Topic: Formation and Evolution of Cool Stars and Brown Dwarfs
Young stars and planetary systems form in molecular clouds. After the initial radial infall an accretion disk develops. The circumstellar envelope depletes and thus the absorption of the central star decreases. For classical T Tauri stars (CTTS, F-K type precursors) and presumably also for their low mass brothers, the young brown dwarfs (BD), the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius by the stellar magnetic field. The inner edge of the disk is ionized by the stellar radiation, so that the accretion has to be funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observable in a wide wavelength range including X-rays and UV excess, optical veiling and different emission lines. Some of the accretion tracers, e.g. H$\alpha$ can be calibrated to measure the accretion rate, in fact, CTTS are traditionally defined by their H$\alpha$ equivalent width. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many (if not all) accreting systems also drive strong outflows which are ultimately powered by accretion. However, the exact driving mechanism is still unclear. Several components could contribute to the outflows: Slow, wide-angle disk winds, X-winds launched close to the inner disk rim, and thermally driven stellar winds. In any case, the outflows contain material of very different temperatures and speeds. The disk wind is cool and can have a molecular component with just a few tens of km/s, while the central component of the outflow can reach a few 100~km/s. In some cases the inner part of the outflow is collimated to a small-angle jet. These jets have an onion-like structure, where the inner components are consecutively hotter and faster. The jets can contain working surfaces, which show up as Herbig-Haro knots and in a some cases can be traced out to several pc from the central star. Accretion and outflows in the CTTS phase do not only determine stellar parameters like the rotation rate on the main-sequence, they also could have a profound impact on the environment of young stars.