This region is illuminated by the strong UV radiation field produced by the star and the accretion–outflow engine. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The temperature of the accretion–outflow engine is between 3000 and 10 7 K. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1 AU) from its parent star. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1 AU) where the gravitational field is strong.
Powerful outflows are known to regulate angular momentum transport during star formation, the so-called accretion–outflow engine. Most of the ISM volume (∼ 65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system – the ISM. Stars form from dense molecular clouds that contain ∼ 30% of the total interstellar medium (ISM) mass. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation: The physics of the ISM.
Planetary systems are angular momentum reservoirs generated during star formation.
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