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Triple evolution: an important channel in the formation of type Ia supernova
Triple evolution: an important channel in the formation of type Ia supernova
Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear explosions in white dwarfs (WDs). Commonly considered formation pathways include two merging WDs (the double-degenerate channel) and a single WD accreting material from a H or He donor (the single-degenerate channel). Since the predicted SN Ia rates from WDs in binaries are thought to be insufficient to explain the observed SN Ia rate, it is important to study similar interactions in higher-order multiple-star systems such as triple systems. We use the evolutionary population synthesis code Multiple Stellar Evolution (MSE) to study the stellar evolution, binary interactions, and gravitational dynamics of the triple-star systems. Also, unlike previous studies, prescriptions are included to simultaneously take into account the single- and double-degenerate channels, and we consider triples across the entire parameter space (including those with tight inner binaries). We explore the impact of typically ignored or uncertain physics such as flybys and common envelope prescription parameters on our results.
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Evolution of binaries containing a hot subdwarf and a white dwarf to double white dwarfs, and double detonation supernovae with hypervelocity runaway stars
Evolution of binaries containing a hot subdwarf and a white dwarf to double white dwarfs, and double detonation supernovae with hypervelocity runaway stars
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 ≈0.05 M⊙, 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 ≈0.18 M⊙. In our double detonation systems, the orbital velocity of the surviving donor star ranges from ≈450km/s to ≈1000km/s. 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.
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The role of triple evolution in the formation of LISA double white dwarfs
The role of triple evolution in the formation of LISA double white dwarfs
Galactic double white dwarfs will be prominent gravitational-wave sources for the Laser Interferometer Space Antenna (LISA). While previous studies have primarily focused on formation scenarios in which binaries form and evolve in isolation, we present the first detailed study of the role of triple stellar evolution in forming the population of LISA double white dwarfs. In this work, we present the first detailed study of the role of triple stellar evolution in forming the population of LISA double white dwarfs. We use the multiple stellar evolution code (MSE) to model the stellar evolution, binary interactions, and the dynamics of triple star systems then use a Milky Way-like galaxy from the TNG50 simulations to construct a representative sample of LISA double white dwarfs. In our simulations about 7 million Galactic double white dwarfs in the LISA frequency bandwidth originate from triple systems, whereas approximately 4 million form from isolated binary stars. The properties of double white dwarfs formed in triples closely resemble those formed from isolated binaries, but we also find a small number of system ∼O(10) that reach extreme eccentricities (>0.9), a feature unique to the dynamical formation channels. Our population produces about 10000 individually resolved double white dwarfs (from triple and binary channels) and an unresolved stochastic foreground below the level of the LISA instrumental noise. About 57% of double white dwarfs from triple systems retain a bound third star when entering the LISA frequency bandwidth. However, we expect the tertiary stars to be too distant to have a detectable imprint in the gravitational-wave signal of the inner binary.
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He-star donor pathway for the hypervelocity star D6–2
He-star donor pathway for the hypervelocity star D6–2
Type Ia supernovae are thermonuclear explosions of white dwarfs, yet the nature of their progenitor systems remains uncertain. Recent discoveries of hypervelocity white dwarfs 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, nearly degenerate C/O cores before explosion. The predicted ejection velocities of ∼ 1000 km/s match the observed velocity of D6-2. Analytical estimates indicate that the thin residual helium envelope can be stripped by the supernova ejecta, yielding a C/O-rich surface consistent with its observed spectrum. 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.
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