Evolution of the Activity of Comet Hale-Bopp (1995 O1) from August 1996 to April 1997

A program for measuring the activity of comet Hale-Bopp was conducted at the Observatoire de Haute-Provence from August 1996 to April 1997. The program consisted of spectroscopic observations in the long-slit mode by the 193 cm telescope and of observations by the 120 cm f/6 Newton telescope equipped with a 1024 x 1024 thinned CCD. A set of four narrow-band filters and a BK7 glass plate were used. Two filters at 527 and 682 nm, Delta lambda = 2 nm were centered on the C_2(1,1) and (0,0) Swan bands. The other two filters at 527 and 682 nm, Delta lambda = 5 nm were employed to obtain the spatial distributions of the intensity and color of dust. Comet Hale-Bopp was active very early. In August 1996, at a heliocentric distance r ~ 3 AU, it already showed four jets. In September, the number of jets had increased to six. An important evolution of the coma processes occured in January 1997. A series of regularly spaced arcs are clearly distinguished in the sunward hemisphere of the coma. A detailed study of the shape of the arcs during the night of April 7-8 is presented. The ratio of the images in the 682 and 527 nm filters provides a measure of the color of the coma compared to the color of the Sun. The images depicting the ratio, i-e the color of dust, show that the arcs have a spiral shape. They are compared with images of the C_2 (1,1)/(0,0) band ratio which provide the spatial distribution of the excitation temperature of C_2. The new type of cometary processes revealed by the presence of spiral arcs produces C_2 with a high degree of excitation and are probably a result of the fragmentation of a fra! ction of the dust grains.