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\nstars, can be used as astrophysical laboratories to constrain the properties of\nfundamental particles like axions. Comparing the measured cooling rates of\nthese stars with the expected values from theoretical models allows us to\nsearch for sources of additional cooling due to the emission of weakly\ninteracting particles. In this paper, we present an independent inference of\nthe mass of the axion using the recent determination of the evolutionary\ncooling rate of R548, the DAV class prototype. We employ a state-of-the-art\ncode which allows us to perform a detailed asteroseismological fit based on\nfully evolutionary sequences. Stellar cooling is the solely responsible of the\nrates of change of period with time ($\\dot\\Pi$) for the DAV class. Thus, the\ninclusion of axion emission in these sequences notably influences the\nevolutionary timescales, and also the expected pulsational properties of the\nDAV stars. This allows us to compare the theoretical $\\dot\\Pi$ values to the\ncorresponding empirical rate of change of period with time of R548 to discern\nthe presence of axion cooling. We found that if the dominant period at 213.13 s\nin R548 is associated with a pulsation mode trapped in the hydrogen envelope,\nour models indicate the existence of additional cooling in this pulsating white\ndwarf, consistent with axions of mass $m_{\\rm a} \\cos^2 \\beta \\sim 17.1$ meV at\na 2$\\sigma$ confidence level. This determination is in agreement with the value\ninferred from another well-studied DAV, G117$-$B15A. We now have two\nindependent and consistent estimates of the mass of the axion obtained from\nDAVs, although additional studies of other pulsating white dwarfs are needed to\nconfirm this value of the axion mass.\n