An independent limit on the axion mass from the variable white dwarf star R548

TL;DR

This study uses pulsating white dwarf stars to independently estimate the axion mass, finding evidence for axions with a mass around 17.1 meV, consistent across different stars, and highlighting the potential of asteroseismology in particle physics.

Contribution

The paper presents a novel asteroseismological method to constrain axion mass using the cooling rates of white dwarf stars, providing an independent estimate consistent with previous findings.

Findings

Evidence for axion cooling in R548 at 2σ confidence level.

Estimated axion mass of approximately 17.1 meV.

Agreement with axion mass estimates from other DAV stars.

Abstract

Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time ($\dot\Pi$) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoretical $\dot\Pi$ values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass $m_{\rm a} \cos^2 \beta \sim 17.1$ meV at a 2$\sigma$ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117$-$B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.