Evolution of binaries containing a hot subdwarf and a white dwarf to double white dwarfs, and double detonation supernovae with hypervelocity runaway stars

TL;DR

This work investigates the fate of tight binaries consisting of a hot subdwarf and a WD by evolving both stars with MESA across a dense grid of initial masses and orbital periods. It identifies regimes leading to double detonation SNe, He novae, or double WD outcomes, and quantifies He-shell masses at ignition (minimum ~$0.05 M_\odot$, maximum ~$0.33 M_\odot$) and potential donor runaway velocities up to ~$1018\,\mathrm{km\,s^{-1}}$. The results highlight how donor mass, accretor mass, and mass-transfer timing (core He burning vs shell/pulses) control detonation conditions, the density of ignition, and the observational signatures including hypervelocity runaways and thick He shells in WD mergers. By providing a dense, publicly accessible grid, the study offers a first-order framework for predicting outcomes of hot subdwarf–WD binaries in surveys like Gaia, LSST, and LISA, and for informing future 3D explosion/merger simulations.

Abstract

Compact binaries containing hot subdwarfs and white dwarfs have the potential to evolve into a variety of explosive transients. These systems could also explain hypervelocity runaway stars such as US 708. We use the detailed binary evolution code MESA to evolve hot subdwarf and white dwarf stars interacting in binaries. We explore their evolution towards double detonation supernovae, helium novae, or double white dwarfs. Our grid of 3120 models maps from initial conditions such as orbital period and masses of hot subdwarf and white dwarf to these outcomes. The minimum amount of helium required to ignite the helium shell that leads to a double detonation supernova in our grid is $\approx 0.05 \, \mathrm{M_{\odot}}$, likely too large to produce spectra similar to normal type Ia supernovae, but compatible with inferred helium shell masses from some observed peculiar type I supernovae. We also provide the helium shell masses for our double white dwarf systems, with a maximum He shell mass of $\approx 0.18\,\mathrm{M_{\odot}}$. In our double detonation systems, the orbital velocity of the surviving donor star ranges from $\approx 450 \, \mathrm{km\,s^{-1}}$ to $\approx 1000 \, \mathrm{km\,s^{-1}}$. Among the surviving donors, we also estimate the runaway velocities of proto-white dwarfs, which have higher runaway velocities than hot subdwarf stars of the same mass. Our grid will provide a first-order estimate of the potential outcomes for the observation of binaries containing hot subdwarfs and white dwarfs from future missions like Gaia, LSST, and LISA.

Figures (13)

• Figure 1: Schematic diagram representing possible evolution channels of a hot subdwarf + WD binary to a double white dwarf, a double detonation supernova or a He nova. Double detonation supernovae and He novae result from systems that undergo mass transfer and later experience thermonuclear instability in the He layer on the surface of the WD. The differentiation between double detonation supernova and He nova is based on the He layer's critical density (see Sect. \ref{['sec:Conditions for ignition']}). Double white dwarfs result from systems that either evolve in isolation or involve episodes of accretion but do not enter unstable nuclear burning.
• Figure 2: Example of a binary with $\mathrm{M_d = 0.5\,\mathrm{M_{\odot}}}$, $\mathrm{M_a = 0.8 \,\mathrm{M_{\odot}}}$, and $\mathrm{P_i = 0.84\, hours}$, where a thermonuclear explosion occurs in the WD (accretor). The background colors yellow, blue, and green represent the gravitational-wave inspiral phase (GW-inspiral), H mass transfer phase (H-MT), and He mass transfer phase (He-MT), respectively. Panels (a) through (f) show the evolution of various parameters: (a) Mass Evolution: $M_a$ (accretor) and $M_d$ (donor); (b) Orbital Period Evolution; (c) Radius Evolution: $R_d$ (donor's radius) and $R_{roche,d}$ (Roche radius of the donor); (d) mass transfer rate; (e) Evolution of surface mass fraction: $X_H$ (hydrogen) and $X_{He}$ (helium) in the donor; (f) He Mass on the Accretor; (g) Luminosity Evolution: $L_a$ (accretor), $L_d$ (donor), and $L_{\text{accretion}}$ (accretion). The accretor gains mass at the rate of $\sim 10^{-8} \, \mathrm{M_{\odot}yr^{-1}}$, resulting in the He ignition in the He layers denser than the assumed critical density for detonation ($> 10^{6} \, \mathrm{g\,cm^{-3}}$).
• Figure 3: Each curve depicts the evolution of the temperature-density profile of the accreting white dwarf at various times, as indicated in the legend, for a system that undergoes double detonation supernova (left panel) and a He nova (right panel). The black star indicates the C/O-He boundary, that moves to higher density as the WD accretes and contracts. In the left panel, the WD accretes at the rate of $\sim 10^{-8} \, \mathrm{M_{\odot}yr^{-1}}$, igniting the high density regions of the He shell ($> 10^{6} \, \mathrm{g\,cm^{-3}}$), and so we classify the outcome as a double detonation supernova. In the right panel, the accretor's evolution is initially dominated by the cooling of the WD. After the accretion starts, the WD accretes at $\sim 10^{-7} \, \mathrm{M_{\odot}yr^{-1}}$. The WD experiences compressional heating, causing the low-density ($< 10^{6} \, \mathrm{g\,cm^{-3}}$) layers to ignite explosively. We then classify this system as a He nova.
• Figure 4: Example of a binary with $\mathrm{M_d = 0.75\, \mathrm{M_{\odot}}}$, $\mathrm{M_a = 0.8\,\mathrm{M_{\odot}}}$, and $\mathrm{P_i = 1.9\, hours}$, where a He nova occurs in the WD (accretor). The panels display the evolution of different parameters as in Fig. \ref{['fig:Explosion']}. The background colors yellow, and green represent the gravitational-wave inspiral phase (GW-inspiral), and mass transfer phase during the He shell burning phase (Shell-MT), respectively. The accretor accretes with a relatively high accretion rate of $\sim 10^{-7} \, \mathrm{M_{\odot}yr^{-1}}$, resulting in the ignition in the He layers less dense than the assumed critical density for detonation ($< 10^{6} \, \mathrm{g\,cm^{-3}}$).
• Figure 5: Evolutionary aspects of a binary with $\mathrm{M_d = 0.7 \,\mathrm{M_{\odot}}}$, $\mathrm{M_a = 0.85 \,\mathrm{M_{\odot}}}$, and $\mathrm{P_i = 2\, hours}$, where the donor evolves into a WD, forming a double white dwarf system. The panels display the evolution of different parameters as in Fig. \ref{['fig:Explosion']}. The background colors yellow, and green represent the gravitational-wave inspiral phase (GW-inspiral), and mass transfer phase during the He shell burning phase (Shell-MT), respectively. This system is detached for most of its lifetime, and undergoes one episode of accretion that fails to ignite the He layer of the accretor, resulting in a double white dwarf. This double white dwarf is expected to merge in 86 million years which might lead to a Type Ia supernova.