Are moons the primary source of white dwarf pollution?

Polluted white dwarf stars offer a unique way to study the bulk compositions of exoplanetary material. These stars show evidence of recent accretion of rocky bodies in the form of metal lines in their spectra. The spectra tells us about the relative elemental abundances of the accreted material. One of the challenges of this method is to determine the origins of the parent bodies for the pollution, be they asteroids, comets, or moons. Motivated by recent observations of excess Be in several polluted WDs attributable to accretion of icy moons, we set out to assess the prevalence of moons as white dwarf polluters through analyses of observed white dwarfs and n-body simulations.

By applying an analytical model for the duration of pollution events, we find that the extrapolated parent body masses resemble those of moons around giant planets in our Solar System more than they do asteroids. We characterize the fraction of polluted white dwarfs currently accreting moons as the fraction of white dwarf systems with at least one moon-hosting giant planet times the fraction of all accretions that are due to moons (as opposed to asteroids). The ratio of observable moon accretions to asteroid accretions is the ratio of the frequency of each type of accretion multiplied by the time scale for the observable accretion event. Combining results from our n-body simulations and previous simulations of asteroid accretions, we estimate that ~70% of the observed polluted white dwarfs are currently accreting moons rather than asteroids.