Radiation Pressure effects on Cometary Dust Tails in Unusual Contexts

The dynamics of cometary dust grains are usually considered to be controlled by the parameter β, which is defined as the ratio of solar radiation pressure force acting on a particle to the gravitational force of the Sun exerted on the same grain, which acts in the opposite direction. As both radiation pressure and gravitational attraction follow inverse square laws, a particle maintains the same β value no matter where it is in the Solar System, assuming that the grain’s physical characteristics are constant. Here, we summarize the results of studies into circumstances where the effective β value of an unchanging grain is not constant; these results suggest that a significant reconsideration of the approaches taken to comet dust dynamical modelling may be required in some contexts.

First, we consider deviations of radiation pressure from an inverse square law in regions close to the Sun. Far from the Sun, our star can be approximated as a point source of light. However, closer in, its angular width in the sky becomes significant, so solar photons do not arrive at a dust grain along parallel paths, decreasing the magnitude of the radiation pressure acting on a dust grain in the radial direction. Furthermore, close to the Sun’s surface, the Sun’s visible disk does not encompass a full solar hemisphere, further decreasing the photon flux at a dust grain. We apply corrections to the effective β value of dust grains to take into account these effects, as well as additional relevant factors. We demonstrate, using Finson-Probstein-based dust tail modelling of observed comets, that the change in the effective β values of dust grains in this near-Sun region leads to observable changes in the morphology of modelled comet tails.

Second, we consider the radiation pressure effects acting on cometary dust in binary star systems. In these contexts, the net gravitation and radiation pressure forces acting on dust grains are not necessarily antiparallel. We present examples of the possible morphologies of exocomets’ dust tails in a small sample of binary star systems. We show through dust tail simulations in these multi-star systems that exocomets with the same dust populations as some example Solar System objects can have markedly different appearances to those they have in orbit about our Sun.