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Evidence for a transitional stage in the formation of a low-mass star isreported, intermediate between the fully embedded and the T Tauri phases.Millimeter aperture synthesis observations in the HCO+ J=1-0 and 3-2, HCN 1-0,13CO 1-0, and C18O 1-0 transitions reveal distinctly different velocity fieldsaround two embedded, low-mass young stellar objects. The 0.6 M(sun) of materialaround TMC 1 (IRAS 04381+2517) closely follows inside-out collapse in thepresence of a small amount of rotation (~3 km/s/pc), while L1489 IRS (IRAS04016+2610) is surrounded by a 2000 AU radius, flared disk containing 0.02M(sun). This disk shows Keplerian rotation around a ~0.65 M(sun) star andinfall at 1.3 (r/100 AU)^-0.5 km/s, or, equivalently, sub-Keplerian motionsaround a central object between 0.65 and 1.4 M(sun). Its density ischaracterized by a radial power law and an exponential vertical scale height.The different relative importance of infall and rotation around these twoobjects suggests that rotationally supported structures grow from collapsingenvelopes over a few times 10^5 yr to sizes of a few thousand AU, and thendecrease over a few times 10^4 yr to several hundred AU typical for T Tauridisks. In this scenario, L1489 IRS represents a transitional phase betweenembedded YSOs and T Tauri stars with disks. The expected duration of this phaseof ~5% of the embedded stage is consistent with the current lack of other knownobjects like L1489 IRS. Alternative explanations cannot explain L1489 IRS'slarge disk, such as formation from a cloud core with an unusually largevelocity gradient or a binary companion that prevents mass accretion onto smallscales. It follows that the transfer and dissipation of angular momentum is keyto understanding the formation of disks from infalling envelopes.Comment: Accepted ApJ. 33 pages, including 10 B/W figures and 1 color figure.  Uses AASTe

From Infall to Rotation around Young Stellar Objects: A Transitional Phase with a 2000 AU Radius Contracting Disk?