2013ApJ...770..153L

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.''3-0.''5 resolutions. Two components are seen in the 350 GHz continuum: (1) a compact emission at the center tracing the dense inner part of the H II region previously detected in a 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 HC₃ N at the center of the circumstellar envelope. The dense core is dusty and may contain millimeter-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 CH₂ CHCN toward the innermost part. Most of the emission detected here arises within ∼630 AU (0.''7) of 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 M_☉/yr. The dense core has a mass of ∼0.47 M_☉ and a dynamical age of ∼400 yr. It could result from a recent enhanced heavy mass-loss episode that ends the asymptotic giant branch phase. The isotopic ratios of ¹²C/¹³C and ¹⁴N/¹⁵N are 9±4 and 150±50, respectively, both lower than the solar values.