ALMA OBSERVATIONS OF THE TRANSITION FROM INFALL MOTION TO KEPLERIAN ROTATION AROUND THE LATE-PHASE PROTOSTAR TMC-1A

Abstract

We have observed the Class I protostar TMC-1A with Atacama\nMillimeter/submillimeter Array (ALMA) in 12CO and C18O (J=2-1), and 1.3-mm dust\ncontinuum emission. Continuum emission with a deconvolved size of 0.50"x0.37",\nperpendicular to the 12CO outflow, is detected. It most likely traces a\ncircumstellar disk around TMC-1A, as previously reported. In contrast, a more\nextended structure is detected in C18O although it is still elongated with a\ndeconvolved size of 3.3"x2.2", indicating that C18O traces mainly a flattened\nenvelope surrounding the disk and the central protostar. C18O shows a clear\nvelocity gradient perpendicular to the outflow at higher velocities, indicative\nof rotation, while an additional velocity gradient along the outflow is found\nat lower velocities. The radial profile of the rotational velocity is analyzed\nin detail, finding that it is given as a power-law \\propto r^{-a} with an index\nof ~0.5 at higher velocities. This suggests that the rotation at higher\nvelocities can be explained as Keplerian rotation orbiting a protostar with a\ndynamical mass of 0.68 Mo (inclination-corrected). The additional velocity\ngradient of C18O along the outflow is considered to be mainly infall motions in\nthe envelope. Position-Velocity diagrams made from models consisting of an\ninfalling envelope and a Keplerian disk are compared with the observations,\nrevealing that the observed infall velocity is ~0.3 times smaller than the free\nfall velocity yielded by the dynamical mass of the protostar. Magnetic fields\ncould be responsible for the slow infall velocity. A possible scenario of\nKeplerian disk formation is discussed.\n