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. We used the multiple stellar evolution code (MSE) to model the stellar evolution, binary interactions, and the dynamics of triple star systems and then used a Milky Way-like galaxy from the TNG50 simulations to construct a representative sample of LISA double white dwarfs. In our simulations, about 7 × 10⁶ Galactic double white dwarfs in the LISA frequency bandwidth originate from triple systems, whereas ∼4 × 10⁶ are in 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 systems, ∼O(10), that reach extreme eccentricities (> 0.9), a feature unique to the dynamical formation channels. Our population produces ∼O(10⁴) 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 the 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.