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Formation of a Keplerian disk in the infalling envelope around L1527 IRS: transformation from infalling motions to Kepler motions

We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of C$^{18}$O ($J=2-1$), SO ($J_N= 6_5-5_4$) and 1.3mm dust continuum toward L1527 IRS, a class 0 solar-type protostar surrounded by an infalling and rotating envelope. C$^{18}$O emission shows strong redshifted absorption against the bright continuum emission associated with L1527 IRS, strongly suggesting infall motions in the C$^{18}$O envelope. The C$^{18}$O envelope also rotates with a velocity mostly proportional to $r^{-1}$, where $r$ is the radius, while the rotation profile at the innermost radius (54 AU) may be shallower than $r^{-1}$, suggestive of formation of a Keplerian disk around the central protostar of 0.3 Mo in dynamical mass. SO emission arising from the inner part of the C$^{18}$O envelope also shows rotation in the same direction as the C$^{18}$O envelope. The rotation is, however, rigid-body like which is very different from the differential rotation shown by C$^{18}$O. In order to explain the line profiles and the position-velocity (PV) diagrams of C$^{18}$O and SO observed, simple models composed of an infalling envelope surrounding a Keplerian disk of 54 AU in radius orbiting a star of 0.3 Mo are examined. It is found that in order to reproduce characteristic features of the observed line profiles and PV diagrams, the infall velocity in the model has to be smaller than the free-fall velocity yielded by a star of 0.3 Mo. Possible reasons for the reduced infall velocities are discussed.