Letter The puzzling detection of the 22 GHz water emission line in Comet Hyakutake at perihelion

Abstract. We observed Comet Hyakutake C/1996 B2 during the period 26 April4 May 1996 around peri-helion searching for the 22 GHz neutral water line by using a fast multichannel spectrometer coupled to the 32 m dish of the Medicina radiotelescope. We report here not only the first certain detection of this radio spectral line in a comet and the unique detection of water in a celestial body at a distance of 0.23 AU from the Sun, but also strong physical anomalies in the behavior of the water molecule velocit-ies. The line is split into two clear and bright components and is positively shifted with respect o the comet rest velocity at velocities varying from 22 to 44 km/s during the four days of observation. As far as we know, a similar behavior of neutral molecules in comets has never been observed before as the outflow velocity should normally not exceed 1 km/s. A qualitative analysis of the data seems to support sublimation of excited water molecules from accel-erated icy grains which form an extended source around the nucleus. The strong coronal activity observed by the SOHO spacecraft in the same period may have contributed to the acceleration mechanism. A future quantitative analysis would be very useful for the study of the complex plasma-grain interaction problem and of the cometary activity at very clos