Direct Monte Carlo and multifluid modeling of the circumnuclear dust coma

This paper describes the first computations of dust distributions in the vicinity of an active cometary nucleus, using a multidimensional Direct Simulation Monte Carlo Method (DSMC). The physical model is simplistic: spherical grains of a broad range of sizes are liberated by H2O sublimation from a selection of nonrotating sunlit spherical nuclei, and submitted to the nucleus gravity, the gas drag, and the solar radiation pressure. The results are compared to those obtained by the previously described Dust Multi-Fluid Method (DMF) and demonstrate an excellent agreement in the regions where the DMF is usable. Most importantly, the DSMC allows the discovery of hitherto unsuspected dust coma properties in those cases which cannot be treated by the DMF. This leads to a thorough reconsideration of the properties of the near-nucleus dust dynamics. In particular, the results show that (1) none of the three forces considered here can be neglected a priori, in particular not the radiation pressure; (2) hitherto unsuspected new families of grain trajectories exist, for instance trajectories leading from the nightside surface to the dayside coma; (3) a wealth of balistic-like trajectories leading from one point of the surface to another point exist; on the dayside, such trajectories lead to the formation of “mini-volcanoes.” The present model and results are discussed carefully. It is shown that (1) the neglected forces (inertia associated with a nucleus rotation, solar tidal force) are, in general, not negligible everywhere, and (2) when allowing for these additional forces, a time-dependent model will, in general, have to be used. The future steps of development of the model are outlined.