The Co-evolution of Disk and Star in Embedded Stages: The Case of the
  Very Low-mass Protostar
  We have observed the CCH (N=3-2, J=7/2-5/2, F=4-3 and 3-2) and SO (6_7-5_6)
emission at a 0"2 angular resolution toward the low-mass Class 0 protostellar
source IRAS 15398-3359 with ALMA. The CCH emission traces the
infalling-rotating envelope near the protostar with the outflow cavity extended
along the northeast-southwest axis. On the other hand, the SO emission has a
compact distribution around the protostar. The CCH emission is relatively weak
at the continuum peak position, while the SO emission has a sharp peak there.
Although the maximum velocity shift of the CCH emission is about 1 km s^-1 from
the systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for the
SO emission. This high velocity component is most likely associated with the
Keplerian rotation around the protostar. The protostellar mass is estimated to
be 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. With
this protostellar mass, the velocity structure of the CCH emission can be
explained by the model of the infalling-rotating envelope, where the radius of
the centrifugal barrier is estimated to be 40 au from the comparison with the
model. The disk mass evaluated from the dust continuum emission by assuming the
dust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting in
the Toomre Q parameter of 0.4-5. Hence, the disk structure may be partly
unstable. All these results suggest that a rotationally-supported disk can be
formed in the earliest stages of the protostellar evolution.
