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