Dynamically Forming Extremely Low-mass White Dwarf Binaries in Wide Orbits

The detection of a 0.2 M (circle dot) extremely low-mass white dwarf (EW) in a wide orbit (P (orb) approximate to 450 days) with a 1.1 M (circle dot) 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) approximate to 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) similar to few 10(2) 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 similar to 10(5) M (circle dot) with a rate of similar to 10(-3) Gyr(-1). Assuming that 50% of all stars form in star clusters and an initial cluster mass function proportional to m (-2), we estimate a galactic formation rate of similar to 4.16 x 10(3) Gyr(-1) for wide EW binaries.