On the formation of Extremely Low-Mass White Dwarf Binaries in wide orbits

Extremely low-mass (ELM) white dwarfs (WDs) have such a low mass (M ≲ 0.3 M⊙) that they cannot be produced via single-stellar evolution within the age of the universe. They are thought to be created in tight binaries via Roche-lobe overflow (RLOF) of their progenitors during red giant branch (RGB), which limits core growth. Almost all observed ELMWDs have WD companions in tight orbits, median orbital period P ≈ 5.4 hr lending support to the above formation channel. The KIC 8145411 system, a 0.2 M⊙ ELMWD in a wide P ≈ 450 day orbit with a 1.1 M⊙ main sequence (MS) star, challenges this understanding. We show that binary-binary interactions in star clusters involving ELMWD binaries with properties typical of the observed systems can naturally create ELMWD binaries in wide orbits similar in properties to the observed KIC 8145411 system. This eliminates any need for novel binary stellar evolution channels to explain KIC 8145411’s formation.