Electron-Capture Supernovae (EC-SNe): An Overview

• Electron-capture supernovae are rare stellar explosions triggered by rapid electron captures in degenerate ONeMg cores near the Chandrasekhar limit.
• They arise via single and binary evolution channels, leading to low explosion energy, faint transients, and low kick velocities for the resulting neutron stars.
• EC-SNe uniquely influence Galactic chemical evolution by producing neutron-rich isotopes and light trans-iron elements that are challenging to form in other supernova types.

Electron-capture supernovae (EC-SNe) are stellar explosions arising from the collapse of degenerate oxygen–neon–magnesium (ONeMg) cores in stars with initial masses generally between 8 and 10 M_$\odot$. The defining astrophysical trigger is the removal of electron degeneracy pressure via rapid electron captures on nuclei such as $^{24}$Mg and $^{20}$Ne once the core approaches the Chandrasekhar limit. This leads to dynamical collapse, typically resulting in a low-mass neutron star, and creates faint, short-lived transients with small ejected masses. EC-SNe represent a distinct pathway in stellar evolution, different from both thermonuclear (Type Ia) and high-mass iron-core-collapse supernovae, contributing uniquely to the formation of neutron star systems and the chemical evolution of the Galaxy (Wang et al., 30 Sep 2025).

1. Progenitor Scenarios and Evolutionary Pathways

Two principal evolutionary channels are recognized for EC-SNe: the single-star and the binary-star pathways.

• Single-star channel: Super-asymptotic giant branch (SAGB) stars (ZAMS masses roughly 8–10 M_$\odot$) can develop degenerate ONeMg cores after off-center carbon burning, followed by core mass growth via helium shell burning. If the core mass reaches a critical value (approximately $1.367\,M_\odot$, robust to moderate variations in initial parameters), electron-capture reactions on $^{24}$Mg eventually trigger collapse (Takahashi et al., 2013). Helium stars formed via stripping of the hydrogen envelope can also form ONe cores under suitable conditions (Wang et al., 30 Sep 2025).
• Binary-star channel: Several sub-channels exist. In close He star binaries, mass transfer (especially Case BB or Case BA, depending on the stage) can strip the star to a metal core just above or near the Chandrasekhar limit, leading to EC-SNe if subsequent evolutionary conditions result in ONe core collapse (Tauris et al., 2015, Guo et al., 10 Jan 2024). Accretion-induced collapse (AIC) from ONe white dwarfs in binaries (either single-degenerate or double-degenerate systems) also represents a viable EC-SN progenitor scenario (Wang et al., 30 Sep 2025). The critical core mass window for EC-SNe is narrow—typically final metal core masses between $\sim1.385$ and $1.43\,M_\odot$ suffice for electron-capture collapse, while lower masses fail to ignite collapse and greater masses evolve to Fe-core-collapse (Guo et al., 10 Jan 2024).

Binary evolution, with parameters such as initial mass, wind mass loss, metallicity, and binary separation, plays a decisive role in determining the outcome. Higher metallicity shifts the minimum He star mass and minimum orbital period required for EC-SNe to higher values, as a result of stronger wind mass loss (Guo et al., 10 Jan 2024).

2. Core Physics: Electron Captures, Flame Propagation, and Collapse

In an ONeMg core near the Chandrasekhar mass, electron-captures on $^{24}$Mg and subsequently on $^{20}$Ne become energetically possible as the central density rises ($\rho_c \gtrsim 10^{10}\ \mathrm{g\,cm}^{-3}$). These reactions reduce the electron fraction $Y_e$ and thus the electron degeneracy pressure. The runaway loss of support causes rapid contraction, further electron captures, and eventually the onset of nuclear statistical equilibrium (NSE). In this regime, an O–Ne deflagration flame, whose propagation is strongly accelerated by the intense pre-bounce electron neutrino ($\nu_e$) radiation from ongoing captures, rapidly consumes the core (Takahashi et al., 2018). The pre-bounce $\nu_e$ luminosity can exceed $10^{51}$ erg s⁻¹, and $\nu_e$-electron scattering ahead of the flame front further enhances the flame speed to $\sim10^8$ cm s⁻¹, one to two orders of magnitude faster than pure conduction would permit.

Hydrodynamic simulations indicate that the fate of the core is highly sensitive to the central density at deflagration onset ($\rho_{c,\mathrm{def}}$) and the $Y_e$ distribution. For log$_{10}(\rho_{c,\mathrm{def}}/[g\,cm^{-3}]) > 10.01$, collapse to a neutron star is the typical outcome (Zha et al., 2019). Lower ignition densities may permit partial thermonuclear disruption (thermonuclear ECSN or "tECSN"), but the conditions for this pathway appear rare given modern electron-capture rates and the effects of convection and semiconvection on mixing (Zha et al., 2019, Jones et al., 2018).

3. Binary Evolution, Double Neutron Star Formation, and Spin Evolution

In binary systems, EC-SNe play a pivotal role in the formation of double neutron star (DNS) systems and the population of recycled pulsars. Population synthesis in NS+He star systems delineates a metallicity-dependent region in the log $P_{\rm orb}$–$M_{\rm He}$ plane that results in EC-SNe, with lower metallicity favoring EC-SNe at lower masses and shorter periods (Guo et al., 10 Jan 2024).

The typical outcome in these systems includes:

• Low NS kick velocities ($\lesssim 50$ km s⁻¹), consistent with the expectation from the low ejecta mass and low explosion energy of EC-SNe.
• DNS orbital properties (orbital periods, eccentricities) that match most observed DNS systems only when these low kicks are applied.
• Recycled (spun-up) pulsars form via accretion preceding collapse; if the He star companion retains a small hydrogen envelope, the NS can accrete additional material during Roche-lobe overflow, permitting "mildly recycled" pulsars with spin periods as low as $\sim 20$ ms, consistent with observations.

Analytic relations demonstrate, for example, the minimum spin period as $$P_{\rm spin}^{\rm min} \approx 0.34\,(\Delta M_{\rm NS}/M_\odot)^{-3/4}\,\mathrm{ms}$$, where $\Delta M_{\rm NS}$ is the mass accreted by the NS (Guo et al., 10 Jan 2024).

4. Nucleosynthesis and Chemical Evolution Implications

EC-SNe contribute distinctively to the chemical evolution of the Galaxy, especially regarding the synthesis of select neutron-rich isotopes and trans-iron elements. Owing to the neutron-rich, low-entropy, and $\alpha$-deficient conditions in the ejecta, EC-SNe are efficient sites for the production of:

• Isotopes such as $^{48}$Ca, $^{50}$Ti, $^{54}$Cr, $^{60}$Fe, and some Zn–Zr isotopes, which are challenging to synthesize in Fe-core-collapse SNe or classical SNe Ia [(Wanajo et al., 2010); (Wanajo et al., 2013); (Wanajo et al., 2013)].
• Light trans-iron elements up to Sr, Y, and Zr (atomic number $Z=38–40$).
• Weak $r$-process elements up to Pd, Ag, and Cd, contingent on the minimum $Y_e$ actually reached in the ejecta (Wanajo et al., 2010).

Population synthesis and galactic chemical evolution modeling suggest that if both core-collapse (cECSNe) and partial thermonuclear (tECSNe) channels operate (with $\sim$0.5–0.7% tECSNe and $\sim$4% cECSNe of all core-collapse SNe), the observed solar inventory of these problematic isotopes—especially $^{48}$Ca, $^{50}$Ti, $^{54}$Cr, $^{58}$Fe, $^{64}$Ni, $^{82}$Se, and $^{86}$Kr—can be matched without tension to other elements (Jones et al., 2019). These nucleosynthetic fingerprints have also been observed in pre-solar meteoritic oxide grains, further supporting the ECSN pathway (Jones et al., 2018).

5. Observational Features, Rate Estimates, and Remnants

Typical observational signatures of EC-SNe include:

• Low explosion energy ($\sim 1-2\times10^{50}$ erg), faint and short-lived optical light curves, low ejecta velocities, and distinctive plateau durations. EC-SNe in binaries stripped of their hydrogen envelopes manifest as ultra-stripped SNe with even lower ejecta mass and rapidly evolving, faint transients resembling SN 2008ha-like events (Tauris et al., 2015, Moriya et al., 2016).
• When a substantial hydrogen envelope persists, EC-SNe can produce events with plateau light curves and spectra classified as Type IIn-P, as suggested for the Crab Nebula’s progenitor (SN 1054) [(Smith, 2013); (Moriya et al., 2014)].
• The observed NS mass distribution shows a low-mass peak at $\sim1.25$ M$_\odot$, which is plausibly explained by EC-SNe in both single-star and binary evolutionary scenarios (Guo et al., 10 Jan 2024, Wang et al., 30 Sep 2025).
• Observational estimates and theoretical modelling both constrain the fraction of EC-SNe to be $1$–$4$% of all core-collapse SNe, with the binary channel (including DNS production) being a significant contributor (Jones et al., 2018, Jones et al., 2019).
• EC-SNe are expected to impart low natal kicks to their NS remnants due to low explosion energies and small ejected masses, facilitating the formation and retention of DNS systems (Guo et al., 10 Jan 2024).
• Recent multi-wavelength studies, notably of SN 2018zd, have provided strong, multi-faceted evidence for EC-SN origin via progenitor identification, circumstellar interaction, nebular composition diagnostic, and nucleosynthesis signatures (Hiramatsu et al., 2020).

6. Uncertainties, Theoretical Challenges, and Future Directions

Outstanding uncertainties in EC-SN research include:

• The precise initial mass range is sensitive to details such as metallicity, mass loss rate prescriptions, and third dredge-up efficiency during late stages of SAGB evolution (Wang et al., 30 Sep 2025).
• The microphysics of electron-capture rates, especially for second-forbidden transitions and Coulomb corrections, significantly affect the critical density and possible bifurcation between neutron star collapse and thermonuclear explosion (Zha et al., 2019).
• The propagation of the O–Ne deflagration flame is complex, influenced by both hydrodynamical mixing and the intense neutrino-electron scattering in the core (Takahashi et al., 2018); multidimensional hydrodynamic treatment is necessary to capture these effects robustly.
• Rate estimates of EC-SNe via population synthesis are converging, but remain sensitive to binary parameters, initial mass function details, and mass transfer stability.
• Integrated modeling and observational campaigns, especially targeting faint, rapidly-evolving Type Ib/c and ultra-stripped SNe, as well as multi-wavelength follow-up of candidate events, are essential for robust identification and quantitative constraint of EC-SN occurrence rates and explosion properties (Wang et al., 30 Sep 2025).

7. Role in Neutron Star System Demographics and Galactic Evolution

EC-SNe are pivotal in shaping the demographics of neutron star systems:

• They provide an alternative channel to form low-mass NSs, especially in binaries where reduced kick velocities promote DNS formation and merger (with significant implications for gravitational wave astronomy) (Guo et al., 10 Jan 2024, Tauris et al., 2015).
• EC-SNe contribute to X-ray binary populations, globular cluster NS retention, and influence the observed NS mass and orbital eccentricity distributions in DNS and high-mass X-ray binaries (Wang et al., 30 Sep 2025).
• Their nucleosynthetic products enrich the interstellar medium with neutron-rich isotopes distinct from Fe-core-collapse yields, marking them as vital contributors to Galactic chemical evolution, especially for select nuclei difficult to synthesize in other channels (Jones et al., 2019).

EC-SNe thus comprise a critical, but relatively rare, outcome of stellar evolution, linking subtle aspects of binary interaction, nuclear microphysics, galactic nucleosynthesis, and neutron star system formation in a manner distinct from both classical iron-core-collapse and thermonuclear supernovae. Ongoing advances in multi-dimensional simulation, nuclear rates, and survey observations are central to refining the quantitative impact and securing robust observational diagnostics of this unique supernova channel.