Bondi-Hoyle-Lyttleton accretion flow in a stratified layer

Abstract

We compute the density and velocity profiles along the tail induced by a body of mass $M$, embedded in the midplane of a vertically-stratified media with scaleheight $H$, adopting a one-dimensional model as in the Bondi-Hoyle-Lyttleton problem. In analogy to what occurs in the case of a homogeneous medium, there exist a family of solutions that satisfy the boundary conditions. A shooting method is employed to isolate those solutions that fulfill a specific set of physical and mathematical constraints. The tail is found to be both densest and slowest when the scaleheight $H$ is equal to the gravitational radius $\xi_{0}\equiv GM/v_{0}^{2}$, where $v_{0}$ its relative velocity with respect to the medium. The location of the stagnation point is evaluated as a function of $H$ and $\xi_{0}$, and an empirical fitting formula is provided. While the distance to the stagnation point is maximized when $H\simeq \xi_{0}$, the mass accretion rate attains its maximum value for $H \ll \xi_{0}$ at fixed surface density. When instead the midplane density is held constant and $H$ is varied, the accretion rate hardly changes once $H$ exceeds about $2\xi_{0}$. Additionally, we investigate how both the drag force resulting from mass accretion and the gravitational drag arising from its tail depend on $H/\xi_{0}$. We highlight how the effect of varying the degree of mixing in the tail influences the resulting drag force. Finally, for the particular case of an infinitely thin layer, we provide a simple analytical solution, which may serve as a useful pedagogical reference.