Axions and the cooling of white dwarf stars

TL;DR

This paper investigates whether axions, hypothetical particles, influence white dwarf cooling by comparing theoretical models with precise observational luminosity functions, suggesting axions with masses around a few meV could be detectable through stellar evolution data.

Contribution

It introduces a method to use white dwarf luminosity functions to test for the existence of axions and constrains their mass based on stellar cooling observations.

Findings

Axions with mass around 5 meV improve model-observation agreement.

Values of axion mass larger than 10 meV are excluded.

White dwarf luminosity function is sensitive to axion emission effects.

Abstract

White dwarfs are the end-product of the lifes of intermediate- and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m_a cos^2 \beta ~ 5 meV, where m_a is the mass of the axion and cos^2 \beta is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existing theoretical and observational uncertainties do not allow yet to confirm the existence of axions, but our results clearly show that if their mass is of the order of few meV, the white dwarf luminosity function is sensitive enough to detect their existence.