THE PUZZLING DETECTION OF THE 22 GHZ WATER MISSION LINE IN COMET HYAKUTAKE AT PERIHELION

We observed Comet Hyakutake C/1996 B2 during the period 26 April-4 May 1996 around perihelion searching for the 22 GHz neutral water line by using a fast multichannel spectrometer coupled to the 32 m dish of th e Medicina radiotelescope. We report here not only the first certain d etection of this radio spectral line in a comet and the unique detecti on 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 molec ule velocities. The line is split into two clear and bright components and is positively shifted with respect to the comet rest velocity at velocities varying from 22 to 44 km/s during the four days of observat ion. As far as we know, a similar behavior of neutral molecules in com ets has never been observed before as the outflow velocity should norm ally not exceed 1 km/s. A qualitative analysis of the data seems to su pport sublimation of excited water molecules from accelerated icy grai ns which form an extended source around the nucleus. The strong corona l activity observed by the SOHO spacecraft in the same period may have contributed to the acceleration mechanism. A future quantitative anal ysis would be very useful for the study of the complex plasma-grain in teraction problem and of the cometary activity at very close distances from the Sun. (C) 1998 Elsevier Science Ltd. All rights reserved.