Abstract
The axion-electron coupling $g_{ae}$ is a generic feature of non-hadronic axion models. This coupling may induce a variety of observable signatures, particularly in astrophysical environments. Here, we revisit the calculation of the axion-electron bremsstrahlung and provide a general formulation valid for a non-relativistic plasma with any level of degeneracy and for any axion mass. We apply our result to the Sun, red giant stars and white dwarfs. In particular, we prove that the approximations used to evaluate the axion emissivity in red giants agree with the exact result within $10\%$, comparable with other uncertainties in these studies. In addition, this prescription allows the red giant and white dwarf bounds to be extended to massive axions.
Highlights
The axion-electron coupling naturally arises in nonhadronic axion models, like the Dine-Fischler-SrednickiZhitnitsky [1,2], and in more general theories, such as grand unified theories and string theory, predicting axionlike particles [3,4,5,6,7]
The largest difference is found in red giants (RGs), where our result is ∼25% lower than the completely degenerate limit and this difference is due to the intermediate electron degeneracy
Since the uncertainties described in Refs. [27,28] have a similar magnitude, the impact of our revised calculation can only be assessed with a detailed reevaluation of the RG bound, to be discussed in a forthcoming work [46]
Summary
The axion-electron coupling naturally arises in nonhadronic axion models, like the Dine-Fischler-SrednickiZhitnitsky [1,2], and in more general theories, such as grand unified theories and string theory, predicting axionlike particles [3,4,5,6,7]. For RGs, our calculated axion emissivity results to be ∼25% lower than the completely degenerate limit This reduction is due to the partial degeneracy of the electron gas in the RG core, which was accounted in Refs. All previous studies have assumed massless axions in the bremsstrahlung process This assumption ceases to be valid if the axion mass ma becomes comparable to the stellar temperature T. PIERLUCA CARENZA and GIUSEPPE LUCENTE and, the astrophysical bounds would be relaxed (see e.g., [31,32]) We quantify this behavior through a calculation which explicitly takes into account the axion mass in the matrix element of the electron-ion bremsstrahlung. In Appendix A, we show the complete matrix element of the electron-ion bremsstrahlung and in Appendix B we summarize the results of the existing literature
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