Constraining the Axion-Photon Coupling with Massive Stars

We point out that stars in the mass window ~ 8-12 Msun can serve as sensitive probes of the axion-photon interaction, g_{A\gamma\gamma}. Specifically, for these stars axion energy losses from the helium-burning core would shorten and eventually eliminate the blue loop phase of the evolution. This would contradict observational data, since the blue loops are required, e.g., to account for the existence of Cepheid stars. Using the MESA stellar evolution code, modified to include the extra cooling, we conservatively find g_{A\gamma\gamma} <~ 0.8 * 10^{-10} GeV^{-1}, which compares favorably with the existing bounds.