We have observed the CCH (N = 3-2, J = 7/2-5/2, F = 4-3 and 3-2) and SO (67-56) 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 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 20-100 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 form in the earliest stages of the protostellar evolution.
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