Fallback Rates in Partial Tidal Disruptions of White Dwarfs by Intermediate-mass Black Holes

Abstract

The fallback rate of debris after the partial tidal disruption of a star by an intermediate-mass black hole (IMBH) might provide important signatures of such black holes rather than supermassive ones. Here using smoothed particle hydrodynamics methods, we provide a comprehensive numerical analysis of this phenomenon. We perform numerical simulations of single partial tidal disruptions of solar-mass white dwarfs in parabolic orbits, with a nonspinning 103 M ⊙ IMBH for various values of the impact parameter, and determine the core mass fractions and fallback rates of debris into the IMBH. For supermassive black holes, in full disruption processes, it is known that the late-time fallback rate follows a power law t −5/3, whereas for partial disruptions, such a rate has recently been conjectured to saturate at a steeper power law t −9/4, independent of the mass of the remnant core. We show here that for IMBHs, partial disruptions significantly alter this conclusion. That is, the fallback rate at late times does not asymptote to a t −9/4 power law, and this rate is also a strong function of the core mass. We derive a robust formula for the late-time fallback rate as a function of the core mass fraction, which is independent of the mass of the white dwarf, as we verify numerically by varying it.