DETECTION OF AMMONIA EMISSION TOWARD OXYGEN-RICH EVOLVED STARS

Using the Effelsberg 100 m telescope, we have discovered emission in the (J, K)=(1, 1) and (2, 2) inversion transitions of ammonia (NH3) toward the circumstelIar envelopes of the Mira variable IK Tau and the OH/IR hypergiant IRC + 10420. Modeling our spectra, we find that [NH3/H-2] abundance ratios of a few times 10(-6) are required to explain the observational results. These values are of the same order as the ammonia abundances previously inferred from infrared absorption measurements toward oxygen-rich evolved stars. The high NH3 abundances implied by our observations present a challenge to models of circumstellar chemistry, which fall short of reproducing the observed values by several orders of magnitude. We also present high-quality spectra of the NH3 (1, 1), (2, 2), and (3, 3) lines toward the peculiar bipolar nebula OH 231.8+4.2, which is the only oxygen-rich (post-)AGB object with previously detected NH3 radio emission. Toward this source the (1, 1) profile looks strikingly different from the profiles of the (2, 2) and (3, 3) lines, which resemble each other closely. Similar to many millimeter-wavelength lines from other molecules, the NH3 (1, 1) line shows high-velocity emission. In contrast, the (2, 2) and (3, 3) lines have narrow line profiles that are quite different from profiles observed in spectra from any other molecule and arise from a warm (T-kin > 50 K) region dose to the central star. The high-velocity material dominating the (1, 1) spectrum is emitted from a cooler (less than or similar to 20 K), more extended region.