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.