Hydrogen Axion Star: Metallic Hydrogen Bound to a QCD Axion BEC

TL;DR

This paper proposes the existence of Hydrogen Axion Stars, astrophysical objects where baryonic matter forms a metallic hydrogen cloud bound to an axion star, potentially observable through their black-body radiation.

Contribution

It introduces the concept of Hydrogen Axion Stars, analyzing their properties and potential observability, a novel astrophysical object not previously considered.

Findings

Hydrogen in HAS likely exists in a liquid metallic state.

HAS can produce significant internal power, making them luminous.

Potential for detection via ultraviolet, optical, and infrared telescopes.

Abstract

As a cold dark matter candidate, the QCD axion may form Bose-Einstein condensates, called axion stars, with masses around $10^{-11}\,M_{\odot}$. In this paper, we point out that a brand new astrophysical object, a Hydrogen Axion Star (HAS), may well be formed by ordinary baryonic matter becoming gravitationally bound to an axion star. We study the properties of the HAS and find that the hydrogen cloud has a high pressure and temperature in the center and is likely in the liquid metallic hydrogen state. Because of the high particle number densities for both the axion star and the hydrogen cloud, the feeble interaction between axion and hydrogen can still generate enough internal power, around $10^{13}~\mbox{W}\times(m_a/5~\mbox{meV})^4$, to make these objects luminous point sources. High resolution ultraviolet, optical and infrared telescopes can discover HAS via black-body radiation.