Dynamical friction with radiative feedback - II. High-resolution study of the subsonic regime

Recent work has suggested that the net gravitational force acting on a massive and luminous perturber travelling through a gaseous and opaque medium can have same direction as the perturber's motion (an effect sometimes called negative dynamical friction). Analytic results were obtained using a linear analysis and were later confirmed by means of non-linear numerical simulations which did not resolve the flow within the Bondi sphere of the perturber, hence effectively restricted to weakly perturbed regions of the flow. Here we present high-resolution simulations, using either 3D Cartesian or 2D cylindrical meshes that resolve the flow within the Bondi sphere. We perform a systematic study of the force as a function of the perturber's mass and luminosity, in the subsonic regime. We find that perturbers with mass M smaller than a few M-c similar to chi c(s)/G are subjected to a thermal force with a magnitude in good agreement with linear theory (. being the thermal diffusivity of the medium, c(s) the adiabatic sound speed, and G the gravitational constant), while for larger masses, the thermal forces are only a fraction of the linear estimate that decays as M-1. Our analysis confirms the possibility of negative friction (hence a propulsion) on sufficiently luminous, low-mass embryos embedded in protoplanetary discs. Finally, we give an approximate expression of the total force at low Mach number, valid both for subcritical (M < M-c) and supercritical (M > M-c) perturbers.