Evolution of Young Stellar Objects

Abstract

This chapter decribes the evolution of stellar objects after the PMF phase when a young stellar object forms via accretion from its circumstellar cloud. It is difficult, if not impossible, to draw a line between the process of protostellar formation and the evolution of a protostar toward a mature star on the main sequence. The dense central object that forms during the collapse of a cloud core is not yet a star, but it is also not a molecular core anymore. One should define what is meant by the terms protostar, PMS star, and young stellar object as these expressions are common in the literature:

• Protostar: the optically thick stellar core that forms during the adiabatic contraction phase and grows during the accretion phase.

• Protostellar system: the entire young stellar system with its infalling envelope and accretion disk. The nomenclature includes binary and multiple systems. The extent of the system is fairly well descibed by its Jeans length.

• PMS star: the premature star (PMS = pre-main sequence) that becomes visible once the natal envelope has been fully accreted and which contracts towards the main sequence.

• PMS stellar system: the PMS stars with their accretion disks and their star/disk interaction regions.

• YSO: the entire stellar system (YSO = young stellar object) throughout all evolutionary phases. However, it is specifically used during the collapse phases and in the case of massive systems, where evolutionary phases cannot easily be distinguished.