Critical Metallicity of Cool Supergiant Formation. II. Physical Origin

Abstract

This study investigates the physical origin of the critical metallicity required for the formation of cool supergiants, as revealed by stellar evolution models. Using grids of stellar models, we show that the terminal-age main-sequence (TAMS) radius, $R_{\mathrm{TAMS}}$, defines a threshold that determines whether a star of a given mass can evolve into the red supergiant (RSG) phase. Metallicity influences the supergiant outcome because it modifies $R_{\mathrm{TAMS}}$ through its effects on opacity and nuclear energy generation, as demonstrated by our stellar models and dimensional analysis based on homology relations. The value of $R_{\mathrm{TAMS}}$ sets the initial radius for post-main-sequence expansion and therefore controls the envelope radius reached at subsequent core-evolution stages. Higher-metallicity stars develop larger $R_{\mathrm{TAMS}}$ and rapidly expand into the stable RSG regime during core helium burning. In contrast, lower-metallicity stars have smaller $R_{\mathrm{TAMS}}$ and advance to more evolved core helium or carbon-burning stages while retaining compact envelopes, thereby preventing expansion into the RSG regime during core helium burning. Our results explain the origin of the critical metallicity and offer insight into the evolution of metal-poor massive stars in the early universe.

18 pages, 18 figures, Accepted for publication in ApJ

References in corpus (13)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The death of massive stars - I. Observational constraints on the progenitors of type II-P supernovae
• Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure
• Very low metallicity massive star models: Pre-SN evolution and primary nitrogen production
• Constraining mixing in massive stars in the Small Magellanic Cloud
• Grids of stellar models with rotation IV. Models from 1.7 to 120 Msun at a metallicity Z = 0.0004
• The impact of convective criteria on the properties of massive stars
• Type IIP Supernova Progenitors and their explodability I: Convective Overshoot, Blue Loops and Surface Composition
• Critical Metallicity of Cool Supergiant Formation. I. Effects on Stellar Mass Loss and Feedback
• A Red Giants' Toy Story
• Type IIP Supernova Progenitors III: Blue to Red Supergiant Ratio in Low Metallicity Models with Convective Overshoot
• Why stars inflate to and deflate from red giant dimensions, II: replies to critics