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, $M_\bullet\mapstoλM_\bullet$ and $n_\infty\mapstoλ<sup>{-1}n_\infty$ enlarge radii and times by $λ$ while preserving dimensionless profiles, optical depths, Eddington ratios, and variability. Here $M_\bullet$ is the central mass, $n_\infty$ the ambient number density, and $λ&gt;0$ 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 $$ε<em>{\rm grow}=\dot M</em>\bullet t_{\rm B}/M_\bullet$$, 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 $ε<em>{\rm grow}=1$ is sonic dilation. A retained law $\dot M</em>\bullet\propto M_\bullet<sup>p$ with $p&gt;0$ reaches this boundary after a finite increase in mass and leaves at most $(pε_0)<sup>{-1}$ additional Bondi times, where $ε_0$ 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.