Discovery of hot ammonia in the proto-planetary nebulae CRL 618 and CRL 2688

The first detection of ammonia toward CRL 618 in all the metastable lines from the (1,1) to the (7,7) is reported. While the (1,1) and (2,2) lines show P Cygni profiles with absorption and emission extended about +/-50 km/s from the systemic velocity of the circumstellar envelope, the higher lines (greater than 5 J) show a narrow absorption (about 2-3 km/s) at a radial velocity smaller than the expansion velocity of the envelope. The detection of the (3,3) and the (4,4) lines of NH3 toward CRL 2668 is also reported. The lower limits derived for the kinetic temperatures are 150 and 270 K for the broad and narrow absorptions in CRL 618, respectively, and 150 K in CRL 2688. The ammonia abundances in these sources, 0.6-10 x 10 exp -6, are up to more than two orders of magnitude higher than in IRC + 10216. It is suggested that the NH3 emission in protoplanetary nebulae arises in the postshock clumps left by the interaction of the fast stellar wind from the central star with the slow expanding circumstellar envelope.