Drawing the line between explosion and collapse in electron-capture supernovae -- I. Impact of conductive flame speeds and ignition conditions on the explosion mechanism

TL;DR

The paper investigates the fate of electron-capture supernovae by performing 56 three-dimensional hydrodynamic simulations of ONe white dwarfs with a level-set flame front. By varying ignition location, central density at ignition, and two laminar flame-speed parameterizations, the authors map the explosion–collapse boundary and identify the physical mechanisms governing each outcome. They find a narrow transition density window, with off-center ignition extending the explosion regime, and show that turbulence and the sinking of neutronized ashes into the core play key roles in deciding the final fate. The study demonstrates that flame physics, electron captures, and 3D hydrodynamics jointly determine whether a tECSN or cECSN occurs, and highlights the need for fully consistent 3D progenitor evolution and ignition modeling to predict ECSN outcomes accurately.

Abstract

Electron-capture supernovae (ECSNe) are commonly thought to result in a collapse to a neutron star. Recent work has shown that a thermonuclear explosion is also a possible outcome. The division between the two regimes has not yet been mapped out. In this study, we investigate the conditions under which the transition from thermonuclear explosion to collapse occurs, and what physical mechanisms drive each outcome. We conducted a parameter study of 56 3D hydrodynamic simulations of ECSN in ONe white dwarfs using a level set based flame model implemented in the Leafs code. We varied both the ignition location and the central density at ignition to determine the conditions of the transition regime. Additionally, we explored two different laminar flame parameterizations and how they impact the simulation outcome. From our parameter study, we find a transition density in the range of $\logρ_c^{ini}=10.0$ and $10.15$ g cm$^{-3}$, depending on the ignition location and utilized laminar flame speed parameterization. Importantly, we find that for sufficiently high central densities, the burned ashes can sink into the core and trap large amounts of neutron-rich material in the bound remnant. In the transition regime between explosion and collapse, we find that the laminar flame speed plays a critical role by suppressing the formation of instabilities and thereby reducing the nuclear energy generation needed to overcome the collapse. We find that a thermonuclear explosion is possible for a wide range of parameters, whereby a more off-center ignition allows for higher central densities to still result in an explosion. Both the conditions at ignition and the flame physics are critical in determining the outcome. Detailed 3D hydrodynamic simulations of the preceding stellar evolution and the ignition process of the thermonuclear flame are necessary to accurately predict the outcome of ECSNe.

Figures (9)

• Figure 1: Illustration of the different ECSN modes observed in our parameter study. Each row shows a separate simulation, with each column representing various characteristic times. In each plot, the top half indicates $Y_e$ and the bottom half $\langle A\rangle$ (note the varying limits in the color bars). Here, we also show contours of uniform $\log \rho\,(\mathrm{g}\,\mathrm{cm}^{-3})$. We note that in the last timestep of $\mathrm{rho}10.4\_\mathrm{r}73$, we no longer fully trust the results of the simulation (e.g., the reappearance of heavier elements in the ashes is likely only an artifact) and only include this timestep as a visual indication of how the ashes get compressed into a sphere.
• Figure 2: Visualization of the impact of different laminar flame speed parameterizations on the example of the $\mathrm{rho}10.093130625\_\mathrm{r}50$(_sfs) model. The left column shows the simulation with the timmes1992a flame speeds, the right column the one with the schwab2020a values. The top two rows show the simulation a $t=0.1\,\mathrm{s}$, the bottom two at $t=0.4\,\mathrm{s}$. Here, we show four different quantities: the density and internal energy with respect to their values at the ignition location (at $t=0.0\,\mathrm{s}$), the mach number of the flow in the radial direction $\mathcal{M}_\mathrm{rise}$, and the electron fraction $Y_e$. Note the different ranges of the $\mathcal{M}_\mathrm{rise}$ color bars.
• Figure 3: Various quantities related to the neutronization of the core in the comparison between the timmes1992a and schwab2020a laminar flame speed parameterizations. First row: Effective $M_\mathrm{Ch}$, computed using the mass weighted value of $Y_e$ over the entire WD. The horizontal dashed line indicates the mass of the initial WD. Second row: Total mass of the ash $M_\mathrm{ash}$, i.e., mass inside the level set. Third row: Maximum density $\rho_\mathrm{max}$ over time. Last row: Maximum total flame speed $v_\mathrm{f,max}$ of the flame anywhere on its surface. Here we also show the values of both the turbulent and laminar flame speed contribution. The vertical dotted lines indicate the time of the first simulation snapshot that where $M_\mathrm{Ch,eff}$ is well below the initial WD mass.
• Figure 4: Total (cumulative) energy generated from nuclear reactions over time as well as energy lost from weak and thermal neutrinos.
• Figure 5: Illustration of the density inversion of the $\mathrm{rho}10.094140625\_\mathrm{r}50$ model for the timmes1992a and schwab2020a flame speed parametrization. Here, we show the density trajectories of tracer particles that are located inside the ignition bubble at $t=0.0\,\mathrm{s}$. The transparent lines are the individual trajectories, whereas the opaque lines are the mean density of all trajectories.