The transport of angular momentum for massive stars I. Formation of slowly rotating WNE stars

TL;DR

This study tackles how massive stars redistribute angular momentum to form slowly rotating WNE stars. It tests two transport channels—internal gravity waves (IGWs) with a diffusion coefficient parameterized by $A$ and a revised Tayler-instability mechanism (TSF) with parameter $\alpha$—within MESA models of $25$–$70\,M_\odot$ at solar metallicity, activated during core helium burning. The results show that IGWs require $A$ on the order of $\ge 10$ and TSF favors $\alpha \approx 0.01$ to produce surface rotations $v_{\rm surf} \lesssim 70$ km s$^{-1}$, demonstrating that both mechanisms can efficiently redistribute angular momentum and lead to slowly rotating WNE stars. This supports the predicted existence of slowly rotating hydrogen-stripped populations and highlights the mass-dependent efficiency of angular momentum transport in massive stars, beyond what is observed in lower-mass stars.

Abstract

The evolutionary scenario of early-type nitrogen-sequence Wolf-Rayet (WNE) stars predicts a slowly rotating subclass that typically forms after the red supergiant (RSG) phase. Their slow rotation rates are attributed to stellar winds that remove angular momentum transferred outward during core contraction. We incorporate improved prescriptions for internal gravity waves and the magnetic Tayler instability into single massive star evolution models. Our simulations successfully produce slowly rotating WNE stars and determine optimal parameters for both mechanisms ($A \ge 10$ for internal gravity waves (IGWs), $α= 0.01$ for revised Tayler instability (TSF)). The results demonstrate that the efficiency of angular momentum transfer in massive stars is significantly enhanced compared to low-mass stars, both processes can self-consistently explain the slow rotation of WNE stars, confirming their efficiency in angular momentum redistribution and providing crucial theoretical support for the existence of this predicted stellar population.

Figures (8)

• Figure 1: Temporal evolution of the surface velocity (upper panel) and the helium core angular velocity (lower panel) during the core helium burning for stellar models of 25–50 $\mathrm{M}_{\odot}$. Models are arranged from longest to shortest evolutionary lifetimes. Different line styles represent varying strengths of angular momentum transport induced by the IGWs. The dark purple curve shows the baseline case without IGW-driven transport. The 25 and 32 $\mathrm{M}_{\odot}$ models do not evolve into the WNE stars, while models with initial masses $\geq$40 $\mathrm{M}_{\odot}$ can form the WNE stars. The left panel shows the results of the IGWs excited from the convective core, while the right panel shows the results of IGWs excited from the base of the convective envelope.
• Figure 2: Evolution of surface velocity for massive ($M \geq 40 \mathrm{M}_{\odot}$) stellar models during core helium burning, for different strengths of IGWs (parameterized by A) that are excited at the convective core boundary. The color scale denotes the surface nitrogen abundance, while blue and gray shaded areas mark the WNL and WNE stages, respectively. The subsequent evolution of the hydrogen-stripped star is shown as a pink dash-dotted curve, where the surface nitrogen abundance exceeds 20 due to complete hydrogen loss.
• Figure 3: Similar to Figure \ref{['fig:abun_N_core']}, but the IGWs excited at the base of the convective envelope.
• Figure 4: Viscosity diffusion coefficient from the IGWs for the 60 $\mathrm{M}_{\odot}$ model ($A$ = 10) at different helium-burning stages. The blue and red curves represent the IGWs excited at the boundary of the convective core and the base of the convective envelope, respectively. The dotted line (labeled 'am_nu_non_rot') indicates the diffusion from other sources (e.g., convection-dominated), shown here as a reference baseline at the convective boundary. For the middle and right panels, the excitation of IGWs is negligible as a consequence of the convective envelope having decreased or lost.
• Figure 5: Specific angular momentum for the 60 $\mathrm{M}_{\odot}$ model as a function of the star mass. Solid curves show the distributions at different evolutionary stages, with labeled surface equatorial velocities. Left panel: results with the IGWs excited by and propagating outward from the convective core. Right panel: baseline case without the IGWs.