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

TL;DR

This study identifies a critical metallicity around Z~10^-3 that causes a significant increase in mass loss for massive stars, influencing their evolution into cool supergiants and affecting stellar feedback in star clusters.

Contribution

It introduces a comprehensive grid of stellar models revealing a metallicity threshold that dramatically alters mass loss and stellar evolution pathways.

Findings

Mass loss jumps dramatically at Z~10^-3.

Stars above Z_c evolve into cool supergiants with strong winds.

Wind feedback energy does not significantly change at Z_c.

Abstract

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 $\sim$2000 models with initial masses ranging between 11 and 60 $M_{\odot}$ 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 $Z_{\rm{c}}$ at $Z \sim 10^{-3}$, where the mass loss exhibits a dramatic jump. If $Z>Z_{\rm{c}}$, massive stars tend to evolve into cool supergiants, and a robust cool wind is operational. In contrast, if $Z<Z_{\rm{c}}$, 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 $10^5$ $M_{\odot}$ star cluster with the Salpeter initial mass function. The kinetic energy released by winds does not exhibit any significant transition at $Z_{\rm{c}}$ 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.