Infalling-Rotating Motion and Associated Chemical Change in the Envelope
  of IRAS 16293-2422 Source A Studied with ALMA
  We have analyzed rotational spectral line emission of OCS, CH3OH, HCOOCH3,
and H2CS observed toward the low-mass Class 0 protostellar source IRAS
16293-2422 Source A at a sub-arcsecond resolution (~0".6 x 0".5) with ALMA.
Significant chemical differentiation is found at a 50 AU scale. The OCS line is
found to well trace the infalling-rotating envelope in this source. On the
other hand, the CH3OH and HCOOCH3 distributions are found to be concentrated
around the inner part of the infalling-rotating envelope. With a simple
ballistic model of the infalling-rotating envelope, the radius of the
centrifugal barrier (a half of the centrifugal radius) and the protostellar
mass are evaluated from the OCS data to be from 40 to 60 AU and from 0.5 to 1.0
Msun, respectively, assuming the inclination angle of the envelope/disk
structure to be 60 degrees (90 degrees for the edge-on configuration). Although
the protostellar mass is correlated with the inclination angle, the radius of
the centrifugal barrier is not. This is the first indication of the centrifugal
barrier of the infalling-rotating envelope in a hot corino source. CH3OH and
HCOOCH3 may be liberated from ice mantles due to weak accretion shocks around
the centrifugal barrier, and/or due to protostellar heating. The H2CS emission
seems to come from the disk component inside the centrifugal barrier in
addition to the envelope component. The centrifugal barrier plays a central
role not only in the formation of a rotationally-supported disk but also in the
chemical evolution from the envelope to the protoplanetary disk.
