[Paper Review] Stellar evolution with rotation XI: Wolf-Rayet star populations at different metallicities

G. Meynet, A. Maeder|ArXiv.org|Aug 18, 2004

Stellar, planetary, and galactic studiesPhysics and Astronomy33 references323 citations

TL;DR

This paper presents rotating massive star models across metallicities (Z = 0.004 to 0.040) to study Wolf-Rayet (WR) populations. Including rotation, rotational mixing, and enhanced mass loss, the models show that rotation lowers the minimum mass for WR evolution, increases WR lifetimes—especially the eWNL phase—and better reproduces observed WR/O star ratios, WN/WC ratios, and Ib/Ic to II supernova fractions than non-rotating models. The results confirm rotation as essential for matching observations across metallicities.

ABSTRACT

Grids of models of massive stars ($M \ge$ 20 $M_\odot$) with rotation are computed for metallicities $Z$ ranging from that of the Small Magellanic Cloud (SMC) to that of the Galactic Centre. The hydrostatic effects of rotation, the rotational mixing and the enhancements of the mass loss rates by rotation are included. The evolution of the surface rotational velocities of the most massive O--stars mainly depends on the mass loss rates and thus on the initial $Z$ value. The minimum initial mass for a star for entering the Wolf--Rayet (WR) phase is lowered by rotation. For all metallicities, rotating stars enter the WR phase at an earlier stage of evolution and the WR lifetimes are increased, mainly as a result of the increased duration of the eWNL phase. Models of WR stars predict in general rather low rotation velocities ($ < 50$ km s$^{-1}$) with a few possible exceptions, particularly at metallicities lower than solar where WR star models have in general faster rotation and more chance to reach the break--up limit.The properties of the WR populations as predicted by the rotating models are in general in much better agreement with the observations in nearby galaxies. The observed variation with metallicity of the fractions of type Ib/Ic supernovae with respect to type II supernovae as found by Prantzos & Boissier (\cite{Pr03}) is very well reproduced by the rotating models, while non--rotating models predict much too low ratios.

Motivation & Objective

• To investigate how stellar rotation affects Wolf-Rayet (WR) star populations across different metallicities.
• To test whether rotating models can reproduce observed WR/O star number ratios, WN/WC ratios, and transition phase fractions in nearby galaxies.
• To examine the impact of rotation on mass loss, surface abundances, and final masses at core collapse.
• To assess the role of rotation in explaining the observed variation in Ib/Ic to II supernova ratios with metallicity.
• To determine if single-star evolution with rotation can account for WR stars in low-metallicity environments like the SMC, challenging binary mass transfer dominance.

Proposed method

• Computing grids of rotating massive star models (M ≥ 20 M☉) for metallicities Z = 0.004 (SMC), 0.008, 0.020 (solar), and 0.040 (Galactic center).
• Incorporating hydrostatic effects of rotation, rotational mixing (e.g., meridional circulation and shear), and enhanced mass loss rates due to rotation.
• Tracking surface hydrogen mass fraction (Xs) and (C+O)/He ratios as functions of luminosity to identify WR phases and subtypes.
• Using the Smith & Maeder (1991) classification to map (C+O)/He ratios to WC subtypes and compare with observations.
• Analyzing evolutionary tracks to determine lifetimes in WR phases, especially eWNL and WC stages, and final masses at supernova explosion.
• Comparing model predictions with observed WR/O ratios, WN/WC ratios, and Ib/Ic to II supernova fractions across metallicities.

Experimental results

Research questions

• RQ1 How does rotation affect the minimum initial mass required for a star to enter the Wolf-Rayet phase across different metallicities?
• RQ2 To what extent do rotating models reproduce the observed number ratio of WR to O-type stars at metallicities from Z = 0.004 to Z = 0.040?
• RQ3 Can rotating models explain the observed distribution of WN and WC subtypes, particularly the absence of late-type WC stars at low metallicity?
• RQ4 How does rotation influence the fraction of WR stars in the transition WN/WC phase, and does this match observations?
• RQ5 Do rotating models reproduce the observed variation in the ratio of Ib/Ic to II supernovae with metallicity, as reported by Prantzos & Boissier (2003)?

Key findings

• Rotation lowers the minimum initial mass required for a star to enter the Wolf-Rayet phase, enabling more massive stars to become WR stars across all metallicities.
• Rotating models increase WR lifetimes, primarily by extending the eWNL phase, and predict surface rotation velocities generally below 50 km s⁻¹, with faster rotation possible at low metallicities.
• At Z = 0.040, stars above 50 M☉ end with final masses of 5–7.5 M☉ at core collapse, whereas at Z = 0.004 (SMC), final masses range from 17–29 M☉, indicating strong metallicity dependence.
• The models reproduce the observed trend that late-type WC stars are only found at high metallicities, as higher metallicity leads to lower luminosity at WC phase entry and longer WN phases.
• Rotating models successfully reproduce the observed variation in the Ib/Ic to II supernova ratio with metallicity, while non-rotating models predict significantly lower ratios.
• The interval of initial masses passing through the LBV phase is found to vary with both metallicity and initial rotation rate (Ω), indicating a complex interplay between mass loss and rotation.