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Dynamical interstellar medium with Gaia and ground-based massive spectroscopic stellar surveys

The ongoing Gaia mission of ESA will provide accurate spatial and kinematical information for a large fraction of stars in the Galaxy. Interstellar extinction and line absorption studies toward a large number of stars at different distances and directions can give a 3-dimensional distribution map of interstellar absorbers, and thus reach a similar spatial perfection. Under certain morphologies (e.g. geometrically thin absorption curtains) one can infer a complete velocity vector from its radial velocity component and so obtain a dynamical information comparable to stars. But observations of a large number of stars at different distances are needed to determine the location of the absorption pockets. Therefore, techniques to measure interstellar absorptions towards (abundant) cool stars are needed. A complex mix of colliding absorption clouds is found in the Galactic plane. Thus, one would wish to start with deep observations to detect the weak, but simpler interstellar absorptions at high Galactic latitudes. Finally, interstellar atomic line absorption studies toward cool stars in the optical are largely limited to Sodium and Potassium doublets, not covered by many surveys, including Gaia. Diffuse interstellar bands can give the same type of information as interstellar atomic absorption lines. A combination of both may also point to differences in dynamics of different components of the interstellar medium. In particular, the Gaia DIB at 862 nm can be used to build absorption maps, as already demonstrated by RAVE. Additionally, several ground-based surveys (e.g APOGEE, Gaia-ESO and Galah) are upgrading this approach. The use of this new information can change our understanding in many areas (e.g. determination of membership of stars in clusters, studies of a few Myr old supernova remnants and investigations of Galactic fountains).