ALMA Observations of the Protostar L1527 IRS: Probing Details of the
  Disk and the Envelope Structures
  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 C18O (J=2-1) line emissions with a ~2 times higher angular
resolution (~0.5") and ~4 times better sensitivity than our ALMA Cycle 0
observations. Continuum emission shows elongation perpendicular to the
associated outflow, with a deconvolved size of 0.53"x0.15". C18O emission shows
similar elongation, indicating that both emissions trace the disk and the
flattened envelope surrounding the protostar. The velocity gradient of the C18O
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 ~74 AU and ~0.45 Mo,
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 ~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. C18O observations can be reproduced by the same geometrical
structures derived from the dust observations, with possible C18O freeze-out
and localized C18O desorption.
