Axions are (still conjectural) non-baryonic particles considered as
candidates for dark matter of the Universe. Their contribution to the dark
matter depends on their mass, a quantity that is not given by the theory
that predicts their existence. At the typical temperatures and densities
found in the cores of white dwarfs, it is expected that copious emission of
axions through bremsstrahlung processes takes place at the deepest regions
of these stars. The mass of the axions determines how strongly they couple
with electrons and, then, how large the axion emissivity is. Since axions
can (almost) freely escape from the interior of white dwarfs, their
existence would increase the cooling rate, with more massive axions
producing larger additional cooling. Pulsating white dwarfs with
hydrogen-rich atmospheres, also known as DAV or ZZ Ceti stars, can be used
as astrophysical laboratories to constrain the properties of fundamental
particles like axions by comparing the measured rates of period changes of
these stars with the expected values from theoretical models.
In this talk, I will review the use of DAV white dwarfs as a tool to
constrain the mass of the axion, and compare the predictions from this
method with the results obtained through other approach that employs the
white dwarf luminosity function (WDLF). In particular, the mass of the axion
derived from pulsating white dwarfs is more than 3 times larger than the
limit derived from the WDLF method. While the origin of this discrepancy is
still unknown, I will focus on the fact that, in order that the results
derived from pulsating white dwarfs be robust, it is necessary that the
modes considered in the analysis be modes trapped in the outer H envelope of
the star. I enumerate a few possibilities connected with the modeling of the
previous evolutionary history of DA white dwarfs that could lead to these
modes be non-trapped.
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