Orbit decay of 2–100 au planetary remnants around white dwarfs with no gravitational assistance from planets

Abstract

ABSTRACT A widely held assumption is that each single white dwarf containing observable rocky debris requires the presence of at least one terrestrial or giant planet to have gravitationally perturbed the progenitor of the debris into the star. However, these planets could have been previously engulfed by the star or escaped the system, leaving behind asteroids, boulders, cobbles, pebbles, sand, and dust. These remaining small bodies could then persist throughout the host star’s evolution into a white dwarf at ≈2–100 au scales, and then be radiatively dragged into the white dwarf without the help of a planet. Here, we identify the parameter space and cooling ages for which this one metal-pollution mechanism is feasible by, for the first time, coupling Poynting–Robertson drag, the Yarkovsky effect, and the YORP effect solely from rapidly dimming white dwarf radiation. We find that this no-planet pollution scenario is efficient for remnant 10−5 to 10−4 m dust up to about 80 au, 10−4 to 10−3 m sand up to about 25 au, and 10−3 to 10−2 m small pebbles up to about 8 au, and perhaps 10−1 to 100 m small boulders up to tens of au. Further, young white dwarf radiation can spin-up large strengthless boulders with radii of 102–103 m to destruction, breaking them down into smaller fragments that then can be dragged towards the white dwarf. Our work hence introduces a planetless metal-pollution mechanism that may be active in some fraction of white dwarf planetary systems.