses have been achieved for the gas phase route, the solid phase route is much less developed, although it is the main path to the more abundant molecule H2, and other species (e.g. CH3OH). In addition, the chemical activity is intimately coupled to gas dynamics, and therefore to its evolution. Chemistry affects the gas motions because the line radiation from polar molecules is the main cooling agent over a broad range of media, controlling their equation of state hence their dynamics. Conversely, the gas dynamics affect chemistry because flows are turbulent, supersonic and variously coupled to the magnetic fields. Combining existing sophisticated chemical codes with gas dynamics is both a vital step required to fully exploit the versatility of molecular line data and a tremendous challenge given the non-linearity of the fluid dynamics and the stiffness of the chemical reactions. The project aims at bringing together theorists and observers in order 1) to develop and test sophisticated tools describing the interaction of molecular gas with radiation (the Meudon-PDR code), and 2) a supersonic perturbation (MHD shock code), 3) simultaneously to provide observation templates for the codes, and 4) enhance the efficiency of interferometer observations for extended sources. 2. Description of the project and methodology On the modelling side, we propose to add two main features to the existing Meudon Photo Dissociation Region (PDR) code: i) the grain surface chemistry, using the recent and efficient formalism known as moment equations and ii) the steady-state solution of turbulent mixing at the H2 front. We will also provide numerical tools to quickly model a 1D non steady-state MHD shock for arbitrary shock parameters, using an extensive chemical network. We will pursue our efforts in investigating the role of interstellar turbulence, in particular its intermittent dissipation regions, on the diffuse ISM chemistry. On the observational side, we will deliver two legacy surveys which will serve as benchmarks to chemical codes: 1) the first comprehensive survey of a key chemical family, the hydrides; 2) the chemical inventory of the Horsehead mane PDR, whose geometry is particularly simple. Two other programs are aimed at probing the structure and kinematics of the interstellar medium: 3) investigation of the small scale structure of line emission in all fields previously probed in line absorption; 4) extensive observations of structures created by the dynamical interaction of massive stars with molecular clouds (e.g. Pillars of Creation in the Eagle Nebula), which could serve as templates for dynamical interfaces. These observations will benefit from the development of the interferometric on-the-fly (OTF) observing mode, which will enable observations of large fields-of-view at high angular resolution with a much better image quality than the standard stop-and-go mosaicing techniques used today. We propose to use our observational projects to scientifically validate this effort and to make this observing mode user-friendly at the PdBI.