Evolution of massive population III stars with rotation and magnetic fields
[Abridged] We present a new grid of massive population III star models including the effects of rotation on the stellar structure and chemical mixing, and magnetic torques for the transport of angular momentum. Based on the grid, we also present a phase diagram for the expected final fates of rotating massive Pop III stars. Our non-rotating models become redder than the previous models in the literature, given the larger overshooting parameter adopted in this study. In particular, convective dredge-up of the helium core material into the hydrogen envelope is observed in our non-rotating very massive star models (>~200 Msun), which is potentially important for the chemical yields. On the other hand, the stars become bluer and more luminous with a higher rotational velocity. With the Spruit-Tayler dynamo, our models with a sufficiently high initial rotational velocity can reach the critical rotation earlier and lose more mass as a result, compared to the previous models without magnetic fields. The most dramatic effect of rotation is found with the so-called chemically homogeneous evolution (CHE), which is observed for a limited mass and rotational velocity range. CHE has several important consequences: 1) Both primary nitrogen and ionizing photons are abundantly produced. 2) Conditions for gamma-ray burst progenitors are fulfilled for an initial mass range of 13 - 84 Msun. 3) Pair instability supernovae of type Ibc are expected for 84 -190 Msun and 4) Both a pulsational pair instability supernova and a GRB may occur from the same progenitor of about 56 - 84 Msun, which might significantly influence the consequent GRB afterglow. We find that CHE does not occur for very massive stars (> 190 Msun), in which case the hydrogen envelope expands to the red-supergiant phase and the final angular momentum is too low to make any explosive event powered by rotation.
💡 Research Summary
This paper presents a comprehensive grid of massive Population III (Pop III) stellar models that simultaneously incorporate rotation‑induced structural effects, rotationally driven chemical mixing, and magnetic torques generated by the Spruit‑Taylor dynamo for angular‑momentum transport. By exploring a wide range of initial masses (from ≈10 M☉ up to >300 M☉) and rotational velocities, the authors construct a detailed phase diagram that predicts the final fate of each model—whether it ends as a core‑collapse supernova, a pair‑instability supernova (PISN), a pulsational PISN, a gamma‑ray burst (GRB), or a direct black‑hole collapse.
Key methodological advances include: (1) adopting a larger convective overshooting parameter than previous Pop III studies, which leads to more extensive core‑envelope mixing; (2) implementing the Spruit‑Taylor dynamo, which creates internal magnetic fields that efficiently redistribute angular momentum from the core to the surface, allowing stars to reach critical rotation earlier and to lose mass via rotationally enhanced winds; and (3) coupling these processes with a state‑of‑the‑art nuclear network to follow the production of light elements, especially primary nitrogen.
The non‑rotating models become significantly redder than earlier calculations because the larger overshoot drives a deep convective dredge‑up of helium‑core material into the hydrogen envelope for very massive stars (> ≈200 M☉). This dredge‑up can enrich the outer layers with carbon and oxygen, potentially altering the chemical yields of the first supernovae.
When rotation is introduced, stars shift toward bluer, more luminous tracks. High initial spin rates trigger strong rotational mixing, and the magnetic torques accelerate angular‑momentum loss once the surface approaches critical rotation. Consequently, mass loss is enhanced compared with previous non‑magnetic models, and the final stellar mass can be substantially reduced before collapse.
The most striking outcome is the occurrence of chemically homogeneous evolution (CHE) in a limited mass‑velocity window. CHE arises when rotational mixing homogenizes the composition throughout the star, preventing the formation of a distinct hydrogen envelope. In this regime, the models produce copious primary nitrogen and emit large quantities of ionizing photons (especially He II), thereby contributing significantly to the re‑ionization of the early universe.
CHE also fulfills the angular‑momentum requirements for collapsar‑type GRB progenitors. The authors find that stars with initial masses between 13 M☉ and 84 M☉ can retain enough core spin to form an accretion disk at collapse, satisfying the conditions for long‑duration GRBs. For masses 84 M☉–190 M☉, the models predict Type Ibc PISNe, where the entire star is disrupted without leaving a remnant. Intriguingly, in the overlapping range of ≈56 M☉–84 M☉, a single star may first undergo a pulsational PISN, ejecting mass in violent pulses, and later collapse to produce a GRB. This dual‑event scenario could imprint distinctive signatures on the GRB afterglow, such as dense circum‑stellar material and late‑time re‑brightening.
For the most massive stars (> ≈190 M☉), CHE does not occur; the hydrogen envelope expands to a red‑supergiant configuration, and magnetic braking removes most of the core angular momentum. These stars are therefore unlikely to generate rotation‑driven explosions; they are expected to collapse directly into massive black holes.
Overall, the paper delivers a nuanced picture of how rotation and magnetic fields reshape the life cycles of the first stars. By quantifying the interplay between overshooting, rotational mixing, magnetic angular‑momentum transport, and mass loss, the authors provide essential inputs for cosmological simulations of early star formation, chemical enrichment, and the sources of high‑energy transients in the primordial universe.