Mapping the Central Region of the PPN CRL 618 at Sub-arcsecond
  Resolution at 350 GHz
  CRL 618 is a well-studied pre-planetary nebula. We have mapped its central
region in continuum and molecular lines with the Submillimeter Array at 350 GHz
at ~ 0.3" to 0.5" resolutions. Two components are seen in 350 GHz continuum:
(1) a compact emission at the center tracing the dense inner part of the H II
region previously detected in 23 GHz continuum and it may trace a fast ionized
wind at the base, and (2) an extended thermal dust emission surrounding the H
II region, tracing the dense core previously detected in HC3N at the center of
the circumstellar envelope. The dense core is dusty and may contain mm-sized
dust grains. It may have a density enhancement in the equatorial plane. It is
also detected in carbon chain molecules HC3N and HCN, and their isotopologues,
with higher excitation lines tracing closer to the central star. It is also
detected in C2H3CN toward the innermost part. Most of the emission detected
here arises within ~ 630 AU (~0.7") from the central star. A simple radiative
transfer model is used to derive the kinematics, physical conditions, and the
chemical abundances in the dense core. The dense core is expanding and
accelerating, with the velocity increasing roughly linearly from ~ 3 km/s in
the innermost part to ~ 16 km/s at 630 AU. The mass-loss rate in the dense core
is extremely high with a value of ~ 1.15x10^{-3} solar mass per year. The dense
core has a mass of ~ 0.47 solar mass and a dynamical age of ~ 400 yrs. It could
result from a recent enhanced heavy mass-loss episode that ends the AGB phase.
The isotopic ratios of 12C/13C and 14N/15N are 9$\pm$4 and 150$\pm50$,
respectively, both lower than the solar values.
