Red giant bound on the axion-electron coupling reexamined

If axions or other low-mass pseudoscalars couple to electrons (``fine structure constant'' ₀) they are emitted from red giant stars by the Compton process +e+a and by bremsstrahlung e+ (Z, A) (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 ₀0. 510^-26 or m₀9 meV/cos^2 lest the red giant core at helium ignition exceed its standard mass by more than 0. 025 scrM_, in conflict with observational evidence. Our bound is the most restrictive limit on ₀, 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.