Probing the formation of planetesimals in the Galactic Centre using Sgr A* flares

TL;DR

This paper investigates how planetesimals form in the Galactic Centre and their role in causing observable flares from Sgr A*, finding that debris discs around stars are the most consistent formation scenario with observed flare rates.

Contribution

It compares two formation scenarios for planetesimals in the Galactic Centre and models their orbital evolution to predict flare rates, favoring formation in debris discs around stars.

Findings

Predicted flaring rate is approximately 0.6 per day.

Flaring rate is nearly independent of perturber distribution.

Formation in debris discs around stars is more consistent with observations.

Abstract

Flares in X-ray and near infrared are observed above the quiescent emission of the supermassive black hole (SBH) in the Galactic Centre (GC) at a rate of approximately once per day. One proposed energy source for these flares is the tidal disruption of planetesimals with radius $\gtrsim 10$ km passing within $\sim$ 1 AU of the SBH. Very little is known about the formation and evolution of planetesimals in galactic nuclei such as the GC, making predictions for flaring event rates uncertain. We explore two scenarios for the formation of planetesimals in the GC: (1) in a large-scale cloud bound to the SBH, and (2) in debris discs around stars. We model their orbital evolution around the SBH using the Fokker-Planck equation and investigate the effect of gravitational interactions with various relevant perturbers. Our predicted flaring rate, $\approx 0.6 \, \mathrm{day^{-1}}$, is nearly independent of the distribution of perturbers. Moreover, it is insensitive to scenarios (1) or (2). The assumed number of planetesimals per star is consistent with debris discs around stars in the Solar neighbourhood. In scenario (1) this implies that the number of planetesimals formed in the large-scale cloud is strongly correlated with the number of stars, and this requires finetuning for our results to be consistent with the observed flaring rate. We favour the alternative explanation that planetesimals in the GC are formed in debris discs around stars, similar to the Solar neighbourhood.