[Paper Review] Stellar evolution with rotation X: Wolf-Rayet star populations at solar metallicity

G. Meynet, A. Maeder|ArXiv.org|Apr 3, 2003

Stellar, planetary, and galactic studies24 references300 citations

TL;DR

This paper investigates the impact of rotation on Wolf-Rayet (WR) star populations at solar metallicity using updated mass loss rates and rotational effects. It shows that rotation significantly extends WR lifetimes—particularly the H-rich WNL phase—lowers the mass threshold for WR formation to ~22 M⊙, enables a transition WN/WC phase below 60 M⊙, and improves agreement with observed WR/O, WN/WC, and transition star ratios, resolving long-standing discrepancies in non-rotating models.

ABSTRACT

We examine the properties of Wolf--Rayet (WR) stars predicted by models of rotating stars taking account of the new mass loss rates for O--type stars and WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis & Lamers \cite{NuLa00}) and of the wind anisotropies induced by rotation. We find that the rotation velocities $v$ of WR stars are modest, i.e. about 50 km s$^{-1}$, not very dependant on the initial $v$ and masses. For the most massive stars, the evolution of $v$ is very strongly influenced by the values of the mass loss rates; below $\sim$12 M$_\odot$ the evolution of rotation during the MS phase and later phases is dominated by the internal coupling. Massive stars with extreme rotation may skip the LBV phase. Models having a typical $v$ for the O--type stars have WR lifetimes on the average two times longer than for non--rotating models. The increase of the WR lifetimes is mainly due to that of the H--rich eWNL phase. Rotation allows a transition WN/WC phase to be present for initial masses lower than 60 M$_\odot$. The durations of the other WR subphases are less affected by rotation. The mass threshold for forming WR stars is lowered from 37 to 22 M$_\odot$ for typical rotation. The comparisons of the predicted number ratios WR/O, WN/WC and of the number of transition WN/WC stars show very good agreement with models with rotation, while this is not the case for models with the present--day mass loss rates and no rotation. As to the chemical abundances in WR stars, rotation brings only very small changes for WN stars, since they have equilibrium CNO values. However, WC stars with rotation have on average lower C/He and O/He ratios. The luminosity distribution of WC stars is also influenced by rotation.

Motivation & Objective

• To resolve discrepancies between non-rotating stellar models and observations of Wolf-Rayet (WR) star populations at solar metallicity.
• To investigate how updated mass loss rates (Vink et al. 2000, 2001; Nugis & Lamers 2000) and rotational effects affect WR star evolution.
• To examine whether rotation can explain the observed number ratios WR/O, WN/WC, and the frequency of transition WN/WC stars.
• To assess the impact of rotation on chemical abundances and luminosity distributions in WR stars.
• To evaluate whether rotation resolves the overprediction of WC stars and underprediction of WR stars in standard models.

Proposed method

• Used rotating stellar evolution models with updated mass loss rates for O-type and WR stars, including wind clumping and bi-stability limits.
• Incorporated rotational mixing and internal coupling to model surface abundances and angular momentum evolution.
• Applied reduced mass loss rates for non-rotating stars by applying a correction factor to empirical data to isolate non-rotating behavior.
• Tracked evolutionary sequences in the Hertzsprung-Russell diagram, focusing on WR phases and surface composition changes.
• Compared model predictions with observed WR/O, WN/WC, and transition WN/WC star ratios in regions of constant star formation.
• Analyzed surface abundance evolution (C/N, C/He, O/He) and luminosity–composition diagrams (e.g., (C+O)/He vs. L/L⊙) to assess WR subtype distributions.

Experimental results

Research questions

• RQ1 How does rotation affect the lifetimes of Wolf-Rayet stars, particularly the WNL and WN/WC phases?
• RQ2 What is the impact of rotation on the observed number ratios of WR stars (WR/O, WN/WC) compared to non-rotating models?
• RQ3 Can rotation explain the observed frequency of transition WN/WC stars (~4.4%) that non-rotating models fail to reproduce?
• RQ4 How does rotation influence the chemical abundances in WR stars, especially in WN and WC subtypes?
• RQ5 What is the effect of rotation on the mass threshold for WR star formation and the luminosity distribution of WC stars?

Key findings

• Rotation increases the average WR lifetime by a factor of two compared to non-rotating models, primarily due to an extended H-rich WNL phase.
• The mass threshold for WR star formation is reduced from 37 M⊙ to 22 M⊙ when rotation is included.
• Rotation enables a transition WN/WC phase for initial masses below 60 M⊙, consistent with the observed 4.4% frequency of such stars.
• Rotation leads to lower C/He and O/He ratios in WC stars compared to non-rotating models, altering their chemical composition.
• Rotating models shift WC star evolutionary tracks in the (C+O)/He vs. luminosity diagram to lower luminosities due to reduced mass at the start of the WC phase.
• The observed WR/O and WN/WC number ratios are well reproduced by rotating models, while non-rotating models with current mass loss rates fail to match observations.