Effects of axions on Population III stars

Aims. Following the renewed interest in axions as a dark matter component, we revisit the effects of energy loss by axion emission on the evolution of the first generation of stars. These stars with zero metallicity are assumed to be massive, more compact, and hotter than subsequent generations. It is hence important to extend previous studies, which were restricted to solar metallicity stars. Methods. Our analysis first compares the evolution of solar metallicity 8, 10, and 12 M-circle dot stars to previous work. We then calculate the evolution of 8 zero-metallicity stars with and without axion losses and with masses ranging from 20 to 150 M-circle dot. Results. For the solar metallicity models, we confirm the disappearance of the blue-loop phase for a value of the axion-photon coupling of g(a gamma) = 10(-10) GeV-1. We show that for ga gamma = 10(-10) GeV-1, the evolution of Population III stars is not much affected by axion losses, except within the range of masses 80-130 M-circle dot. Such stars show significant differences in both their tracks within the T-c-p(c) diagram and their central composition (in particular Ne-20 and Mg-24). We discuss the origin of these modifications from the stellar physics point of view, and also their potential observational signatures.