2014ApJ...793....1Y

We have conducted ALMA observations in the 1.3 mm continuum and ¹²CO (2-1), C¹⁸O (2-1), and SO (5₆-4₅) 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 ¹²CO and C¹⁸O emission, and its outer radius (∼700 AU) and mass (∼0.005 M_☉) are comparable to those of disks around T Tauri stars. The protostellar mass is estimated to be 1.6 M_☉ with the inclination angle of 66°. In addition to the Keplerian disk, there are blueshifted and redshifted off-axis protrusions seen in the C¹⁸O 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 ∼5x10^–7 M_☉/yr. The specific angular momentum of the infalling flows (∼2.5x10^–3 km/s pc) is comparable to that at the outer radius of the Keplerian disk (∼4.8x10^–3 km/s pc). The SO emission is elongated along the disk major axis and exhibits a linear velocity gradient along the axis, which is interpreted to mean that the SO emission primarily traces a ring region in the flared Keplerian disk at radii of ∼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.