Critical Metallicity of Cool Supergiant Formation. I. Effects on Stellar-mass Loss and Feedback

This paper systematically studies the relation between metallicity and mass loss of massive stars. We perform one-dimensional stellar evolution simulations and build a grid of ∼2000 models with initial masses ranging between 11 and 60 M _⊙ and absolute metallicities Z between 0.00001 and 0.02. Steady-state winds, comprising hot main-sequence winds and cool supergiant winds, are the main drivers of the mass loss of massive stars in our models. We calculate the total mass loss over the stellar lifetime for each model. Our results reveal the existence of a critical metallicity Z _c at Z ∼ 10⁻³, where the mass loss exhibits a dramatic jump. If Z > Z _c, massive stars tend to evolve into cool supergiants, and a robust cool wind is operational. In contrast, if Z < Z _c, massive stars usually remain as blue supergiants, wherein the cool wind is not activated and the mass loss is generally weak. Moreover, we calculate the wind feedback in a 10⁵ M _⊙ star cluster with the Salpeter initial mass function. The kinetic energy released by winds does not exhibit any significant transition at Z _c because the wind velocity of a cool supergiant wind is low and contributes little to the kinetic energy. The effects of critical metallicity provide implications for the fates of metal-poor stars in the early universe.