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 1.3 mm 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 ( \sim 54 AU ) may be shallower than r ^ { -1 } , suggestive of formation of a Keplerian disk around the central protostar of \sim 0.3 ~ { } M _ { \sun } 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 M _ { \sun } 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 M _ { \sun } .
Possible reasons for the reduced infall velocities are discussed .