Random fluctuations versus Poynting–Robertson drag on interplanetary dust grains

Abstract

A particle moving through a sea of stochastic impulses gains energy from the sea; this is a long-established property of brownian motion, given theoretical backing in the fluctuation–dissipation theorem¹. However, this energy gain is generally neglected in describing circumstellar dust grains as slowly spiralling into the central star under Poynting–Robertson (P–R) drag. Stochastic forces on circumsolar grains arise from low-frequency fluctuations in surface charge and interplanetary fields, as has long been recognized^2,3, but previous studies have treated the mean orbital energy as constant^4,5 or slowly decreasing⁶. I show here that stochastic heating can balance and indeed overwhelm the dissipative P–R drag for small grains (≲1 µm scale radius). Furthermore, the consequent drift to the outer Solar System may cancel the strong collisional source inferred⁷ for micrometre and sub-micrometre fragments, explaining how their present distribution is stable in time. More generally, the stochastic heating mechanism combined with collisional fragmentation sets limits on dust accretion onto cool stars.