Observations of the OH radical in comet C/1996 B2 (Hyakutake) with the Nançay radio telescope

The 18-cm lines of the OH radical were monitored in C/1996 B2 (Hyakutake) with the Nançay radio telescope between 1 March and 18 May 1996. Besides providing regular measurements of the water production rate, the C/1996 B2 campaign was useful to test and improve existing models of the OH density, excitation and tranfer because the comet passed successively close to the Earth and close to the Sun. At the time of closest approach to Earth, a rough mapping was made by observing at offset positions several beamwidths (3.5′) from the nucleus together with the centre position. The OH line intensity at the centre was comparble to those at offset positions, providing direct evidence for collisional quenching of the OH maser in the inner coma. After removing the Greenstein effect, the line profiles suggest a slight excess outgassing towards the Sun. We are able to detect the OH satellite lines at 1612 and 1721 MHz. A weak Zeeman splitting of the 1667 and 1665 MHz transitions was observed corresponding to an average line-of-sight magnetic field in the range +5 to +10 nT.