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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 protostellarsource IRAS 15398-3359 with ALMA. The CCH emission traces theinfalling-rotating envelope near the protostar with the outflow cavity extendedalong the northeast-southwest axis. On the other hand, the SO emission has acompact distribution around the protostar. The CCH emission is relatively weakat 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 fromthe systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for theSO emission. This high velocity component is most likely associated with theKeplerian rotation around the protostar. The protostellar mass is estimated tobe 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. Withthis protostellar mass, the velocity structure of the CCH emission can beexplained by the model of the infalling-rotating envelope, where the radius ofthe centrifugal barrier is estimated to be 40 au from the comparison with themodel. The disk mass evaluated from the dust continuum emission by assuming thedust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting inthe Toomre Q parameter of 0.4-5. Hence, the disk structure may be partlyunstable. All these results suggest that a rotationally-supported disk can beformed in the earliest stages of the protostellar evolution.

The Co-evolution of Disk and Star in Embedded Stages: The Case of the Very Low-mass Protostar