Impact of accretor size on the morphology of supersonic Bondi-Hoyle-Lyttleton accretion flows

TL;DR

This study investigates how the size of a moving accretor influences the flow structures and accretion behavior in supersonic Bondi-Hoyle-Lyttleton flows through hydrodynamic simulations, revealing the importance of resolving key physical scales.

Contribution

It provides a detailed analysis of the impact of accretor size on flow morphology and accretion, emphasizing the necessity of resolving the Hoyle-Lyttleton radius in simulations.

Findings

Smaller accretors form bow shocks and spherical atmospheres.

Accretors smaller than the Hoyle-Lyttleton radius produce Mach cones.

Larger accretors show a flow transition from supersonic to subsonic.

Abstract

Fast-moving accretors are ubiquitous in astrophysics. Their interaction with surrounding gas leaves characteristic imprints, forming structures like bow shocks, Mach cones, and density trails. We study how various physical processes affect the flow structure around an accretor with a one-way surface, its accretion rate, and accretion anisotropy. These processes correspond to distinct length scales: the Bondi radius, the bow shock's stand-off distance, and the Hoyle-Lyttleton radius. We conducted adiabatic hydrodynamic simulations using a spherical coordinate grid centred on the accretor. By varying the accretor's (numerical) size across scales -- from much smaller than the stand-off distance to much larger than the Bondi radius -- we analyse how these spatial scales affect steady-state flow physics. All simulations reach a steady state. When the accretor is smaller than the stand-off distance, a bow shock forms ahead, and a nearly spherically symmetric atmosphere develops within. Accretors smaller than the Hoyle-Lyttleton radius produce a Mach cone, while larger ones exhibit a supersonic-to-subsonic flow transition on larger scales. Fully resolved simulations align with Hoyle-Lyttleton theory, showing slightly anisotropic accretion with enhanced inflow from behind. In contrast, larger accretors approach the geometrical limit, accreting mainly from the flow direction, with a low-density 'shadow' forming behind. The accretor's size strongly influences small- and large-scale morphologies. Resolving the Hoyle-Lyttleton radius is essential for capturing large-scale flow characteristics. Resolving the stand-off distance is needed only to study the bow shock: since it determines the shock's position, its non-resolution does not affect large-scale flow morphology.