Red giant bound on the axion-electron coupling reexamined

Abstract

If axions or other low-mass pseudoscalars couple to electrons (``fine structure constant'' $α_a$) they are emitted from red giant stars by the Compton process $γ+e\to e+a$ and by bremsstrahlung $e+(Z,A)\to (Z,A)+e+a$. We construct a simple analytic expression for the energy-loss rate for all conditions relevant for a red giant and include axion losses in evolutionary calculations from the main sequence to the helium flash. We find that $α_a\lapprox0.5\mn(-26)$ or $m_a\lapprox 9\,\meV/\cos^2β$ lest the red giant core at helium ignition exceed its standard mass by more than $0.025\,\MM_\odot$, in conflict with observational evidence. Our bound is the most restrictive limit on $α_a$, but it does not exclude the possibility that axion emission contributes significantly to the cooling of ZZ~Ceti stars such as G117--B15A for which the period decrease was recently measured.