Young stellar objects and protostellar disks

Abstract

Stars form out of molecular clouds with densities of 104 cm− 3 and temperatures of 10 K. Clouds fragment into molecular cores covering a broadrange of masses that give rise to the formation stellar associations, clusters andsingle stars. High mass star formation rapidly removes the environmental gas anddust having a large impact into cloud evolution. Low mass star formation is lessaggressive with the environmental cloud. Young stellar objects (YSOs)display a wide range of properties depending on their mass, age and starforming environment. Very massive YSOs generate ultracompact HII regionsembedded in the cores of molecular clouds. Their key observational propertiesare related with the action of strong and highly photoionizing radiationfields in the surrounding molecular cloud. They evolve in time scales offew Myrs into the main sequence. Low mass YSOs progress to the mainsequence at a slower pace; the gravitational collapse is slowed down by theaction of the molecular cloud magnetic field and their most conspicuousobservational properties are related with dissipation of the magnetic energycarried out during star formation and enhanced by shear during the starbuilding up Protostellar disks are a by-product of star formation. They are formed as angular momentum reservoirs during the gravitational accretion process that gives rise to the formation of new stars. They are observed around all types of YSOs however their properties and evolution depend on the nature of the parent star/stars. The opacity of the disk and its selfshielding properties against the stellar and the environmental radiation field determine the lifetime of protostellar disks that end as debris disks and planetary systems. Disks play a key role in the generation of the powerful bipolar outflows observed during star formation. Bipolar outflows are observed from the very early stages (104 years) of star formation and from all the mass spectrum of YSOs This chapter attempts to summarize our current understanding of single star formation and the joint evolution of the star-disk system towards the main sequence.