Jets and Arcs in the Three H-Comets: Halley, Hyakutake and Hale-Bopp

J. Clairemidi, C. Laffont, P. Rousselot, G. Moreels (Observatoire de Besancon)

A program of observations of comet Hale-Bopp conducted from the Observatoire de Haute-Provence in 1996-97 showed the presence of jets and arcs. We used a 120 cm telescope equipped with a 1024 x 1024 CCD camera and four narrow-band filters centered at 513 nm (C tex2html_wrap_inline11 1,1 band), 516 nm (C tex2html_wrap_inline11 0,0 band), 527 nm (visual continuum) and 682 nm (red continuum). A series of regularly spaced arcs enrolled in a spiral is clearly apparent in the images on April 7-8, 1997.

Images of comet Hyakutake taken on March 31, 1996 were processed by a software algorithm which subtracts from the original image: i) the same image having suffered a rotation by an angle tex2html_wrap_inline15 3 degrees (differential rotation) and ii) the same image having been smoothed (mask). Jets and arcs comparable to the ones observed in comet Hale-Bopp distinctly appear in the sunward hemisphere of the coma. In the anti-sunward direction, two condensation clusters are located along the thin tail.

In the case of Halley, the monochromatic maps of the coma provided by the Vega TKS spectrometer on March 9, 1986, were used to compute the spatial distribution of two color ratios: blue/visual and red/visual. Regularly spaced arcs are also present in the display.

The presence of arcs in the sunward hemisphere was detected in the three H-Comets. The arcs are detected in the spatial distribution of the dust-scattered intensity, the color ratios and the C tex2html_wrap_inline11 (1,1)/(0,0) ratio. Their presence shows that in modeling the dust distribution in the coma, mechanisms such as the fragmentation of grains and centrifugal force should be taken into account, in addition to the radiation pressure and drag effects.