Bounds on hadronic axions from stellar evolution

We consider in detail the effect of the emission of ‘‘hadronic’’ invisible axions (which do not couple to electrons) from the interior of stars on stellar evolution. To this end we calculate plasma emission rates for axions due to the Primakoff process for the full range of conditions encountered in a giant star. Much attention is paid to plasma, degeneracy, and screening effects. We reconsider the solar bound by evolving a 1.0 $M_{\odot }$ star to solar age and lowering the presolar helium abundance so as to obtain the correct present-day luminosity of the Sun. The previous bound on the axion-photon coupling of $G_9$≲2.5 (corresponding to $m_a$≲17 eV R where R is a model-dependent factor of order unity) is confirmed, where $G_9$ is the coupling constant G in units of ${10}^{\mathrm{-}9}$ ${\mathrm{GeV}}^{\mathrm{-}1}$. We then follow the evolution of a 1.3$M_{\odot }$ star from zero age to the top of the giant branch.