We have observed the Class I protostar TMC-1A with Atacama Millimeter/submillimeter Array ( ALMA ) in the emissions of ^ { 12 } CO and C ^ { 18 } O ( J = 2 - 1 ) , and 1.3-mm dust continuum .
Continuum emission with a deconvolve size of 0 \hbox { $ . ^ { \prime \prime } $ } 50 \times 0 \hbox { $ . ^ { \prime \prime } $ } 37 , perpendicular to the ^ { 12 } CO outflow , is detected .
It most likely traces a circumstellar disk around TMC-1A , as previously reported .
In contrast , the C ^ { 18 } O a more extended structure is detected in C ^ { 18 } O although it is still elongated with a deconvolved size of 3 \hbox { $ . ^ { \prime \prime } $ } 3 \times 2 \hbox { $ . ^ { \prime \prime } $ } 2 , indicating that C ^ { 18 } O traces mainly a flattened envelope surrounding the disk and the central protostar .
C ^ { 18 } O 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 \propto r ^ { - a } with an index of \sim 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 M _ { \odot } ( inclination corrected ) .
The additional velocity gradient of C ^ { 18 } O 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 \sim 0.3 times smaller than 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 .