We have conducted ALMA observations in the 1.3 mm continuum and ^ { 12 } CO ( 2–1 ) , C ^ { 18 } O ( 2–1 ) and SO ( 5 _ { 6 } –4 _ { 5 } ) lines toward L1489 IRS , a Class I protostar surrounded by a Keplerian disk and an infalling envelope .
The Keplerian disk is clearly identified in the ^ { 12 } CO and C ^ { 18 } O emission , and its outer radius ( \sim 700 AU ) and mass ( \sim 0.005 M _ { \sun } ) are comparable to those of disks around T Tauri stars .
The protostellar mass is estimated to be 1.6 M _ { \sun } with the inclination angle of 66° .
In addition to the Keplerian disk , there are blueshifted and redshifted off-axis protrusions seen in the C ^ { 18 } O emission pointing toward the north and the south , respectively , adjunct to the middle part of the Keplerian disk .
The shape and kinematics of these protrusions can be interpreted as streams of infalling flows with a conserved angular momentum following parabolic trajectories toward the Keplerian disk , and the mass infalling rate is estimated to be \sim 5 \times 10 ^ { -7 } M _ { \sun } yr ^ { -1 } .
The specific angular momentum of the infalling flows ( \sim 2.5 \times 10 ^ { -3 } km s ^ { -1 } pc ) is comparable to that at the outer radius of the Keplerian disk ( \sim 4.8 \times 10 ^ { -3 } km s ^ { -1 } pc ) .
The SO emission is elongated along the disk major axis and exhibits a linear velocity gradient along the axis , which is interpreted as that the SO emission primarily traces a ring region in the flared Keplerian disk at radii of \sim 250–390 AU .
The local enhancement of the SO abundance in the ring region can be due to the accretion shocks at the centrifugal radius where the infalling flows fall onto the disk .
Our ALMA observations unveiled both the Keplerian disk and the infalling gas onto the disk , and the disk can further grow by accreting material and angular momenta from the infalling gas .