[Paper Review] Do electron-capture supernovae make neutron stars? First multidimensional hydrodynamic simulations of the oxygen deflagration

Samuel Jones, F. K. Roepke+4 more|arXiv (Cornell University)|Feb 18, 2016

Gamma-ray bursts and supernovae72 references56 citations

TL;DR

This study presents the first multidimensional hydrodynamic simulations of oxygen deflagration in oxygen-neon cores, using a level-set flame model to investigate whether electron-capture supernovae and accretion-induced collapse produce neutron stars. Results show that at intermediate and low ignition densities, the cores explode with nearly a solar mass unbound, leaving stable remnants below the Chandrasekhar mass; only at the highest ignition density (log₁₀ρ_c = 10.3) does collapse into a neutron star occur.

ABSTRACT

In the classical picture, electron-capture supernovae and the accretion-induced collapse of oxygen-neon white dwarfs (ONeWDs) undergo an oxygen deflagration phase before gravitational collapse produces a neutron star (NS). These types of core collapse events are postulated to explain several astronomical phenomena. In this work, the deflagration phase is simulated for the first time using multidimensional hydrodynamics, with the aim of gaining new insight into the explosive deaths of $8-10~M_\odot$ stars and ONeWDs that accrete material from a binary companion star. The main aim is to determine whether these events are thermonuclear or core-collapse supernova explosions, and hence whether NSs are formed by such phenomena. The deflagration is simulated in ONe cores with three different central ignition densities. The intermediate density case is perhaps the most realistic, being based on recent nuclear physics calculations and 1D stellar models. The 3D hydrodynamic simulations presented in this work begin from a centrally confined flame structure using a level-set-based flame approach and are performed in $256^3$ and $512^3$ numerical resolutions. In the simulations with intermediate and low ignition density, the cores do not appear to collapse into NSs. Instead, almost a solar mass of material becomes unbound from the cores, leaving bound remnants. These simulations represent the case in which semiconvective mixing during the electron-capture phase preceding the deflagration is inefficient. The masses of the bound remnants double when Coulomb corrections are included in the EoS, however they still do not exceed the effective Chandrasekhar mass and, hence, would not collapse into NSs. The simulations with the highest ignition density ($\log_{10}ρ_{ m c}=10.3$), representing the case where semiconvective mixing is very efficient, show clear signs that the core will collapse into a NS.

Motivation & Objective

• To determine whether electron-capture supernovae and accretion-induced collapse of ONe white dwarfs produce neutron stars.
• To investigate the role of multidimensional hydrodynamics and turbulent flame propagation in oxygen-deflagration-driven explosions.
• To assess the impact of central ignition density, semiconvective mixing efficiency, and equation of state corrections on core remnant masses.
• To evaluate whether the deflagration phase leads to core collapse or explosive unbinding.

Proposed method

• Simulations employ a level-set-based flame approach to model turbulent deflagration in 3D hydrodynamics, avoiding direct resolution of the flame front.
• Three central ignition densities are used: low (log₁₀ρ_c = 10.0), intermediate (log₁₀ρ_c = 10.15), and high (log₁₀ρ_c = 10.3), with the intermediate case informed by 1D stellar models and nuclear physics.
• Numerical resolution is tested at 256³ and 512³ grid points to assess convergence and numerical robustness.
• The equation of state includes Coulomb corrections to assess their impact on remnant mass and stability.
• Perturbations are introduced to seed hydrodynamical instabilities, particularly Rayleigh-Taylor, to test sensitivity to asymmetries.
• Simulations track electron fraction (Yₑ) and energy release to evaluate nuclear burning progress and explosion energetics.

Experimental results

Research questions

• RQ1 Does oxygen deflagration in degenerate ONe cores lead to core collapse and neutron star formation, or does it result in a thermonuclear explosion?
• RQ2 How does the central ignition density influence the outcome—explosive unbinding or gravitational collapse?
• RQ3 To what extent do semiconvective mixing efficiency and equation of state corrections affect the final remnant mass?
• RQ4 Can multidimensional hydrodynamics reproduce the flame asymmetries and turbulent burning observed in 1D models?
• RQ5 What is the role of initial flame geometry and perturbations in determining the explosion dynamics and nucleosynthesis?

Key findings

• At intermediate and low ignition densities (log₁₀ρ_c = 10.15 and 10.0), the cores undergo explosive burning, ejecting nearly one solar mass of material and leaving bound remnants below the Chandrasekhar mass.
• Even with Coulomb corrections included, the bound remnants in the intermediate and low-density cases do not exceed the effective Chandrasekhar mass, preventing neutron star formation.
• Only at the highest ignition density (log₁₀ρ_c = 10.3), corresponding to highly efficient semiconvective mixing, does the core show clear signs of gravitational collapse into a neutron star.
• The Rayleigh-Taylor instability drives significant asphericity in the flame front, even in the laminar regime, indicating that 3D hydrodynamics is essential for accurate modeling.
• Turbulent burning speeds exceed laminar speeds only after ~460 ms in the intermediate-density case, suggesting that turbulence enhances burning but does not initiate collapse.
• The simulations demonstrate that 1D models are insufficient for predicting global outcomes due to unresolvable asymmetries and hydrodynamic instabilities.