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 phi. The white-dwarf luminosity function yields g(B) <= 7 x 10(-13) and g(L) <= 4 x 10(-16), based on bremsstrahlung from C-12 and O-16 collisions with electrons. In models with a Higgs portal, this also implies a bound on the scalar-Higgs mixing angle sin theta <= 2 x 10(-10). Our new bounds apply for m(phi) <= 1 keV and are among the most restrictive ones, whereas for m(phi) <= 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 gB. 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.