Dilute and dense axion stars
Abstract
Axion stars are hypothetical objects formed of axions, obtained as localized and coherently oscillating solutions to their classical equation of motion. Depending on the value of the field amplitude at the core θ_{0}  ≡  θ (r = 0) , the equilibrium of the system arises from the balance of the kinetic pressure and either selfgravity or axion selfinteractions. Starting from a general relativistic framework, we obtain the set of equations describing the configuration of the axion star, which we solve as a function of θ_{0} . For small θ_{0}  ≲ 1, we reproduce results previously obtained in the literature, and we provide arguments for the stability of such configurations in terms of first principles. We compare qualitative analytical results with a numerical calculation. For large amplitudes θ_{0}  ≳ 1, the axion field probes the full nonharmonic QCD chiral potential and the axion star enters the dense branch. Our numerical solutions show that in this latter regime the axions are relativistic, and that one should not use a single frequency approximation, as previously applied in the literature. We employ a multiharmonic expansion to solve the relativistic equation for the axion field in the star, and demonstrate that higher modes cannot be neglected in the dense regime. We interpret the solutions in the dense regime as pseudobreathers, and show that the lifetime of such configurations is much smaller than any cosmological time scale.
 Publication:

Physics Letters B
 Pub Date:
 February 2018
 DOI:
 10.1016/j.physletb.2017.12.010
 arXiv:
 arXiv:1710.08910
 Bibcode:
 2018PhLB..777...64V
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 11 pages, 4 figures. v2: added references, matches published version