Hale–Bopp: What Makes a Big Comet Different? Coma Dynamics: Observations and Theory

Abstract

Comet Hale–Bopp was the largest comet by almost any definition,observed at least since the advent of modern observing techniques. Ina more typical comet both the chemical and dynamical influences ofcollisional processes are limited by the short time a parcel of gassublimated from the nucleus remains in the dense part of the coma. Theresulting large size of the collisional coma in comet Hale–Bopp had important consequences on the dynamics of the coma,which in turn has important consequences on how observations areinterpreted with standard models. Measured velocities of typical gasspecies (mostly the observed radicals) as well as dust were largerthan normal comets. Conversely, velocities of superthermalatomic hydrogen were smaller than normal because of the samecollisional processes. Furthermore, as a consequence, dust particles,which are dragged by the outflowing gas, were also accelerated tolarger velocities. Such larger velocities are not simply aninteresting curiosity in their own right, because nearly allobservations of dust and gas are interpreted with models of the comathat depend directly on some measurement or assumption with regard tovelocity. In this presentation both observations and theory regardingthe dynamical conditions in the coma of comet Hale–Bopp aresummarized.