Abstract:

Planetary systems are angular momentum reservoirs generated during star formation. Powerful outflows are known to regulate angular momentum transport during star formation; the so-called "accretion-outflow engine". Elementary physical considerations point out that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1AU) where the gravitational field is strong. The temperature of the accretion-outflow engine is between 3000 K and 300,000 K. The spectral lines of the most abundant atoms and ions at these temperatures are in the ultraviolet (UV) together with the electronic transitions of the most abundant molecules (H2, CO, OH, CS, S2, CO2+, C 2, O2, O3). After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at the ultraviolet wavelengths. According to recent numerical simulations, terrestrial planets begin to form in the inner disk at this time. IUE, HST (with its GHRS or STIS UV instruments) and FUSE allowed us to begin to grasp the enormous potential of high resolution spectroscopy for the study of the physics of accretion and outflow, including the inner region of young planetary disks. A spectroscopic UV mission is instrumental to make progress in this very important field of astrophysics.