On the Importance of the Convective Urca Process in 3D Simulations of a Simmering White Dwarf

Abstract

Type Ia supernovae are bright thermonuclear explosions that are important to numerous areas of astronomy. However, the origins of these events are poorly understood. One proposed setting is that of a near Chandrasekhar mass white dwarf that undergoes runaway carbon burning in the core. During the thousand years leading up to the explosion, the white dwarf undergoes a simmering phase where slow carbon burning heats the core and drives convection. A poorly understood aspect of this phase is the convective Urca process, which links convection with weak nuclear reactions. We use the low Mach number code MAESTROeX to perform full 3D simulations as is required to accurately capture the turbulent convection. We present simulations with and without the A=23 convective Urca process, which have relaxed to a steady state. We characterize the effects of the convective Urca process on the neutrino losses, the nuclear energy generation, and the convective boundary. We find that the size of the convection zone is substantially reduced by the convective Urca process, though convection still extends past the Urca shell. Our findings on the structure of the convective zone and the compositional changes can be used to inform 1D stellar models that track the longer-timescale evolution.

Figures (12)

• Figure 1: Top plot: Star temperature vs radius profiles. Bottom plot: Mass fraction of the Urca species vs radius. The grey curves represent the initial state of the simulations in B2025, the blue curves the initial state of the FN1 and NB1 simulations, and the green curves the initial state of the FN2 simulation. The dashed curves represent the ${}^{23}\mathrm{Ne}$ and the solid ones the ${}^{23}\mathrm{Na}$. The vertical red line indicates the position of the Urca shell.
• Figure 2: 2D slice through the center of the WD colored by the value of $X({}^{12}\mathrm{C})$ at the end of the simulation. The solid and dashed black lines represent the Urca shell and the boundary where $X({}^{23}\mathrm{Na}) = X({}^{23}\mathrm{Ne})$ respectively. The black streamlines indicate the trajectories of the convective flows. The left slice represents the FN2 simulation, and the right one the NB1 simulation.
• Figure 3: Spherically averaged $X({}^{12}\mathrm{C})$ profiles vs radial bin for each simulation. The green curve is from FN1 simulation. The blue curve from the FN2 simulation. And the orange curve from NB1 simulation. The red vertical dash-dot line indicates the location of the Urca Shell. The vertical dashed lines indicate the $R_{\mathrm{conv}}$ for the FN2 (blue) and NB1 (orange) simulations
• Figure 4: The left plot tracks the mass contained in the convection zone, $M_{\mathrm{conv}}$, over simulation time. The right plot shows the $U_{\mathrm{rms}}$ of the convection zone over time. We plot all three simulations with FN1 in green, FN2 in blue and NB1 in orange.
• Figure 5: Spherically averaged convective gradient profiles vs radial bin for each simulation. The dashed curves represent the ratio of the real gradient to an adiabatic $(\nabla / \nabla_{\mathrm{ad}}$. The solid curves represent the ratio of the real gradient to the Ledoux gradient $(\nabla / \nabla_{\mathrm{Led}}$. The green curve is from FN1 simulation. The blue curve from the FN2 simulation. And the orange curve from NB1 simulation. The red vertical dash-dot line indicates the location of the Urca Shell.