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Structure and Chemistry

of the Interstellar Medium

(Ce texte n'est pas disponible en français, nous nous en excusons.)


The SCHISM project in a nutshell

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 modelling tools and analysis methods appropriate to all accessible environments? 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) 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 full 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 benefit from 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 to develop and test sophisticated tools describing the interaction of molecular gas with radiation (the Meudon-PDR code) and a velocity perturbation (MHD shock code), and simultaneously to provide observation templates for the codes, and 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 interface. We will also provide numerical tools to quickly model a one-dimensional 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 pillars created by the dynamical interaction of massive stars with a 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 observing mode, which will enable observation 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.

3. Expected results

The development and the public release of (1) the interferometric on-the-fly observing mode, (2) versatile numerical codes which couple (A) gas-phase and grain-surface chemistry and (B) gas chemistry and dynamics, and (3) legacy surveys used as benchmarks to chemical models, would each be a particularly meaningful accomplishment. It would provide the astrophysical community with tools not yet freely available.

The studies will be conducted in collaboration among the three partners, and we anticipate original insights and major progresses in the understanding of (1) the chemical conditions of formation of H2, (2) the formation routes of organic molecules like CH3OH, (3) the origin of CH+ in the diffuse gas, a 70-year old problem, (4) the structure and dynamics of the dense and diffuse phases of the interstellar medium.

3 expected results : peut etre utilise pour le paragraphe : retombees.

Objectifs 1000 caractères

The projects aims at bringing together theorists and observers in order to develop and test sophisticated tools describing the interaction of molecular gas with radiations (the Meudon-PDR code) and a velocity perturbation (MHD shock code), and simultaneously to provide observation templates for the codes, and enhance the efficiency of interferometer observations for extended sources. We want to investigate the coupling of chemistry with the gas dynamics, by studying template problems : MHD shock, the intermittent dissipation of turbulence, and the turbulent mixing at a PDR interface. We also aim at improving the description of the solid state processes in interstellar chemistry as such processes control the formation of H2 and other key molecules. We plan to complement the theoretical activities by extensive ground based (IRAM) and space born (Herschel-PRISMAS) observation programs, which will provide templates for models and long lasting data bases.

Programme de travail : 1000 caracteres

The theoretical activities will last 4 years. Two new features will be implemented in the Meudon-PDR code : i) an accurate description of the solid state chemistry on grain surfaces relevant for the understanding of the formation of H2, and of organic molecules, ii) the coupling of dynamics and chemistry by turbulent mixing at the PDR interface. A numerical tool will be developed, that will be able to compute a MHD shock model fast, for arbitrary initial conditions. Mapping and deconvolution algorithms for interferometric OTF will be prepared in 2009, and tested and scientifically validated in 2010. The analysis and publications of the extensive observation surveys (Horsehead, PRISMAS, diffuse ISM, pillars) will start with the ground based data (end 2009), the Herschel data being released approximatively one year after the observations are performed. The release of the data bases is scheduled for the end of the SCHISM project (2012).


Programme ANR SCHISM
Jérôme Pety

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