SHOCK-ENHANCED AMMONIA EMISSION IN THE EGG NEBULA

We present high angular resolution observations of the NH3(1, 1), (2, 2), and (3, 3) inversion transitions from the Egg nebula, the archetypical protoplanetary nebula. The spatial distribution and kinematics of the emission in all three lines show four distinct components or lobes that are aligned with the polar and equatorial directions. The kinematics of the NH3 emission is also found to follow a clear pattern: redshifted emission in the south and west and blueshifted emission in the north and east. The morphology and spatial kinematics of NH3 emission are shown to have strong similarity to that observed previously in molecular hydrogen emission and CO emission which arise from the shocked molecular gas. We also find that the higher lying inversion transition NH3 (2, 2) and (3, 3) are stronger in the polar direction in comparison to the lower transition NH3 (1, 1). We conclude that the NH3 emission traces the warm molecular gas, which is shocked and heated by the interaction between the high-velocity outflows and the surrounding envelope. The presence of strong ammonia emission associated with the shock fronts and the lack of the emission at the center of the nebula indicate that the abundance of ammonia is significantly enhanced by shocks, a situation very similar to that found in outflows from protostars.