Unveiling a Few Astronomical Unit Scale Rotation Structure around the Protostar in B335

We report a kinematic structure in the innermost envelope of the low-mass Class 0 protostar IRAS 19347+0727 in the Bok globule B335 observed at the best angular resolution ever achieved for this source with ALMA. This is based on observations of complex organic molecule emission in the 1.2 mm band, which selectively traces a hot and dense area around the protostar. The distribution of the CH ₃ OH and HCOOH emission is resolved, and a clear velocity gradient is observed. Moreover, the direction of the gradient is found to be different between these two molecules. These features are well explained by the model of an infalling and rotating gas, but not by the model of a Keplerian motion. The protostellar mass and the radius of the centrifugal barrier are determined to be 0.02-0.06 M and <5 au, respectively. The different directions of the gradients observed for CH ₃ OH and HCOOH are interpreted as the different sizes of their distributions. On the other hand, the SiO emission seems to trace a compact region at the closest vicinity of the protostar, which is a launching point of the outflow or a shocked region caused by the gas accretion onto the protostar. These results first reveal the transition zone from the infalling motion to the rotating motion in this representative isolated protostellar source, which has long been employed as a testbed for star formation studies.