Stellar limits on scalars from electron-nucleus bremsstrahlung

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