Osservatorio Astronomico di Palermo Giuseppe S. Vaiana

STAR AND PLANETARY SYSTEMS FORMATION AND EVOLUTION

Young stars are particularly active and this bears important consequences for the surrounding environment. In the first million years after their formation, following the gravitational collapse of a cloud of gas and dust, stars are characterized by ejection of collimated outflows and are surrounded by dense plasma disks, which they interact with through accretion of mass, responsible also for the formation of planets.

Young stars, during their early evolution, accrete material from a surrounding circumstellar disk. The accretion process is regulated by the stellar magnetic field, that disrupts the inner disk and guides the infalling material to the central star along its flux tubes. The accreting material arrives near the stellar surface with high velocities. There, because of the impact with the denser stellar atmosphere, a strong shock forms. This shock, located at the base of the accretion stream, heats the accreting material, producing there an accretion spot hotter than the surrounding photosphere. The shock heated material cools down by radiating in the X-ray, UV, and optical bands. Hence accretion produces significant radiation in several wavelength bands, superimposed to the normal photospheric emission. The accretion process can be investigated by monitoring its high-energy radiation (UV and X-rays).

Studying the accretion process and its high-energy radiation is important to understand the physics of pre-main-sequence low-mass stars and their circumstellar environment. In fact accretion affects several aspects of the central star (rotation, magnetic activity, luminosity); accretion controls the mass and angular momentum exchange between the star and the disk; moreover accretion, because of the resulting high-energy radiation, might influence the physics and the chemistry of the circumstellar disk, and likely also its lifetime.

As a consequence, stellar coronal X-rays may influence the accretion process, while - in a sort of feedback mechanism - accretion itself is a source of X-rays. A joint study of high-energy phenomena and disk/accretion properties of young stars is, therefore, fundamental to understanding star and planet formation. OAPa scientists have been actively engaged in searching for the origin of magnetic activity and its effects on the environment surrounding the star. They take part in or lead several large international projects based on observations made with the most powerful space- and ground-based instruments (e. g. Chandra and XMM-Newton). They have developed and applied detailed magnetohydrodynamic (MHD) and hydrodynamic models describing the interaction of the circumstellar disk with the central star (through accretion of mass) and the evolution of protostellar jets. Other critical issue is to understand as the environment conditions may affect the stellar evolution. In particular the presence of massive stars may accelerate the disk evolution, with important consequence on planet formation time scales.