Recent searches for the radio lines of NH₃ in comets

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK e-mail: hatchell@astro.ex.ac.uk
³ Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 1 February 2005
Accepted: 9 May 2005

Abstract

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 , could be detected in these comets. We derive the following 3σ upper bounds on the NH₃ production rate , and comparing to the corresponding water production rates, percentage NH₃ abundances relative to H₂O:  s^-1 (0.63%) for C/2001 A2 (LINEAR),  s^-1 (0.13%) for C/2001 Q4 (NEAT),  s^-1 (0.74%) for C/2002 T7 (LINEAR) and  6.3  10²⁶ 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.