Sub-Keplerian accretion onto circumstellar disks
  Models of the formation, evolution and photoevaporation of circumstellar
disks are an essential ingredient in many theories of the formation of
planetary systems. The ratio of disk mass over stellar mass in the
circumstellar phase of a disk is largely determined by the angular momentum of
the original cloud core from which the system was formed. While full 3D or 2D
axisymmetric hydrodynamical models of accretion onto the disk automatically
treat all aspects of angular momentum, this is not so trivial for 1D and
semi-2D viscous disk models. Since 1D and semi-2D disk models are still very
useful for long-term evolutionary modelling of disks with relatively little
numerical effort, we investigate how the 2D nature of accretion affects the
formation and evolution of the disk in such models. A proper treatment of this
problem requires a correction for the sub-Keplerian velocity at which accretion
takes place. We develop an update of our semi-2D time-dependent disk evolution
model to properly treat the effects of sub-Keplerian accretion. The new model
also accounts for the effects of the vertical extent of the disk on the
accretion streamlines from the envelope. The disks produced with the new method
are smaller than those obtained previously, but their mass is mostly unchanged.
The new disks are a few degrees warmer in the outer parts, so they contain less
solid CO. Otherwise, the results for ices are unaffected. The 2D treatment of
the accretion results in material accreting at larger radii, so a smaller
fraction comes close enough to the star for amorphous silicates to be thermally
annealed into crystalline form. The lower crystalline abundances thus predicted
correspond more closely to observed abundances than did earlier model
predictions. We argue that thermal annealing followed by radial mixing must be
responsible for at least part of the observed crystalline material.
