Axion Constraints from White Dwarf Cooling in 47 Tucanae

Abstract

We analyse the cooling of white dwarfs in the globular cluster 47 Tucanae to look
for evidence of axion emission affecting the rate of white dwarf cooling. If axions exist and
couple to electrons, then axions could be produced at an appreciable rate in the electrondegenerate core of a white dwarf through axion bremsstrahlung from electrons. The emission
of these axions would provide an additional cooling mechanism for white dwarfs that would
affect the cooling rate, and hints of axions have been suggested based on observations of
anomalous cooling reported for white dwarfs in the Galactic disc and halo. We performed
stellar evolution simulations of white dwarf cooling that accounted for the additional energy
loss due to axion bremsstrahlung from electrons, producing a suite of white dwarf cooling
models for different values of the axion-electron coupling constant, as well as the white dwarf
mass and envelope thickness. These cooling models are compared to observations of white
dwarfs in 47 Tucanae from the Hubble Space Telescope through an unbinned likelihood
analysis. The optimal model found by this analysis corresponds to the case of no axion
emission with a thick white dwarf envelope, and we find a new bound on the axion-electron
coupling of gaee ≤ 0.81 × 10−13 at 95% confidence level. This improves upon the previous
white dwarf cooling bound for this coupling and excludes the range of values favoured by the
axion hints from the anomalous cooling of Galactic white dwarfs.