Collapse of Magnetized White Dwarfs as site of Heavy Element Formation and Kilonova Signal

Abstract

We present the first end-to-end calculation connecting the accretion-induced collapse (AIC) of a magnetized, rapidly rotating white dwarf to observable kilonova signatures, combining 2D general-relativistic neutrino-magnetohydrodynamic simulations, followed by radiation hydrodynamics with in-situ nuclear network and 2D Monte Carlo radiative transfer with spatially resolved heating rates. Unlike all previous unmagnetized AIC models - which predicted proton-rich, $^{56}$Ni-dominated ejecta - strong magnetic fields eject ${\sim 0.2 M_\odot}$ of neutron-rich material $(\langle Y_e \rangle \sim 0.24)$ on dynamical timescales, before neutrino irradiation can raise the electron fraction, enabling strong $r$-process nucleosynthesis up to and beyond the third peak. The resulting kilonova is lanthanide-rich $(X_{\rm lan} \approx 6\%)$ and dominated by near-infrared emission. We compute synthetic light curves in the LSST and JWST bands and find striking agreement, without parameter tuning, between the observations of AT 2023vfi/GRB 230307A and our broadband light curves for polar viewing angles. These results establish magnetized AIC as a viable channel for heavy $r$-process element production and a compelling progenitor candidate for long-duration gamma-ray bursts with kilonova signatures.

Figures (11)

• Figure 1: Schematic overview of our computational pipeline for modeling AIC transient. The simulation proceeds through three stages: (1) GR$\nu$MHD evolution of the WD collapse and explosion with Gmunu, (2) ray-by-ray radiation hydrodynamics coupled with in-situ nucleosynthesis using SNEC+SkyNet, and (3) Monte Carlo radiative transfer with Sedona to produce synthetic observables.
• Figure 2: 2D snapshots in the $rz$-plane at $t_{\rm pb} = 0.2$, $0.6$, and $1.1~{\rm s}$ (left to right), showing the rest-mass density $\rho$ (top), electron fraction $Y_e$ (middle), and plasma $\beta_{\rm mag} \equiv P_{\rm gas}/P_{\rm mag}$ (bottom). A magnetically-dominated ($\beta_{\rm mag} < 1$), low-density outflow develops along the polar axis, while the bulk of the neutron-rich material ($Y_e \lesssim 0.25$) is concentrated at mid-latitudes, between the polar outflow and the equatorial disk. Solid, dashed, and dotted lines represent density contours of $10^{11} \mathrm{g\, cm^{-3}},10^{10}\mathrm{g\, cm^{-3}}$ and $10^{9}\mathrm{g\, cm^{-3}}$, respectively.
• Figure 3: Joint distributions of ejecta mass at $t_{\rm{pb}} = 1.1$ s as a function of electron fraction $Y_e$ versus asymptotic velocity $v_\infty$ (left), specific entropy $s$ (center), and polar angle $\theta$ (right). The color scale encodes the ejected mass in each bin, revealing correlations between composition and kinematics: the bulk of the neutron-rich ($Y_e \lesssim 0.25$) material is concentrated at moderate velocities ($v_\infty \sim 0.05$--$0.3\,c$), low entropies ($s \lesssim 20\,k_B$/baryon), and polar latitudes. Mass-weighted averages are indicated in each panel.
• Figure 4: Mass distributions of the ejecta at $t_{\rm{pb}} = 1.1$ s as a function of electron fraction $Y_e$, specific entropy $s$, asymptotic velocity $v_\infty$, and polar angle $\theta$ (left to right). The distributions are obtained by integrating the 2D histograms of Fig. \ref{['Fig: 2D hist']} over the complementary variable. The ejecta are dominated by neutron-rich material ($Y_e \sim 0.15$--$0.25$), with a secondary proton-rich component near $Y_e \sim 0.5$. Most of the mass has low entropy ($s \sim 3$--$20\,k_B$/baryon) and moderate velocities ($v_\infty \sim 0.05$--$0.2\,c$).
• Figure 5: Mass-weighted average abundance pattern of the AIC ejecta (solid line) compared with the solar $r$-process residuals (circles; Prantzos:2019bpv). Abundances are normalized to the second-peak region ($130 \leq A \leq 140$).