Modeling dust halos in comet Hale-Bopp (1995 O1): Existence of two active nuclei unequivocally implied

The morphology of the comet's expanding, nearly concentric dust halos, whic h were observed extensively in 1997 February-April, is modeled by employing a Monte Carlo computer-simulation technique. A satisfactory model for the halos in late March requires only a slight correction to the spin-vector po sition determined previously from the diurnal evolution of a jet on Februar y 28. A surprising result is the finding that, with this spin constraint, t he halos observed in late February, only a few days before the jet, cannot be accounted for by ejecta from any combination of dust sources on the nucl eus. Instead, the presence of a source on another nearby object-a second nu cleus-is clearly implied. The spin vectors of the two nuclei are found to h ave subtended an angle greater than 90 degrees, but the tilt of their equat orial planes to the comet's heliocentric orbital plane may have practically been the same. Dust emission from the second nucleus is also shown to acco unt for some of the fine jet features observed in late March. Thus, the exi stence of an active satellite nucleus, independent evidence for which was r eported elsewhere, is strongly suggested by the comet's dust morphology.