Limits on the neutrino magnetic dipole moment from the luminosity function of hot white dwarfs

Abstract

Context. 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. Aims. 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 (μν). Methods. We used a state-of-the-art stellar evolution code to compute a grid of white dwarf cooling sequences under the assumptions of different values of μν. Then we constructed theoretical WDLFs for different values of μν and performed a χ2-test to derive constraints on the value of μν. Results. We find that the WDLFs derived from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey do not yield consistent results. The discrepancy between the two WDLFs suggests that the uncertainties are significantly underestimated. Consequently, we 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 μν should be μν < 5 × 10 -12 eħ/(2mec). In particular, a χ2-test on the averaged WDLF suggests that observations of the disk WDLF exclude values of μν > 5 × 10 -12eħ/(2mec) at more than a 95% confidence level, even when conservative estimates of the uncertainties are adopted. This is close to the best available constraints on μν from the physics of globular clusters. Conclusions. 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. Once this is done, we believe the Galactic disk WDLF will offer constraints on the magnetic dipole moment of the neutrino similar to the best available constraints obtainable from globular clusters.