Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-05-20
Astronomy and Astrophysics
Astrophysics
8 pages, 3 figures
Scientific paper
10.1051/0004-6361:20052801
Radio observations in the ammonia inversion lines of four comets, C/2001 A2 (LINEAR), 153P/Ikeya-Zhang, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), were performed at the Effelsberg 100-m Radio Telescope during their respective close approaches to Earth. None of the four lowest energy metastable lines (J,K=J),J=1--4, could be detected in these comets. We derive the following 3 sigma upper bounds on the NH_3 production rate, and c omparing to the corresponding water production rates, percentage NH_3 abundances relative to H_2O: Q(NH_3) < 1.9x10^26 s^-1 (0.63%) for C/2001 A2 (LINEAR), Q(NH_3) < 2.7x10^26 s^-1 (0.13%) for C/2001 Q4 (NEAT), Q(NH_3) < 2.3x10^27 s^-1 (0.74%) for C/2002 T7 (LINEAR) and Q(NH_3) <= 6.3x10^26 s^-1 (0.63%) for Comet 153P/Ikeya-Zhang. At 0.74% or less, the ammonia-to-water ratios are factors of 2 below the value for C/1995 O1 (Hale-Bopp) and 1P/Halley, suggesting chemical diversity between comets. The 18-cm lines of OH were clearly detected in the two comets observed during the 2004 campaign, thereby validating the cometary ephemerides.
Bird Michael K.
der Tak Floris F. S. van
Hatchell Jennifer
Sherwood W. A.
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