Red giant stellar collisions in the Galactic Centre

TL;DR

This study investigates whether stellar collisions with black holes can explain the observed absence of certain bright giant stars in the Galactic Centre, combining stellar evolution models and collision rate simulations.

Contribution

It provides a detailed analysis showing collisions can explain the missing mid-brightness giants but not the brightest ones, constraining black hole populations.

Findings

Collisions can explain missing giants in the 10.5<K<12 band.

Depleting brighter giants requires an unrealistically large black hole population.

Black hole collisions cannot account for the depletion of the brightest giants.

Abstract

We show that collisions with stellar--mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low--mass giants and AGB stars in the range 1-3 M$_{\odot}$. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculations, restricted three--body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main--sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the $10.5<K<12$ band. However, depleting the brighter ($K<10.5$) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the $10.5<K<12$ giants in a region much larger than is observed. We conclude that collisions with stellar--mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar--mass black holes in the Galactic Centre.