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

TL;DR

Addressing the origin of frequent, short-lived flares from Sgr A*, the paper revisits tidal disruption and aerodynamic evaporation of small planetary bodies as a flare source. It couples material-strength–aware disruption with a meteor-like fireball luminosity model, constrained by data from OSIRIS-REx and Hayabusa2, predicting flare luminosities in the range $L\sim10^{34}-10^{36}$ erg s$^{-1}$ and durations matching observations. The flare statistics follow a power law with index $\alpha_L\simeq 1.83$ and a duration–luminosity scaling $t_{\rm f}\propto L^{-1/3}$, consistent with data. A reservoir of planets and small bodies around the central young stars, akin to a super-Oort cloud, can supply disruptions at a rate sufficient to account for the observed flares.

Abstract

Sgr A*, the supermassive black hole at the center of the Milky Way, exhibits frequent short-duration flares with luminosity greater than 1e34 erg/s across multiple wavelengths. The origin of the flares is still unknown. 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*. We refined previous models by incorporating material strength constraints on the tidal disruption limit and by evaluating the evaporation dynamics of the resulting fragments. 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 OSIRIS-REx and JAXA Hayabusa2), we estimated the survivability of fragments under aerodynamic heating and computed their expected luminosity from ablation, modeled as fireball flares analogous to meteor events. We find that planetary fragments can approach as close as 8 gravitational radii, consistent with observed flare locations. The fireball model yields luminosities from 1e34 to 1e36 erg/s for fragments whose parent bodies are a few kilometers in size. The derived flare frequency vs. luminosity distribution follows a power law with index 1.83, in agreement with observed values (1.65 - 1.9), while the flare duration scales as L^(-1/3), consistent with observations. We consider the 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 and long-period Neptunes, we show that this planetary reservoir can supply the observed flares.

Figures (3)

• Figure 1: Tidal downsizing and evaporation of planetary bodies. The maximum size of a planetary body surviving tidal forces is denoted by the gradient-filled regions; the color denotes the cohesive strength. The blue curve traces the maximum size of survivable small bodies under the tidal effect. Rubble piles ($>$100 m) are typically assumed to have a cohesion of 1 Pa, whereas monolithic bodies ($<$100 m) exhibit much higher cohesion, often exceeding $10^4~$Pa. The vertical gray line marks the classical Roche limit. The red line represents the minimum size of survivable small bodies under evaporation via gas drag during a single flyby. The shaded region represents the parameter space where small bodies can persist against both tidal disruption and hydrodynamic ablation.
• Figure 2: Luminosity produced by asteroids at $\sim$$8~R_\bullet$ based on the estimate of the bulk energy (red) and the fireball luminosity (blue). The luminosity of the observed flares is represented by the gray region.
• Figure 3: Predicted UV–optical emission spectra of photoionized asteroid material irradiated by ionizing continuum with $\phi = 2\times10^{15}~\mathrm{photons~s^{-1}~cm^{-2}}$. The four panels correspond to different combinations of gas density, $n_{\mathrm{H}} = 10^{6}$ (upper two panels) or $10^{7}~\mathrm{cm^{-3}}$ (bottom) and total hydrogen column density, $N_{\mathrm{H}} = 10^{18}$ (left) and $10^{19}~\mathrm{cm^{-2}}$ (right). Prominent emission lines are indicated by vertical dashed lines. We adopt a metallicity of $Z = 100\, Z_{\odot}$ and a covering factor of 5%. These spectra illustrate how the emission-line strengths vary with gas density and column depth in the evaporating asteroidal clouds.