He-star donor pathway for the hypervelocity star D6-2

TL;DR

This study probes the progenitors of Type Ia supernovae by modeling hot subdwarf + CO white dwarf binaries undergoing helium accretion with MESA, coupled to population synthesis via MSE. The donors evolve into compact CO-core objects with thin helium envelopes, and their SN ejecta can strip the envelope, yielding CO-rich surfaces; ejection velocities span roughly $450$–$1000\, \mathrm{km\,s^{-1}}$, with D6-2 representing the high-velocity extreme. The authors find that this hot subdwarf + WD channel can account for D6-2’s kinematics and surface composition, while contributing about ${\sim}(1.69 \pm 0.06)\times10^{-5}$ to the Ia rate per unit stellar mass (roughly 1% of the total Ia rate). The results suggest a continuum of surviving donors, from subdwarfs to cooling white dwarfs, and imply that a single Type Ia progenitor class can explain the observed range of hypervelocity remnants (e.g., US 708, LP 40-365) with testable predictions for future observations and detailed 3D impact simulations.

Abstract

Type Ia supernovae are thermonuclear explosions of white dwarfs, yet the nature of their progenitor systems remains uncertain. Recent discoveries of hypervelocity stars provide unique constraints, as these stars likely represent the surviving companions of such explosions. Using detailed binary evolution models computed with MESA and population synthesis with MSE, we investigate the outcomes of hot subdwarf + white dwarf binaries undergoing helium accretion. We find that donors can nearly exhaust their helium and form compact, C/O cores before explosion. The predicted ejection velocities span a broad distribution reaching up to $\sim 1000\,\mathrm{km\,s^{-1}}$, with D6-2 representing the extreme high-velocity tail of this population. We estimate analytically that the thin residual helium envelope can be stripped by the supernova ejecta, producing a C/O-rich surface composition consistent with the observed spectrum. The Type Ia supernova rate from this channel is ${\sim}(1.69\pm0.06)\times10^{-5}\,\mathrm{M_\odot^{-1}}$, consistent with 1% of the observed Type Ia supernova rate. Hot subdwarf + white dwarf binaries containing nearly exhausted He-star donors can therefore naturally explain the velocity and composition of D6-2 while providing a quantitatively consistent contribution to the observed Type Ia supernova rate. Our models predict a distribution of surviving donor remnants with various core He fractions and with ejection velocities extending down to $\sim 450\,\mathrm{ km\,s^{-1}}$. The orbital velocities of donor stars in this progenitor channel naturally yield orbital velocities consistent with US 708, LP 40-365 stars, and D6-2, indicating that a single class of thermonuclear supernova progenitors can account for their entire range of ejection velocities.

Figures (6)

• Figure 1: Ejection properties of donors in hot subdwarf + white dwarf binaries. Left: Schematic illustrating the formation of C/O core donor stars with varying degrees of degeneracy in CO white dwarf + hot subdwarf binaries. Right: Radius versus orbital velocity of the donors at the time of accretor explosion, with the color bar indicating the degree of degeneracy at the time of explosion. The grey region shows the velocity range observed for D6-2. Some of our models reach this velocity range, demonstrating that evolved donors in hot subdwarf + white dwarf systems can naturally reproduce the kinematic properties of D6-2.
• Figure 2: Ejection velocity of donor stars as a function of their C/O core mass at the time of the companion’s explosion. The color bar indicates the total donor mass. The shaded region marks the measured velocity of D6-2. Models within this region possess C/O core masses of $\sim 0.1 - 0.2 \,\rm M_\odot$, consistent with the observed properties of D6-2.
• Figure 3: An example of a binary with $\mathrm{M_d = 0.4\,\mathrm{M_{\odot}}}$, $\mathrm{M_a = 0.9 \,\mathrm{M_{\odot}}}$, and $\mathrm{P_i = 2.16\, hours}$, where a thermonuclear explosion occurs in the white dwarf (accretor). The background colors yellow and green represent the gravitational-wave inspiral phase (GW-inspiral) and the mass transfer phase (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 $X_{He}$ (He) 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 4: Internal composition profiles of the donor star at the time of explosion (panel a) compared to a canonical He white dwarf (panel b). The x-axis shows the enclosed mass coordinate, and the y-axis indicates mass fractions of key isotopes. The hot subdwarf donor has already converted $\gtrsim 90 \%$ of its He into carbon and oxygen, in contrast to the He-dominated composition of the He white dwarf model.
• Figure 5: Mass distribution of the white dwarf at the time of the explosion. Most of the white dwarfs undergo double detonation around the peak $\gtrsim 1\rm M_\odot$.