[Paper Review] Populations of rotating stars. - I. Models from 1.7 to 15 Msun at Z = 0.014, 0.006, and 0.002 with {\Omega}/{\Omega}crit between 0 and 1

C. Georgy, Sylvia Ekström+5 more|arXiv (Cornell University)|Mar 10, 2013

Stellar, planetary, and galactic studiesPhysics and Astronomy28 references124 citations

TL;DR

This paper presents a comprehensive grid of 270 rotating stellar evolution models for 1.7–15 M⊙ stars at metallicities Z = 0.014, 0.006, and 0.002, with initial rotation rates up to 95% of critical. Using an improved version of the Geneva stellar evolution code that tracks angular momentum loss and equatorial mass loss during critical rotation, the study reveals that fast-rotating low-mass stars (1.7 M⊙) develop small convective cores, and metal-poor stars (Z = 0.002) show strong nitrogen enrichment (up to 0.4 dex) at mid-MS, with enhanced carbon and oxygen yields at high rotation due to efficient rotational mixing.

ABSTRACT

B-type stars are known to rotate at various velocities, including very fast rotators near the critical velocity as the Be stars. In this paper, we provide stellar models covering the mass range between 1.7 to 15 Msun, which includes the typical mass of known Be stars, at Z = 0.014, 0.006, and 0.002 and for an extended range of initial velocities on the zero-age main sequence. We used the Geneva stellar-evolution code, including the effects of shellular rotation, with a numerical treatment that has been improved so the code can precisely track the variation in the angular momentum content of the star as it changes under the influence of radiative winds and/or mechanical mass loss. We discuss the impact of the initial rotation rate on the tracks in the Hertzsprung-Russell diagram, the main-sequence (MS) lifetimes, the evolution of the surface rotation and abundances, as well as on the ejected masses of various isotopes. Among the new results obtained from the present grid we find that 1) fast-rotating stars with initial masses around 1.7 Msun present at the beginning of the core hydrogen-burning phase quite small convective cores with respect to their slowly rotating counterparts. This fact may be interesting to keep in mind in the framework of the asteroseismic studies of such stars. 2) The contrast between the core and surface angular velocity is higher in slower rotating stars. The values presently obtained are in agreement with the very few values obtained for B-type stars from asteroseismology. 3) At Z = 0.002, the stars in the mass range of 1.7 to 3 Msun with a mean velocity on the MS of the order of 150 km/s show N/H enhancement superior to 0.2 dex at mid-MS, and superior to 0.4 dex at the end of the MS phase. At solar metallicity the corresponding values are below 0.2 dex at any time in the MS.

Motivation & Objective

• To provide a detailed, high-resolution grid of rotating stellar models covering the mass range of B-type and Be stars.
• To improve the tracking of angular momentum loss and equatorial mass loss in stars rotating near critical velocity.
• To investigate the impact of initial rotation rate and metallicity on stellar evolution, surface abundances, and mass ejection.
• To enable synthetic population synthesis with realistic initial rotation and metallicity distributions.

Proposed method

• Used the Geneva stellar evolution code with shellular rotation and improved numerical treatment for angular momentum evolution.
• Incorporated radiative and mechanical mass loss anisotropy, particularly during critical rotation phases.
• Tracked the evolution of surface velocity, effective temperature, and chemical abundances (N, C, O) throughout the main sequence.
• Computed equatorial mass loss rates for stars at or near critical rotation using a modified wind model.
• Simulated models across 10 masses and 9 initial rotation rates (Ω/Ωcrit from 0 to 0.95) at three metallicities (Z = 0.014, 0.006, 0.002).
• Calculated ejected masses of H, He, C, N, O, and CO core masses, comparing non-rotating to rapidly rotating models.

Experimental results

Research questions

• RQ1 How does initial rotation rate affect the size of convective cores in low-mass (1.7 M⊙) stars during core hydrogen burning?
• RQ2 What is the impact of metallicity on nitrogen surface enrichment in rotating stars, particularly at mid- and end-main sequence?
• RQ3 How does rotational mixing influence the yields of carbon and oxygen in stars at different metallicities and rotation rates?
• RQ4 To what extent does critical rotation lead to enhanced equatorial mass loss and non-spherical mass ejection?
• RQ5 How do blue loops in post-main-sequence evolution depend on rotation rate and metallicity?

Key findings

• Fast-rotating 1.7 M⊙ stars develop significantly smaller convective cores at the start of core hydrogen burning compared to slowly rotating counterparts, a result relevant for asteroseismic modeling.
• The contrast between core and surface angular velocity is higher in slower-rotating stars, with computed values consistent with asteroseismic measurements of B-type stars.
• At Z = 0.002, stars with mean main-sequence surface velocities of ~150 km·s⁻¹ show nitrogen-to-hydrogen (N/H) enhancements exceeding 0.4 dex by the end of the main sequence, while at solar metallicity, such enrichments remain below 0.2 dex.
• Rotational mixing increases the ejected mass of carbon by a factor of up to 2.15 and oxygen by a factor of up to 3.36 in SMC-metallicity models (Z = 0.002) with initial rotation at 95% of critical velocity.
• The CO core mass increases by 43% in SMC-metallicity models (Z = 0.002) and by 27–25% in LMC and Galactic models, respectively, when initial rotation reaches 95% of critical velocity.
• At Z = 0.002, all models with initial rotation ωini > 0.60 show observable surface nitrogen enrichment by mid-main sequence, whereas at solar metallicity, such enrichment is only detectable in the most rapidly rotating stars (ωini > 0.8) for 1.7 M⊙ models.