Alma observations of the transition from infall motion to keplerian rotation around the late-phase protostar tmc-1a

We have observed the Class I protostar TMC-1A with the Atacama Millimeter/submillimeter Array (ALMA) in the emissions of ¹²CO and C¹⁸O (J = 2-1) and 1.3 mm dust continuum. Continuum emission with a deconvolved size of 0.″50 × 0.″37, perpendicular to the ¹²CO outflow, is detected. It most likely traces a circumstellar disk around TMC-1A, as previously reported. In contrast, a more extended structure is detected in C¹⁸O, although it is still elongated with a deconvolved size of 3.″3 × 2.″2, indicating that C¹⁸O traces mainly a flattened envelope surrounding the disk and the central protostar. C¹⁸O shows a clear velocity gradient perpendicular to the outflow at higher velocities, indicative of rotation, while an additional velocity gradient along the outflow is found at lower velocities. The radial profile of the rotational velocity is analyzed in detail, finding that it is given as a power law ∝r^-a with an index of ∼0.5 at higher velocities. This indicates that the rotation at higher velocities can be explained as Keplerian rotation orbiting a protostar with a dynamical mass of 0.68 (inclination corrected). The additional velocity gradient of C¹⁸O along the outflow is considered to be mainly infall motions in the envelope. Position-velocity diagrams made from models consisting of an infalling envelope and a Keplerian disk are compared with the observations, revealing that the observed infall velocity is ∼0.3 times smaller than the free-fall velocity yielded by the dynamical mass of the protostar. Magnetic fields could be responsible for the slow infall velocity. A possible scenario of Keplerian disk formation is discussed.