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 NH3 toward the protoplanetary nebula CRL 618. The integrated flux density in the continuum indicates that the source has not experienced any sys tematic increase since 1980. The continuum emission is elongated in th e east-west direction and shows an elliptical shell structure, reminis cent of an extremely young planetary nebula. While the southern part o f the elliptical shell is rather uniform, the northern portion shows c lumpy structure. The walls of the shell are unresolved by our 0.066'' beam. The continuum structure indicates that the H II region is not is othermal, but the electron temperature increases toward the edges. Amm onia is detected only in absorption toward the continuum source. The d eepest absorption occurs at -50 km s-1, blueshifted with respect to th e radial velocity of the approaching part of the expanding envelope (a bout -40 km s-1). The ammonia lines show a bipolar distribution with t he blueshifted (radial velocities less than or similar to -45 km s-1) and redshifted (radial velocities greater than or similar to -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 o f 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 f rom the center to the western and eastern edges of the H II region. Th e data indicate that the ammonia absorption does not arise in the slow expanding asymptotic giant branch (AGB) envelope, but from the high-v elocity molecular outflow. The NH3 fractional abundance increases by m ore than three orders of magnitude from less than or similar to 8 x 10 (-9) in the AGB envelope to at least 2 x 10(-5) in the molecular outfl ow. The chemistry of NH3 in protoplanetary nebulae is briefly discusse d.