  The evolution of white dwarfs is a simple gravothermal process of cooling.
Since the shape of their luminosity function is sensitive to the characteristic
cooling time, it is possible to use its slope to test the existence of
additional sources or sinks of energy, such as those predicted by alternative
physical theories. The aim of this paper is to study if the changes in the
slope of the white dwarf luminosity function around bolometric magnitudes
ranging from 8 to 10 and previously attributed to axion emission are,
effectively, a consequence of the existence of axions and not an artifact
introduced by the star formatio rate. We compute theoretical luminosity
functions of the thin and thick disk, and of the stellar halo including axion
emission and we compare them with the existing observed luminosity functions.
Since these stellar populations have different star formation histories, the
slope change should be present in all of them at the same place if it is due to
axions or any other intrinsic cooling mechanism. The signature of an unexpected
cooling seems to be present in the luminosity functions of the thin and thick
disks, as well as in the halo luminosity function. This additional cooling is
compatible with axion emission, thus supporting the idea that DFSZ axions, with
a mass in the range of 4 to 10 meV, could exist. If this were the case, these
axions could be detected by the future solar axioscope IAXO.
