Bondi Accretion onto a Luminous Object

Abstract

Abstract Spherical accretion of ionized gas onto a gravitating object is examined under the influence of central radiation. In classical Bondi accretion onto a non-luminous source with mass $ M$, the accretion rate $ \dot{M}_\mathrm{B}$ is expressed as $ \dot{M}_\mathrm{B}= \lambda(\gamma) \times 4\pi G^2 M^2\rho_\infty c_{\mathrm{s}\infty}^{-3}$, where $ \rho_\infty$ is the density at infinity and $ c_{\mathrm{s}\infty}$ the sound speed at infinity. We first found that the normalized accretion rate $ \lambda(\gamma)$ is approximated by $ \lambda(\gamma) = -(5/4)\gamma+ (19/8)$, instead of a rigorous expression. When the central object is a “spherical” source, the accretion rate $ \dot{M}$ reduces to $ \dot{M}_\mathrm{B} (1-\Gamma)^2$, where $ \Gamma$ is the central luminosity normalized by the Eddington one. If the central luminosity is produced by the accretion energy, the steady canonical luminosity is determined and the normalized luminosity does not exceed unity, as expected. On the other hand, when the central object is a “disk” source, such as an accretion disk, the accretion rate becomes $ \dot{M}/\dot{M}_\mathrm{B} = 1 -2\Gamma_\mathrm{d} + (4/3)\Gamma_\mathrm{d}^2$ for $ \Gamma_\mathrm{d} \leq 1/2$, and $ \dot{M}/\dot{M}_\mathrm{B} = 1 /(6\Gamma_\mathrm{d})$ for $ \Gamma_\mathrm{d} > 1/2$, where $ \Gamma_\mathrm{d}$ is the normalized disk luminosity. We also found steady canonical solutions, where the normalized luminosity can exceed unity for sufficiently large accretion rates. The anisotropic radiation field of accretion disks greatly modifies the accretion nature.