Stellar limits on scalars from electron-nucleus bremsstrahlung

We revisit stellar energy-loss bounds on the Yukawa couplings g$_{B,L}$ of baryophilic and leptophilic scalars ϕ. The white-dwarf luminosity function yields g$_{B}$ ≲ 7 × 10$^{-13}$ and g$_{L}$ ≲ 4 × 10$^{-16}$, based on bremsstrahlung from $^{12}$C and $^{16}$O collisions with electrons. In models with a Higgs portal, this also implies a bound on the scalar-Higgs mixing angle sinθ ≲ 2 × 10$^{-10}$. Our new bounds applyfor mϕ ≲ 1 keV and are among the most restrictive ones, whereas for mϕ ≲ 0.5 eV, long-range force measurements dominate. Besides a detailed calculation of the bremsstrahlung rate for degenerate and semi-relativistic electrons, we prove with a simple argument that non-relativistic bremsstrahlung by the heavy partner is suppressed relative to that by the light one by their squared-mass ratio. This large reduction was overlooked in previous much stronger bounds on g$_{B}$. In an appendix, we provide fitting formulas (few percent precision) for the bremsstrahlung emission of baryophilic and leptophilic scalars as well as axions for white-dwarf conditions, i.e., degenerate, semi-relativistic electrons and ion-ion correlations in the “liquid” phase.