[Paper Review] Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones

I. Brott, S. E. de Mink+7 more|UvA-DARE (University of Amsterdam)|Feb 2, 2011

Stellar, planetary, and galactic studies51 references403 citations

TL;DR

This paper presents a comprehensive grid of rotating massive main-sequence star evolutionary models and isochrones calibrated using VLT-FLAMES survey data. It introduces a novel method to calibrate convective overshooting via observed rotation rate drops at low surface gravity, and quantifies rotational mixing effects, showing that fast rotators exhibit quasi-chemically homogeneous evolution, with surface nitrogen enrichment exceeding initial Galactic levels even in low-metallicity environments like the SMC and LMC.

ABSTRACT

We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in the Galaxy. Each grid covers masses ranging from 5 to 60 Msun and initial rotation rates between 0 and about 600 km/s. To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities log g < 3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of the stellar interior found for the most massive fast rotators. We provide a detailed set of isochrones for rotating stars. Rotation as an initial parameter leads to a degeneracy between the age and the mass of massive main sequence stars if determined from its observed location in the Hertzsprung-Russell diagram. We show that the consideration of surface abundances can resolve this degeneracy.

Motivation & Objective

• To develop a dense grid of evolutionary models for rotating massive main-sequence stars across diverse metallicities.
• To calibrate convective overshooting using observed rotation rate drops in B stars with log g < 3.2, linking rotational evolution to main-sequence width.
• To quantify rotational mixing efficiency using nitrogen abundance measurements from massive stars in the Large Magellanic Cloud.
• To provide detailed isochrones that account for rotation as a key initial parameter, resolving age-mass degeneracy in Hertzsprung-Russell diagrams.
• To predict surface abundance evolution, including CNO, Li, Be, B, F, and Na, across mass and rotation ranges.

Proposed method

• Computed evolutionary tracks for stars between 5 and 60 M⊙ with initial rotation rates from 0 to 600 km/s.
• Used three initial compositions matching the Galaxy, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) metallicities.
• Calibrated overshooting parameter (α_ov) by matching observed drop in projected rotation rates at log g < 3.2, yielding α_ov = 0.34 ± 0.1 pressure scale heights.
• Calibrated rotational mixing efficiency using nitrogen surface abundance data from B stars in the LMC, ensuring model predictions align with observations.
• Generated isochrones by combining evolutionary tracks with age and rotation as parameters, showing broadened sequences in the Hertzsprung-Russell diagram.
• Tracked surface abundance evolution, including CNO, Li, Be, B, F, and Na, to assess the impact of rotational mixing over time.

Experimental results

Research questions

• RQ1 What is the appropriate level of convective overshooting in massive main-sequence stars, and how can it be constrained observationally?
• RQ2 How does rotational mixing affect surface abundances, particularly nitrogen, in massive stars across different metallicities?
• RQ3 To what extent does rotation induce quasi-chemically homogeneous evolution in the most massive and rapidly rotating stars?
• RQ4 Can surface abundances and projected rotational velocities break the degeneracy between age and mass in massive main-sequence stars when using Hertzsprung-Russell diagrams?
• RQ5 How do the predicted isochrones of rotating stars differ from classical non-rotating isochrones in terms of location and spread in luminosity-effective temperature space?

Key findings

• The convective overshooting parameter was calibrated to α_ov = 0.34 ± 0.1 pressure scale heights using the observed drop in rotation rates at log g < 3.2.
• Rotating models of 15 and 40 M⊙ in the SMC and LMC reach surface nitrogen abundances at terminal main-sequence that exceed the initial Galactic composition, despite lower initial metallicity.
• Stars with initial rotation rates of 400–500 km/s remain on the main sequence at 4 Myr, while nonrotating models of 50–60 M⊙ have already evolved off, indicating extended main-sequence lifetimes due to rotation.
• The steep drop in boron abundance along nonrotating isochrones at ~35 M⊙ in the LMC is linked to efficient stellar wind mass loss removing outer layers.
• Quasi-chemically homogeneous evolution is observed in the most massive, fastest-rotating stars, where internal mixing homogenizes composition and delays core evolution.
• Isochrones for rotating stars span a broad range of effective temperatures at fixed luminosity, invalidating unique age-mass determinations from Hertzsprung-Russell diagrams alone without additional abundance constraints.