Jets and arcs in the coma of Comet Hale-Bopp from August 1996 to April 1997

An observation program for measuring the activity of Comet Hale-Bopp was conducted at the Observatoire de Haute Provence from August 1996 to April 1997. Narrow band images were obtained from the 120 cm telescope. A set of four filters at 513, 516, 527 and 682 nm was used to measure the C-2 emission in the (1,1) and (0,0) band and the intensity of the dust-scattered continuum. The luminosity of the central part of the coma was measured in the continuum and C-2 emissions, as a function of heliocentric distance before perihelion. The activity of the comet considerably increased between October 1996 and February 1997. The images obtained in August, September and October 1996 show the presence of strong jets. In February, March and April 1997, series of roughly concentric arcs were observed in the sunward hemisphere of the coma. The arcs were also detected in the spatial distribution of the I(682 nm)/I(527 nm) red color ratio and the I(513 nm)/I(516 nm) C-2 emission ratio. Using the I(513 nm)/I(516 nm) ratio as a parameter to measure the degree of excitation of C-2 molecules, we can show that the local production of gas, probably resulting from dust fragmentation, should be taken into account.