From infall to rotation around young stellar objects: A transitional phase with a 2000 AU radius contracting disk?

Evidence for a transitional stage in the formation of a low-mass star is reported, intermediate between the fully embedded and the T Tauri phases. Millimeter aperture synthesis observations in the HCO+ J = 1-0 and 3-2, HCN 1-0, (CO)-C-13 1-0, and (CO)-O-18 1-0 transitions reveal distinctly different velocity fields around two embedded, low-mass young stellar objects. The 0.6 M-circle dot of material around TMC 1 (IRAS 04381 +/- 2517) closely follows inside-out collapse in the presence of a small amount of rotation (similar to3 km s(-1) pc(-1)), while L1489 IRS (IRAS 04016 +/- 2610) is surrounded by a 2000 AU radius, flared disk containing 0.02 M-circle dot. This disk shows Keplerian rotation around a similar to0.65 M-circle dot star and infall at 1.3(r/100 AU)(-0.)5 km s(-1), or, equivalently, sub-Keplerian motions around a central object between 0.65 and 1.4 M-circle dot. Its density is characterized by a radial power law and an exponential vertical scale height. The different relative importance of infall and rotation around these two objects suggests that rotationally supported structures grow from collapsing envelopes over a few times 10(5) yr to sizes of a few thousand AU, and then decrease over a few times 10(4) yr to several hundred AU typical for T Tauri disks. In this scenario, L1489 IRS represents a transitional phase between embedded young stellar objects and T Tauri stars with disks. The expected duration of this phase of similar to5% of the embedded stage is consistent with the current lack of other known objects like L1489 IRS. Alternative explanations cannot explain L1489 IRS's large disk, such as formation from a cloud core with an unusually large velocity gradient or a binary companion that prevents mass accretion onto small scales. It follows that the transfer and dissipation of angular momentum is key to understanding the formation of disks from infalling envelopes.