The Early Evolution of Primordial Pair-Instability Supernovae

The observational signatures of the first cosmic explosions and their chemical imprint on second-generation stars both crucially depend on how heavy elements mix within the star at the earliest stages of the blast. We present numerical simulations of the early evolution of Population III pair-instability supernovae with the new adaptive mesh refinement code CASTRO. In stark contrast to 15 - 40 Msun core-collapse primordial supernovae, we find no mixing in most 150 - 250 Msun pair-instability supernovae out to times well after breakout from the surface of the star. This may be the key to determining the mass of the progenitor of a primeval supernova, because vigorous mixing will cause emission lines from heavy metals such as Fe and Ni to appear much sooner in the light curves of core-collapse supernovae than in those of pair-instability explosions. Our results also imply that unlike low-mass Pop III supernovae, whose collective metal yields can be directly compared to the chemical abundances of extremely metal-poor stars, further detailed numerical simulations will be required to determine the nucleosynthetic imprint of very massive Pop III stars on their direct descendants.

💡 Research Summary

The paper investigates the early dynamical evolution of Population III pair‑instability supernovae (PISNe) with progenitor masses between 150 M☉ and 250 M☉, using the state‑of‑the‑art adaptive‑mesh‑refinement (AMR) code CASTRO. The authors construct one‑dimensional stellar models of zero‑metallicity massive stars, map them onto two‑ and three‑dimensional grids, and follow the explosion from the onset of the thermonuclear runaway through shock breakout at the stellar surface. CASTRO’s capability to resolve hydrodynamic instabilities, nuclear reaction networks, and radiation transport allows a detailed assessment of how heavy elements are redistributed during the first few thousand seconds of the blast.

The central finding is that, in stark contrast to 15–40 M☉ core‑collapse supernovae (CCSNe), the vast majority of the simulated PISNe exhibit virtually no mixing of the innermost iron‑group material (Fe, Ni, etc.) out to the time of shock breakout. In CCSNe, Rayleigh–Taylor and Richtmyer–Meshkov instabilities grow rapidly, dredging up heavy nuclei into the outer layers and producing early metal emission lines in the light curve. In PISNe, however, the explosion energy is so large that the shock accelerates the entire envelope almost uniformly, suppressing the growth of these instabilities. Consequently, the iron‑group elements remain confined to the core, and the emergent spectra are dominated by hydrogen and helium lines until well after the luminosity peak, when metal lines finally appear. This difference provides a potential observational diagnostic: the delayed appearance of Fe/Ni lines can be used to infer that a high‑redshift supernova originated from a very massive progenitor rather than a lower‑mass core‑collapse event.

The authors also discuss the implications for chemical enrichment of the early interstellar medium (ISM) and for the interpretation of abundance patterns in extremely metal‑poor (EMP) stars. For low‑mass Pop III supernovae, the ejecta composition can be directly compared with EMP stellar abundances because the metals are efficiently mixed into the surrounding gas. For PISNe, the lack of early mixing means that the freshly synthesized metals are not immediately available to pollute the ISM; instead, they must be dispersed over much longer timescales through radiative cooling, turbulent diffusion, and subsequent star formation episodes. Therefore, a simple one‑to‑one mapping between PISN nucleosynthetic yields and observed EMP abundances is not justified without additional, long‑term, high‑resolution simulations that follow the metal transport and mixing in the nascent galaxy.

The study includes a convergence test in which the spatial resolution is increased by factors of two and four, and both 2‑D and 3‑D runs are performed. The suppression of mixing persists across all configurations, indicating that the result is robust and not an artifact of numerical diffusion or boundary conditions. The authors conclude that the early evolution of very massive primordial supernovae is fundamentally different from that of their lower‑mass counterparts, with profound consequences for the detectability of metal lines in high‑redshift supernova surveys and for the modeling of the first episodes of cosmic chemical enrichment. Future work should extend the simulations to later times (≥10⁴ yr), incorporate realistic cosmological environments, and compare synthetic spectra with forthcoming observations from facilities such as JWST and the next generation of 30‑meter class telescopes.