  Recent determinations of the white dwarf luminosity function (WDLF) from very
large surveys have extended our knowledge of the WDLF to very high
luminosities. This, together with the availability of new full evolutionary
white dwarf models that are reliable at high luminosities, have opened the
possibility of testing particle emission in the core of very hot white dwarfs,
where neutrino processes are dominant. We use the available WDLFs from the
Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the value
of the neutrino magnetic dipole moment ($\mu_\nu$). We constructed theoretical
WDLFs for different values of $\mu_\nu$ and performed a $\chi^2$-test to derive
constraints on the value of $\mu_\nu$. We also constructed a unified WDLF by
averaging the SDSS and SSS and estimated the uncertainties by taking into
account the differences between the WDLF at each magnitude bin. Then we
compared all WDLFs with theoretical WDLFs.Comparison between theoretical WDLFs
and both the SDSS and the averaged WDLF indicates that $\mu_\nu$ should be
$\mu_\nu<5\times 10^{-12}\, e\hbar/(2m_e c)$. In particular, a $\chi^2$-test on
the averaged WDLF suggests that observations of the disk WDLF exclude values of
$\mu_\nu>5\times 10^{-12}e\hbar/(2m_e c)$ at more than a 95\% confidence level,
even when conservative estimates of the uncertainties are adopted. Our study
shows that modern WDLFs, which extend to the high-luminosity regime, are an
excellent tool for constraining the emission of particles in the core of hot
white dwarfs. However, discrepancies between different WDLFs suggest there
might be some relevant unaccounted systematic errors. A larger set of
completely independent WDLFs, as well as more detailed studies of the
theoretical WDLFs and their own uncertainties, is desirable to explore the
systematic uncertainties behind this constraint.
