Dense Axion Stars

TL;DR

This paper introduces a new class of dense axion stars, expanding the understanding of axion star configurations beyond the previously known dilute solutions, with potential implications for axion dark matter phenomena.

Contribution

It presents a novel branch of dense axion star solutions using an effective field theory approximation, showing gravity balanced by mean-field pressure, unlike traditional dilute axion stars.

Findings

Discovery of a dense axion star branch with masses up to one solar mass.

Dense axion stars are stabilized by mean-field pressure, not just kinetic pressure.

Collapse of dilute axion stars could lead to dense axion star formation.

Abstract

If the dark matter particles are axions, gravity can cause them to coalesce into axion stars, which are stable gravitationally bound systems of axions. In the previously known solutions for axion stars, gravity and the attractive force between pairs of axions are balanced by the kinetic pressure. The mass of these dilute axion stars cannot exceed a critical mass, which is about $10^{-14} M_\odot$ if the axion mass is $10^{-4}$~eV. We study axion stars using a simple approximation to the effective potential of the nonrelativistic effective field theory for axions. We find a new branch of dense axion stars in which gravity is balanced by the mean-field pressure of the axion Bose-Einstein condensate. The mass on this branch ranges from about $10^{-20} M_\odot$ to about $M_\odot$. If a dilute axion star with the critical mass accretes additional axions and collapses, it could produce a bosenova, leaving a dense axion star as the remnant.