Condensations and diffuse source of C2 in Comet Hyakutake C/1996 B2

An observation program of Comet Hyakutake C/1996 B2 was conducted in the long-slit spectrometric and narrow-band imagery modes at the Observatoire de Haute Provence on March 24–25 and March 30-April 2, 1996. Images in the continuum taken on March 31, at 19 h 30 UT, show dust jets in the solar-side hemisphere and the presence of two condensationsat r = 2000 and 8000 km (±200 km) along a line in the anti-solar direction. Images in the C2 (1,1) (at 513 nm) and (0,0) (at 516 nm) bands show the presence of an arc perpendicular to the anti-solar direction at r = 2000 km (±200 km). A bi-dimensional visualisation of the 513 nm516 nm intensity ratio for C2 and of the dust “red” colour parameter given by the 682 nm527 nm intensity ratio shows that both parameters are correlated in the inner coma at r < 5000 km. Synthetic spectra of C2 are calculated by using a model in which the vibrational and rotational temperatures are adjusted in order to obtain the best fit to the measured spectra. It is shown that both temperatures increase with increasing cometocentric distance. The C2513 nm516 nm ratio shows a V-type behaviour centred on the nucleus, with the exception of a region around 1200 km where it reaches a maximum value. It is suggested that the condensations are dust particle clusters released by the nucleus, which create a diffuse source producing the C2 molecules at a high excitation temperature. The colour of dust in this scheme suggests that the diffuse source contains a higher proportion of small grains than the surrounding coma.