2023ApJ...949..102K

The detection of a 0.2 M_⊙ extremely low-mass white dwarf (EW) in a wide orbit (P_orb ≈ 450 days) with a 1.1 M_⊙ main-sequence companion, KIC 8145411, challenges our current understanding of how EWs form. The traditional channel for EW formation, via mass transfer from the EW's progenitor, is expected to form EW binaries in tight orbits. Indeed, the majority of known EWs are found in tight binaries with a median P_orb ≈ 5.4 hr. Using numerical scattering experiments, we find that binary-binary strong encounters in star clusters can sufficiently widen the orbit of a typical EW binary, to explain the observed wide orbit of the KIC 8145411 system. The P_orb distribution for EW binaries produced through binary-binary encounters is bimodal: one mode corresponds to the initial orbital period of the EW binary, while the other is near P_orb ∼ few 10² days, similar to the orbital period of the KIC 8145411 system. We find that the production of wide EW binaries that are also ejected from the cluster peaks at a star cluster mass of ∼10⁵M_⊙ with a rate of ∼10^–3 Gyr^–1. Assuming that 50% of all stars form in star clusters and an initial cluster mass function ∝m^–2, we estimate a galactic formation rate of ∼4.16 × 10³ Gyr^–1 for wide EW binaries.