A Similarity Theorem and Its Breakdown in Atomic Black Hole Accretion

Abstract

Atomic gas in a point-mass potential possesses an exact similarity that survives time dependence, two-body atomic microphysics, and a specified class of radiation and feedback laws. At fixed ambient temperature and composition, $M_\bullet\mapsto\lambda M_\bullet$ and $n_\infty\mapsto\lambda^{-1}n_\infty$ enlarge radii and times by $\lambda$ while preserving dimensionless profiles, optical depths, Eddington ratios, and variability. Here $M_\bullet$ is the central mass, $n_\infty$ the ambient number density, and $\lambda>0$ the scale factor. We prove this rescaling unique within the class. The symmetry also locates its boundary during rapid growth. Define the fractional mass gained in one Bondi time as $\epsilon_{\rm grow}=\dot M_\bullet t_{\rm B}/M_\bullet$, where $\dot M_\bullet$ is the retained rate and $t_{\rm B}$ the Bondi time. This quantity equals $\dot R_{\rm B}/c_\infty$, the expansion speed of the Bondi radius $R_{\rm B}$ in units of the ambient sound speed $c_\infty$; hence $\epsilon_{\rm grow}=1$ is sonic dilation. A retained law $\dot M_\bullet\propto M_\bullet^p$ with $p>0$ reaches this boundary after a finite increase in mass and leaves at most $(p\epsilon_0)^{-1}$ additional Bondi times, where $\epsilon_0$ is the initial loading. If retained, the canonical hyper-Eddington example has already crossed. Independently, no nontrivial stationary growing profile preserves both the atomic similarity and its self-consistent flux. The theorem therefore unifies radiating Bondi and feedback-regulated scalings and identifies where a relaxed fixed-mass continuation loses control.