Red-giant evolution, metallicity, and new bounds on hadronic axions

We explore stellar cooling by nuclear axion emission, identifying those special isotopes that dominate this process for temperatures {similar to}10{sup 7}--10{sup 9} K. We argue that such nuclear energy-loss mechanisms are distinctive because the effects track metallicity. Three observables associated with evolution of stars along the red-giant and horizontal branches are shown to impose new and restrictive constraints on axions in the hadronic window.