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

Abstract

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 ∼105 M ⊙ 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.

White Dwarfs Revealed in Gaia’s Candidate Compact Object Binaries

“…These four ELMWD candidates from our analysis show eccentricity values around 0.1 and a period of 500-1000 days. These eccentricities and orbital periods may indicate creation via dynamical processes Khurana et al (2023).…”

“…If created from a primordial binary in the field, the age and metallicity of the DR's progenitor and the LC must be the same. In a small fraction of cases, the DR-LC binary may have been created inside star clusters via dynamical processes and ejected from them into the field (e.g., Chatterjee et al 2017;Khurana et al 2023). For dynamically created DR-LC binaries, although the DR's progenitor and LC may not have been together by birth, all stars in a star cluster are roughly coeval with usually small spreads in metallicity and age (e.g., Milone et al 2020).…”

No Longer Impossible: The Self-lensing Binary KIC 8145411 is a Triple

“…A schematic diagram of this scenario is shown on the left panel of Figure 1. In view of this challenge, alternative formation histories, such as a merger of an inner binary (Masuda et al 2019) or more recently, dynamical assembly (Khurana et al 2023), have been considered.…”