The formation of a Keplerian disk around a late-stage protostar
Protostars surrounded by their infalling envelopes evolve into Classical T Tauri stars (CTTSs). In the course of the evolution from protostars to CTTSs, it is considered that infall motions decay while Keplerian rotation becomes more dominant. Protoplanetary disks, which are important as the cradle of planets, are considered to form as Kepler rotation becomes more dominant. The formation process of the protoplanetary disk is, however, still not understood well. In order to reveal how Keplerian disks form, we have observed a late-phase protostar TMC-1A (Class I, Tbol=118 K) with ALMA Cycle 0 in C18O (J=2–1), SO (J,N=6,5–5,4), and 1.3 mm continuum emission. The C18O integrated intensity map shows an elongated structure perpendicular to the direction of the associated outflow. A clear velocity gradient along the major axis of the elongated structure is observed, indicating rotation of the elongated structure having a rotation axis perpendicular to the elongation. Overall, the rotation law derived from the observations is consistent with Keplerian rotation as identified in previous works. The specific angular momentum 1.7×10^–3 pc km/s is larger than a typical value for Class 0 objects, suggesting that more angular momentum is transferred from the envelope into the disk in the later-protostar phase. On the other hand, a clear velocity gradient can be also seen along the minor axis of the elongated structure. Additionally, the peak positions in channel maps at middle and high (|ΔV| > 1 km/s) velocities delineate a S-shaped warp centered at the protostar position. These results suggest that there is still an infall motion in the late-phase protostar, as well as rotation. SO shows more compact emission than C18O, and its velocity structure coincides with that of C18O at high velocities (|ΔV|=2–3 km/s) while a rigid-like velocity law can be seen at low velocities.