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As a cold dark matter candidate, the QCD axion may form Bose-Einsteincondensates, called axion stars, with masses around $10^{-11}\,M_{\odot}$. Inthis paper, we point out that a brand new astrophysical object, a HydrogenAxion Star (HAS), may well be formed by ordinary baryonic matter becominggravitationally bound to an axion star. We study the properties of the HAS andfind that the hydrogen cloud has a high pressure and temperature in the centerand is likely in the liquid metallic hydrogen state. Because of the highparticle number densities for both the axion star and the hydrogen cloud, thefeeble interaction between axion and hydrogen can still generate enoughinternal power, around $10^{13}~\mbox{W}\times(m_a/5~\mbox{meV})^4$, to makethese objects luminous point sources. High resolution ultraviolet, optical andinfrared telescopes can discover HAS via black-body radiation.

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