  Low-mass protostars may accrete most of their material through short-lived
episodes of rapid disk accretion; yet until recently evolutionary tracks for
these protostars assumed only constant or slowly-varying accretion. Important
initial steps toward examining the potential effects of rapid accretion were
recently made by Baraffe, Chabrier, & Gallardo, who showed that in the limit of
low-temperature ("cold") accretion, protostars may have much smaller radii than
found in previous treatments. Such small radii at the end of protostellar
accretion would have the effect of making some young stars appear much older -
perhaps as much as 10 Myr - than they really are. However, we argue that very
rapid disk accretion is unlikely to be cold, because observations of the
best-studied pre-main sequence disks with rapid disk accretion outbursts - the
FU Ori objects - have spectral energy distributions which imply large, not
small, protostellar radii. In addition, theory indicates models that at high
accretion rates, protostellar disks become internally hot and geometrically
thick, making it much more likely that hot material is added to the star. In
addition, the very large luminosity of the accretion disk is likely to
irradiate the central star strongly, heating up the outer layers and
potentially expanding them. Nevertheless, the Baraffe et al. calculations
emphasize the importance of initial protostellar radii for subsequent
evolution.
