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Session 62 - Comet Hyakutake Encounter.
Display session, Wednesday, June 12
Great Hall,
Ammonia is thought to contain a significant fraction of the nitrogen in cometary atmospheres. Unfortunately, the only direct evidence is a single detection of the 1.25cm (3,3) line toward Comet IRAS-Araki-Alcock by Altenhoff et al.(Aamp;A, 125, L19, 1983) and the in situ measurements of the peaks near mass 17 with the Giotto spacecraft ion mass spectrometer (IMS) and neutral mass spectrometer (NMS) for P/Halley. We report detection of the (1,1) and the (3,3) transitions of ammonia in Comet 1996/B2 (Hyakutake) with the 43m telescope of the National Radio Astronomy Observatory in Green Bank, West Va, on 1996 March 23/24.
Indirect estimates of the ammonia abundance can be made from optical spectra of NH_2 and NH if they are presumed to be photo - fragments of NH_3 only. From the NH_2 studies, a number of workers have obtained estimates in the range 0.1 - 0.5% for P/Halley. (All abundances herein are referred to that of water.) From a study of ultraviolet data on NH for P/Halley, Feldman et al. (ApJ, 404, 348, 1993) estimated an ammonia abundance in the range 0.4 -- 0.8%.
Analysis of the Giotto IMS data lead to an estimate of 1 -- 2% by Allen et al. (Aamp;A, 187, 502, 1987); and estimates from the NMS data lead to an estimate of 1.5% by Meier et al. (Aamp;A, 287, 268, 1994). Acceptance of these values would imply another source for NH and NH_2 in comets.
>From the strength of the measured radio lines in Comet Hyakutake, we find that the ammonia abundance is about 0.3%. This value is much less than the 1.5% derived from the Giotto data for P/Halley, and comparable to the indirect determinations from optical NH_2 lines.
