Rotation in the Protostellar Envelopes around IRAS 04169+2702 and IRAS 04365+2535: The Size Scale for Dynamical Collapse

We report interferometric observations of two embedded protostar candidates, IRAS 04169+2702 and IRAS 04365+2535 (TMC-1A), in the Taurus molecular cloud. The C 18 O J = 1-0 emission from IRAS 04169+2702 reveals a flattened envelope 2200 AU × 1100 AU in size; there is a velocity gradient along the elongation axis, which in turn is perpendicular to the outflow direction. Since the rotational velocity corrected for inclination, 0.23 km s -1 at 370 AU, gives an unacceptably small dynamical mass of 0.02 M ☉ within that radius, we speculate that there is additional radial motion, possibly infall, in the flattened envelope. Around IRAS 04365+2535, a compact 13 CO J = 1-0 condensation ~1400 AU in size was detected. Again, the velocity gradient is perpendicular to the associated molecular outflow, but a rotation velocity of 0.87 km s -1 at 580 AU radius is consistent with the condensation being a rotationally supported disk. Combining our new data for the two sources with published observations of rotationally supported disks and infalling envelopes around five young stars associated with the Taurus molecular cloud enables us to compare local specific angular momenta of a significant sample of these sources on scales of 200-2000 AU with those of dense cores on 6000-80,000 AU (0.03-0.4 pc) scales. The specific angular momenta for infalling envelopes and rotationally supported disks are relatively constant, ~10 -3 km s -1 pc, and are typically an order of magnitude smaller than those for dense cores. These results can be explained if the dynamical collapse of dense star-forming cores takes place inside radii of ~0.03 pc while the region outside this radius remains dynamically stable.