White dwarfs are the final remnants of low and intermediate mass
stars and their evolution is essentially a long lasting process of cooling.
The tool that allows to compare the theoretical models with the observations
is their luminosity function, that is, the number of stars per unit volume
and luminosity interval. The shape of the bright branch of this function
is only sensitive to the average cooling rate and, thus, it is possible to
use it to check for the possible existence of additional non standard sources
or sinks of energy able to modify the expected 'normal evolution'. Despite
the recent improvements introduced by cosmological surveys like the SDSS
and the SCSS catalogues, one of the main difficulties to achieve this goal
is still the size of the samples. Gaia, launched in 2013, will allow to
obtain, in a next future, the fundamental properties of ~400,000
white dwarfs.
Such a sample will be statistically significant and will allow the detection of
small deviations from the normal cooling process. As an example, we describe
here the case of axions, a not yet detected weakly interacting particle
introduced to solve the so called CP-problem of the Standard Model of
particles. In particular, we show that their inclusion noticeably improves
the agreement between the theoretical and observational white dwarf luminosity
functions, thus providing a first hint that axions could exist. This
improvement is not only valid for the luminosity function obtained with all three
catalogues but also for the luminosity function of the galactic thin and thick
disks, suggesting that the change of the luminosity function shape is due to an intrinsic
property of white dwarfs and not to a fluctuation of the star formation rate.
The best fit is obtained for axion masses around
6 meV, and values larger than 16 meV could probably be excluded.