PROSAC: A Submillimeter Array survey of low-mass protostars. II. The
  mass evolution of envelopes, disks, and stars from the Class 0 through I
  stages
  The key question about early protostellar evolution is how matter is accreted
from the large-scale molecular cloud, through the circumstellar disk onto the
central star. A sample of 20 Class 0 and I protostars has been observed in
continuum at (sub)millimeter wavelengths at high angular resolution with the
Submillimeter Array. Using detailed dust radiative transfer models, we have
developed a framework for disentangling the continuum emission from the
envelopes and disks, and from that estimated their masses. For the Class I
sources in the sample, HCO+ 3-2 line emission has furthermore been observed
with the Submillimeter Array. Four of these sources show signs of Keplerian
rotation, constraining the masses of the central stars. Both Class 0 and I
protostars are surrounded by disks with typical masses of about 0.05 M_sun. No
evidence is found for a correlation between the disk mass and evolutionary
stage of the young stellar objects. This contrasts the envelope mass, which
decreases sharply from 1 M_sun in the Class 0 stage to <0.1 M_sun in the Class
I stage. Typically, the disks have masses that are 1-10% of the corresponding
envelope masses in the Class 0 stage and 20-60% in the Class I stage. For the
Class I sources for which Keplerian rotation is seen, the central stars contain
70-98% of the total mass in the star-disk-envelope system, confirming that
these objects are late in their evolution through the embedded protostellar
stages. Theoretical models tend to overestimate the disk masses relative to the
stellar masses in the late Class I stage. The results argue in favor of a
picture in which circumstellar disks are formed early during the protostellar
evolution and rapidly process material accreted from the larger scale envelope
onto the central star.
