  (Abridged) With the SMA we have made high angular-resolution (~1" = 160 AU)
observations of the protobinary system IRAS 16293-2422 in the J = 4-3 lines of
HCN and HC^15N, and in the continuum at 354.5 GHz. The HCN (4-3) line was also
observed using the JCMT to supply missing short spacing information.
Submillimeter continuum emission is detected from the individual binary
components with a separation of ~5". The HC^15N (4-3) emission has revealed a
compact (~500 AU) flattened structure (P.A. = -16 degree) associated with
Source A, and shows a velocity gradient along the projected minor axis, which
can be interpreted as an infalling gas motion. Our HCN image including the
short-spacing information shows an extended (~3000 AU) circumbinary envelope as
well as the compact structure associated with Source A. A toy model consisting
of a flattened structure with radial infall towards a 1 Msun central star
reproduces the HCN/HC^15N position-velocity diagram along the minor axis of the
HC^15N emission. In the extended envelope there is also a North-East (Blue) to
South-West (Red) velocity gradient across the binary alignment, which is likely
to reflect gas motion in the swept-up dense gas associated with the molecular
outflow from Source A. At Source B, there is only a weak compact structure with
much narrower line widths (~2 km/s) seen in the optically-thin HC^15N emission
than that at Source A (>10 km/s), and there is no clearly defined bipolar
molecular outflow associated with Source B. These results imply the different
evolutionary stages between Source A and B in the common circumbinary envelope.
