[Paper Review] Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)

Sylvia Ekström, C. Georgy+10 more|arXiv (Cornell University)|Oct 23, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy801 citations

TL;DR

This paper presents a homogeneous grid of rotating stellar evolution models for masses from 0.8 to 120 M☉ at solar metallicity (Z = 0.014), incorporating updated opacities, nuclear reaction rates, and mass-loss prescriptions that account for Eddington and critical velocity limits. The key result is that enhanced mass loss during the red supergiant phase—driven by rotation and increased mass-loss rates—causes stars above 15–20 M☉ to evolve back into the blue region of the Hertzsprung-Russell diagram, significantly altering the blue-to-red supergiant ratio and lowering the minimum mass for Wolf-Rayet star formation.

ABSTRACT

[abridged] Many topical astrophysical research areas, such as the properties of planet host stars, the nature of the progenitors of different types of supernovae and gamma ray bursts, and the evolution of galaxies, require complete and homogeneous sets of stellar models at different metallicities in order to be studied during the whole of cosmic history. We present here a first set of models for solar metallicity, where the effects of rotation are accounted for in a homogeneous way. We computed a grid of 48 different stellar evolutionary tracks, both rotating and non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun. For each of the stellar masses considered, electronic tables provide data for 400 stages along the evolutionary track and at each stage, a set of 43 physical data are given. These grids thus provide an extensive and detailed data basis for comparisons with the observations. The rotating models start on the ZAMS with a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of the central carbon-burning phase, the early AGB phase, or the core helium-flash for, respectively, the massive, intermediate, and both low and very low mass stars. The initial abundances are those deduced by Asplund and collaborators, which best fit the observed abundances of massive stars in the solar neighbourhood. We update both the opacities and nuclear reaction rates, and introduce new prescriptions for the mass-loss rates as stars approach the Eddington and/or the critical velocity. We account for both atomic diffusion and magnetic braking in our low-mass star models. [...]

Motivation & Objective

• To provide a complete, homogeneous set of rotating and non-rotating stellar evolution models across a wide mass range (0.8–120 M☉) at solar metallicity (Z = 0.014) for use in astrophysical population synthesis and galaxy evolution studies.
• To improve the physical realism of stellar models by incorporating updated opacities, nuclear reaction rates, and mass-loss prescriptions that account for Eddington and critical velocity limits.
• To investigate the impact of rotation on stellar evolution, including rotational mixing, surface abundance changes, and rotational velocity evolution.
• To explore how enhanced mass loss during the red supergiant phase affects the blue-to-red supergiant ratio and the minimum mass required for a star to become a Wolf-Rayet star.
• To make the full model grids and isochrones publicly available through the CDS and Geneva Observatory databases for broad community use.

Proposed method

• Computed 48 evolutionary tracks (rotating and non-rotating) for 48 different masses spanning 0.8 to 120 M☉ at Z = 0.014, with 400 evolutionary stages per track.
• Used initial rotation rate of 40% of critical velocity (υ_ini/υ_crit = 0.4) on the zero-age main sequence, with consistent treatment of rotational mixing and angular momentum transport.
• Incorporated updated opacity tables based on the Asplund et al. (2005) solar abundance mixture and revised nuclear reaction rates.
• Implemented a new mass-loss prescription that increases mass loss when luminosity approaches the Eddington limit or when critical velocity is reached, particularly during the red supergiant phase.
• Applied atomic diffusion and magnetic braking in low-mass models (≤2 M☉), and included rotational mixing in intermediate and massive stars.
• Generated electronic tables with 43 physical quantities per evolutionary stage, enabling detailed comparisons with observations and interpolation for isochrone fitting.

Experimental results

Research questions

• RQ1 How does rotation affect the evolution of stars from 0.8 to 120 M☉ at solar metallicity, particularly in terms of surface abundances, rotational velocities, and HR diagram positions?
• RQ2 To what extent does enhanced mass loss during the red supergiant phase—driven by rotation and Eddington-limited conditions—affect the blue-to-red supergiant ratio?
• RQ3 What is the minimum initial mass required for a single star to evolve into a Wolf-Rayet star, and how does rotation alter this threshold?
• RQ4 How do the updated mass-loss prescriptions, especially near the Eddington limit, influence the evolution of massive stars through core helium burning and beyond?
• RQ5 Can rotating models reproduce the observed main-sequence width, surface compositions, and rotational velocities simultaneously, without relying on ad hoc parameters like enhanced overshoot?

Key findings

• Rotating models successfully reproduce the observed main-sequence width, surface abundances, and rotational velocity evolution across the mass range, indicating that rotational mixing is essential for consistency with observations.
• Enhanced mass loss during the red supergiant phase—triggered by supra-Eddington luminosities in outer layers—causes stars above 15–20 M☉ to lose significant hydrogen envelopes and evolve back into the blue part of the Hertzsprung-Russell diagram.
• The blue-to-red supergiant lifetime ratio increases from 0.2 (non-rotating, Schaller et al. 1992) to 1.5 for 20 M☉ models and from 0.10 to 5.3 for 25 M☉ models when rotation is included, indicating a strong shift toward blue supergiant dominance.
• The minimum initial mass for a star to become a Wolf-Rayet star is reduced from 32 M☉ (Schaller et al. 1992) to 20 M☉ in the present rotating models, consistent with observed WR populations at solar metallicity.
• The core helium-burning phase in rotating models spends a larger fraction of time in the blue part of the HR diagram (Y ≲ 0.3–0.6), due to combined effects of rotational mixing and enhanced mass loss, altering the evolutionary timescales and final fate of massive stars.
• The models provide a self-consistent framework where a single initial rotation rate (υ_ini/υ_crit = 0.4) reproduces multiple observational constraints across the mass range, supporting the physical plausibility of the model assumptions.