Unveiling the Evolutionary Sequence from Infalling Envelopes to
  Keplerian Disks around Low-Mass Protostars
  We performed SMA observations in the C18O (2-1) emission line toward six
Class 0 and I protostars, to study rotational motions of their surrounding
envelopes and circumstellar material on 100 to 1000 AU scales. C18O (2-1)
emission with intensity peaks located at the protostellar positions is detected
toward all the six sources. The rotational velocities of the protostellar
envelopes as a function of radius were measured from the Position-Velocity
diagrams perpendicular to the outflow directions passing through the
protostellar positions. Two Class 0 sources, B335 and NGC 1333 IRAS 4B, show no
detectable rotational motion, while L1527 IRS (Class 0/I) and L1448-mm (Class
0) exhibit rotational motions with radial profiles of Vrot ~ r^{-1.0+/-0.2} and
~ r^{-1.0+/-0.1}, respectively. The other Class I sources, TMC-1A and L1489
IRS, exhibit the fastest rotational motions among the sample, and their
rotational motions have flatter radial profiles of Vrot ~ r^{-0.6+/-0.1} and ~
r^{-0.5+/-0.1}, respectively. The rotational motions with the radial dependence
of ~ r^{-1} can be interpreted as rotation with a conserved angular momentum in
a dynamically infalling envelope, while those with the radial dependence of ~
r^{-0.5} can be interpreted as Keplerian rotation. These observational results
demonstrate categorization of rotational motions from infalling envelopes to
Keplerian-disk formation. Models of the inside-out collapse where the angular
momentum is conserved are discussed and compared with our observational
results.
