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.
