The 150 AU Structure of the Radio Continuum and the Ammonia Bipolar Outflow in CRL 618

We present high angular resolution (0".096 x 0".066) observations of the radio continuum emission at 23 GHz and of the (2, 2) line of NH₃toward the protoplanetary nebula CRL 618. The integrated flux density in the continuum indicates that the source has not experienced any systematic increase since 1980. The continuum emission is elongated in the east-west direction and shows an elliptical shell structure, reminiscent of an extremely young planetary nebula. While the southern part of the elliptical shell is rather uniform, the northern portion shows clumpy structure. The walls of the shell are unresolved by our 0".066 beam. The continuum structure indicates that the H II region is not isothermal, but the electron temperature increases toward the edges. Ammonia is detected only in absorption toward the continuum source. The deepest absorption occurs at -50 km s-1, blueshifted with respect to the radial velocity of the approaching part of the expanding envelope (about -40 km s-1). The ammonia lines show a bipolar distribution with the blueshifted (radial velocities ≲ -45 km s-1) and redshifted (radial velocities ≳ -45 km s-1) absorptions found toward the eastern and western parts of the H II region, respectively. The most blueshifted gas is found at the eastern edge of the continuum source, possibly indicating that acceleration of the high-velocity gas is taking place at scales of a few 100 AU. The optical depth of the absorption lines is not uniform, but increases from the center to the western and eastern edges of the H II region. The data indicate that the ammonia absorption does not arise in the slow expanding asymptotic giant branch (AGB) envelope, but from the high-velocity molecular outflow. The NH3 fractional abundance increases by more than three orders of magnitude from 8 × 10-9 in the AGB envelope to at least 2 × 10-5 in the molecular outflow. The chemistry of NH3 in protoplanetary nebulae is briefly discussed.