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

Abstract

Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to place contraints on the properties of funtamental particles like axions, by measuring their cooling rates and searching for extra cooling sources. In this paper, we present an independent inference of the mass of the axion using the most recent determination of the rate of change of the period of the largest amplitude mode (213 s) of R548, the prototype of this class of stars. This is done employing a state- of-the-art code which allows us to perform a detailed asteroseismological fit based of fully evolutionary sequences. The inclusion of the axions in the evolutionary sequences notably influences the evolutionary timescales, and thus the pulsational properties of these stars. In particular, the rates of period change of the modes are remarkably affected. This allows us to compare the theoretical values with the observed rate of period change of R548. We found that, if the period at 213 s of R548 is associated to a pulsation mode trapped in the hydrogen envelope, then the models indicate the existence of extra cooling in this pulsating white dwarf, consistent with axions of mass ma cos2 β aprox. 17.1 meV, in full agreement with the value inferred from another well-studied variable white dwarf with hydrogen-rich atmosphere, G117-B15A. Thus, we now have two independent estimates of the mass of the axion obtained from pulsating white dwarfs, which agree each other, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.