DISCOVERY OF HOT AMMONIA IN THE PROTOPLANETARY NEBULAE CRL-618 AND CRL-2688

We report the first detection of ammonia toward CRL 618 in all the metastable lines from the (1,1) to the (7,7). The line profiles are remarkable: while the (1,1) and (2,2) lines show P Cygni profiles with absorption and emission extending almost-equal-to +/- 50 km s-1 from the systemic velocity of circumstellar envelope, the higher lines (J > 5) show a narrow absorption (DELTA-upsilon almost-equal-to 2-3 km s-1) at a radial velocity smaller than the expansion velocity of the envelope. We also present the detection of the (3,3) and the (4,4) lines of NH3 toward CRL 2688. The line profiles in this source are also much broader (almost-equal-to 80 km s-1) than the terminal velocity of the envelope. 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(-6), are up to more than two orders of magnitude higher than in IRC + 10216. We suggest 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.