Three-Dimensional Hydrodynamic Bondi-Hoyle Accretion. V. Specific Heat Ratio 1.01, Nearly Isothermal Flow

Abstract

We investigate the hydrodynamics of three-dimensional classical Bondi-Hoyle accretion. A totally absorbing sphere of different sizes (1, 0.1 and 0.02 accretion radii) moves at different Mach numbers (0.6, 1.4, 3.0 and 10) relative to a homogeneous and slightly perturbed medium, which is taken to be an ideal, nearly isothermal, gas ($γ=1.01$). The hydrodynamics is modeled by the ``Piecewise Parabolic Method'' (PPM). The resolution in the vicinity of the accretor is increased by multiply nesting several $32^3$-zone grids around the sphere, each finer grid being a factor of two smaller in zone size than the next coarser grid. grids. For small Mach numbers (0.6 and~1.4) the flow patterns tend towards a steady state, while in the case of supersonic flow (Mach~3 and~10) and small enough accretors (radius of~0.1 and~0.02 accretion radii), an unstable Mach cone develops, destroying axisymmetry. The shock cones in the supersonic models never clear the surface of the accretors (they are tail shocks, not bow shocks) and the opening angle is smaller (compared to models with larger $γ$) especially for the highly supersonic models. The densities in the shock cone is larger for models with smaller $γ$. The fluctuations of the accretion rates and flow structures are weaker than in the corresponding models with larger $γ$. The hydrodynamic drag of all models with accretor sizes of 0.1~$R_{\rm A}$ or smaller acts in an accelerating direction, while the gravitational drag is always decelerating and larger than the hydrodynamic drag (thus the net force is decelerating).