RED GIANT BOUND ON THE AXION-ELECTRON COUPLING REEXAMINED

If axions or other low-mass pseudoscalars couple to electrons ("fine structure constant" αa) they are emitted from red giant stars by the Compton process γ+e→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 αa≤0.5×10-26 or ma≤9 meV/cos2β 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 αa, 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. © 1995 The American Physical Society.