LYMAN-ALPHA OBSERVATIONS OF COMET HYAKUTAKE WITH SWAN ON SOHO

The SWAN instrument on board SOHO isa Lyman-alpha (L alpha) photometer able to map the sky intensity with a resolution of 1 degrees, and a c apability of microstepping (0.1 degrees). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its impr ints on the interplanetary sky background, but was in addition extensi vely used to map the L alpha emission of several comets since its laun ch in December 1995. Here we report observations of comet C/1996 B2 (H yakutake). Its L alpha emission cloud extended over more than 60 degre es while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distanc e to the Sun, pre-perihelion. The derived H2O production rate was foun d in fair agreement with other derivations (IUE and ground-based in th e IR and UV), validating the L alpha method. The H2O production by SWA N was related to several other measurements of minor constituents in o rder to derive new values of abundance of CO, HCN, H2CO, CH3OH and CH3 CN. Most important, the D/H ratio in comet Hyakutake is now found at 3 x 10(-4), as in comet Halley, while a previous estimate based on a wr ong H2O number had indicated a value twice lower, with important cosmo gonic consequences. The time evolution showed a fast surge on 21 March , coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the deta iled comparison of H column densities (observed vs model) as a functio n of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 Ma rch, and then a progressive filling-in of the H envelope. After the su rge, there was a plateau for 16 days around 1.8 x 10(29) H2O mol s(-1) ,and then an increase following approximately a R-2 law. This behavior is interpreted as the surge and plateau corresponding to the fragment ation and total disruption/evaporation of a fragment of the nucleus, o f approximately 200 m. Finally, it is argued that the first detection of ethane C2H6 in this comet (IR observations) might have been the res ult of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich c omets as argued by other authors. (C) 1998 Elsevier Science Ltd. All r ights reserved.