Abstract
It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities M Bol≲ 8) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than ma cos 2β≳ 5 meV (i.e. axion-electron coupling constant gae≳ 1.4× 10-13). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed χ2-tests to have a quantitative measure of the agreement between the theoretical WDLFs — computed under the assumptions of different axion masses and normalization methods --- and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ma cos 2β≳ 10 meV; gae≳ 2.8× 10-13) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.
Highlights
Of the WDLF [12, 21] computed the impact of the axion emission in the WDLF by adopting a perturbative approach on the axion emission. [12, 22] find that the inclusion of the DFSZaxion emissivity, with ma cos2 β ∼ 5 meV, in the evolutionary models of white dwarfs would improve the agreement between the theoretical and observational WDLFs, and that axion masses ma cos2 β > 10 meV are clearly excluded
It is worth noting for the aim of the present work that LPCODE has recently been tested against other well-known stellar evolution code and it was found that uncertainties in white dwarf cooling times arising from different numerical implementations of the stellar evolution equations were below 2% [40]
For the sake of completeness DFSZ-type axion emission by both Compton and Bremsstrahlung processes were included in LPCODE, only Bremsstrahlung processes are relevant in white dwarfs
Summary
The stellar evolution computations presented in this work have been performed with LPCODE stellar evolution code, which has been used to study different problems related to the formation and evolution of white dwarfs —e.g. [28,29,30]. The stellar evolution computations presented in this work have been performed with LPCODE stellar evolution code, which has been used to study different problems related to the formation and evolution of white dwarfs —e.g. In addition the effects of time dependent element diffusion during the white dwarf evolution is taken into account following treatment of [39] for multicomponent gases. It is worth noting for the aim of the present work that LPCODE has recently been tested against other well-known stellar evolution code and it was found that uncertainties in white dwarf cooling times arising from different numerical implementations of the stellar evolution equations were below 2% [40]. [40] found that, at log L/L −1.5, differences up to 8% can be found due to gravothermal differences in the initial white dwarf models, even when the same chemical stratification is adopted
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