Tidal disruption and evaporation of rubble-pile and monolithic bodies as a source of flaring activity in Sgr A^★

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
² Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
³ Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, 48109 USA
⁴ Department of Physics, University of California, Santa Barbara, USA
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, USA
⁶ Institute for Advanced Studies, Tsinghua University, Beijing, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 May 2025
Accepted: 19 October 2025

Abstract

Context. Sgr A^★, the supermassive black hole at the center of the Milky Way, exhibits frequent short-duration flares (e.g., with luminosity > 10³⁴ erg s⁻¹) across multiple wavelengths. The origin of the flares is still unknown.

Aims. We revisited the role of small planetary bodies, originally from the stellar disk, and their tidally disrupted fragments as a source of flaring activity in Sgr A^★. In particular, we refined previous models by incorporating material strength constraints on the tidal disruption limit and by evaluating the evaporation dynamics of the resulting fragments.

Methods. We analyzed the tidal fragmentation and gas-induced fragmentation of small planetary bodies with rubble-pile and monolithic structures. Using constraints from recent space missions (e.g., NASA’s OSIRIS-REx and JAXA’s Hayabusa2 missions), we estimated the survivability of fragments under aerodynamic heating and computed their expected luminosity from ablation, modeled as fireball flares analogous to meteor events.

Results. We find that planetary fragments can approach as close as 8 R_• due to material strength, where R_• denotes the gravitational radius consistent with flare locations inferred from observations. The fireball model yields luminosities from 10³⁴ to 10³⁶ erg/s for fragments whose parent bodies are a few kilometers in size. The derived flare frequency–luminosity distribution follows a power law with the power index 1.83, in agreement with observed values (1.65–1.9), while the flare duration scales as t_f ∝ L^−1/3, consistent with observational constraints. We considered the discovered young stars around Sgr A^★ as the planetary reservoir. Given a small-body population analogous in mass to the primordial Kuiper belt and the common existence of close-in super-Earths as well as long-period Neptunes, we show that this planetary reservoir can provide an adequate supply for the observed flares.

Conclusions. The tidal disruption and thermal evaporation of small bodies offer a plausible explanation for the observed flare properties of Sgr A^★.