Axions and the cooling of white dwarf stars

TL;DR

This study tests whether axions can alter white dwarf cooling by incorporating axion energy losses into WD evolution and comparing to the high-precision SDSS white dwarf luminosity function. It shows that the bright portion of the luminosity function is governed by the averaged cooling rate and is relatively insensitive to star formation history, making it a clean probe of extra cooling channels. By varying the axion mass parameter through $m_a\cos^2\beta$, the authors find a best-fit near $5$ meV and exclude masses above $10$ meV, aligning with prior bounds and suggesting a potential, though not yet conclusive, signal for axions. The work highlights cosmological implications and calls for improved observational data, refined neutrino-rate calculations, and independent pulsation-based tests to strengthen evidence for or against axions.

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 β~ 5 meV, where m_a is the mass of the axion and cos^2 βis 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.

Figures (4)

• Figure 1: Luminosity functions of white dwarfs. Filled squares correspond to the luminosity function of white dwarfs (DA and non-DA types) obtained using the reduced proper motion method har06 and open squares to that obtained using spectroscopically identified DA white dwarfs deg08. The solid lines are the theoretical luminosity functions obtained for different ages of the Galaxy --- from left to right: 10, 11, 12 and 13 Gyr --- and a constant star formation rate. The dotted lines are the same for an exponentially decreasing star formation rate --- $\psi(t)=\exp{(-t/\tau)}$ with $\tau=0.5$, 3 and 5 Gyr --- and the same age of the Galaxy, 11 Gyr. All the luminosity functions have been normalized to the same observational data point.
• Figure 2: Energy losses for a $0.61 \, M_{\sun}$ white dwarf as a function of the bolometric magnitude. The dashed lines represent the axion luminosity for different values of $m_a \cos^2 \beta$ --- from top to bottom: $m_a \cos^2 \beta$ =10, 5, 1, 0.1 and 0.01 meV. The thick solid line is the photon luminosity, while the thin solid line shows the neutrino luminosity.
• Figure 3: White dwarf luminosity functions for different values of the axion mass. The luminosity functions have been computed assuming $m_a \cos^2 \beta$ = 0 (solid line), 5 (dashed line) and 10 (dotted line) meV.
• Figure 4: Value of $\chi^2$ as a function of the mass of the axion for the case in which the initial--final mass relationship of cat08 (solid line) and that of wood (dotted line) are used.