We have newly observed the Class 0/I protostar L1527 IRS using the Atacama Large Millimeter/submillimeter Array ( ALMA ) during its Cycle 1 in 220 GHz dust continuum and C ^ { 18 } O ( J = 2 - 1 ) line emissions with a \sim 2 times higher angular resolution ( \sim 0 \farcs 5 ) and \sim 4 times better sensitivity than our ALMA Cycle 0 observations .
Continuum emission shows elongation perpendicular to the associated outflow , with a deconvolved size of 0 \farcs 53 \times 0 \farcs 15 .
C ^ { 18 } O emission shows similar elongation , indicating that both emissions trace the disk and the flattened envelope surrounding the protostar .
The velocity gradient of the C ^ { 18 } O emission along the elongation due to rotation of the disk/envelope system is re-analyzed , identifying Keplerian rotation proportional to r ^ { -0.5 } more clearly than the Cycle 0 observations .
The Keplerian-disk radius and the dynamical stellar mass are kinematically estimated to be \sim 74 AU and \sim 0.45 M _ { \sun } , respectively .
The continuum visibility is fitted by models without any annulus averaging , revealing that the disk is in hydrostatic equilibrium .
The best-fit model also suggests a density jump by a factor of \sim 5 between the disk and the envelope , suggesting that disks around protostars can be geometrically distinguishable from the envelope from a viewpoint of density contrast .
Importantly , the disk radius geometrically identified with the density jump is consistent with the radius kinematically estimated .
Possible origin of the density jump due to the mass accretion from the envelope to the disk is discussed .
C ^ { 18 } O observations can be reproduced by the same geometrical structures derived from the dust observations , with possible C ^ { 18 } O freeze-out and localized C ^ { 18 } O desorption .