Axion Stars in the Infrared Limit

Joshua Eby; Peter Suranyi; Cenalo Vaz; L. C. R. Wijewardhana

doi:10.1007/JHEP03(2015)080

paper

Axion Stars in the Infrared Limit

arXiv:1412.3430 · doi:10.1007/JHEP03(2015)080

Abstract

Following Ruffini and Bonazzola, we use a quantized boson field to describe condensates of axions forming compact objects. Without substantial modifications, the method can only be applied to axions with decay constant, $f_{a}$ , satisfying $δ = (f_{a} / M_{P})^{2} ≪ 1$ , where $M_{P}$ is the Planck mass. Similarly, the applicability of the Ruffini-Bonazzola method to axion stars also requires that the relative binding energy of axions satisfies $Δ = 1 - (E_{a} / m_{a})^{2} ≪ 1$ , where $E_{a}$ and $m_{a}$ are the energy and mass of the axion. The simultaneous expansion of the equations of motion in $δ$ and $Δ$ leads to a simplified set of equations, depending only on the parameter, $λ = δ / Δ$ in leading order of the expansions. Keeping leading order in $Δ$ is equivalent to the infrared limit, in which only relevant and marginal terms contribute to the equations of motion. The number of axions in the star is uniquely determined by $λ$ . Numerical solutions are found in a wide range of $λ$ . At small $λ$ the mass and radius of the axion star rise linearly with $λ$ . While at larger $λ$ the radius of the star continues to rise, the mass of the star, $M$ , attains a maximum at $λ_{max} ≃ 0.58$ . All stars are unstable for $λ > λ_{max}$ . We discuss the relationship of our results to current observational constraints on dark matter and the phenomenology of Fast Radio Bursts.

arXiv comment14 pages, 2 figures

Axion Stars in the Infrared Limit

Abstract

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