ar X iv : a st ro - p h / 99 02 33 5 v 1 2 3 Fe b 19 99 EVOLUTION OF DISK ACCRETION

We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate ˙ M and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the current best estimates of ˙ M for Classical T Tauri stars and for objects still surrounded by infalling envelopes are given. We argue that the low mass accretion rates of the latter objects require episodes of high mass accretion rate to build the bulk of the star. Similarity solutions for viscous disk evolution suggest that the inner disk mass accretion rates can be self-consistently understood in terms of the disk mass and size if the viscosity parameter α ∼ 10 −2. Close companion stars may accelerate the disk accretion process, resulting in accretion onto the central star in ≤ 1Myr; this may help explain the number of very young stars which are not currently surrounded by accretion disks (the weak emission T Tauri stars). I. INTRODUCTION The initial angular momenta of star-forming molecular cloud cores must be responsible for the ultimate production of binary (and multiple) stellar systems and circumstellar disks. Unless the protostellar cloud core is very slowly rotating, much or most of the stellar mass is likely to land initially on a disk, and must subsequently be accreted from the disk onto the protostellar core. In the early phases of this accretion, the circumstellar disks may be relatively massive in comparison with the central protostar, and so gravitational instabilities may be important in angular momentum transport and consequent disk evolution. At later phases, disk evolution is likely to be driven by viscous processes, perhaps limited by condensation of bodies. Stellar magnetic fields may ultimately halt the accretion of material onto the central