# Formation and mass growth of axion stars in axion miniclusters

## TL;DR

This paper investigates the formation and mass evolution of axion stars within axion miniclusters, demonstrating their properties, merger behaviors, and mass growth dynamics consistent with wave condensation theories.

## Contribution

It provides the first detailed numerical analysis of axion star formation and mass growth inside miniclusters, confirming theoretical models of wave condensation and core-halo relations.

## Key findings

- Axion stars exhibit solitonic core properties similar to ultralight bosonic dark matter halos.
- Mergers follow an empirical law for mass conservation during axion star coalescence.
- Mass growth over time aligns with wave condensation models, predicting a saturation related to minicluster mass.

## Abstract

We study the formation and the subsequent mass growth of axion stars inside axion miniclusters. Numerically solving the Schr\"odinger-Poisson equations with realistic initial conditions we find that the axion stars exhibit similar properties to solitonic cores in ultralight bosonic dark matter halos in terms of their radial density profiles and large-amplitude oscillations. A merger of two axion stars confirms a previously found empirical law for the mass of the merged axion star. Monitoring the axion star masses over time, we observe a mass growth consistent with the mass increase of Bose stars in the kinetic regime reported by Levkov et al., confirming that the mass evolution of axion stars can be understood in terms of wave condensation. Based on this result, we predict a saturation of mass growth in relation to the minicluster mass consistent with the core-halo mass relation previously found for ultra-light bosonic dark matter halos.