http://arxiv.org/abs/2209.06231
The detection of a $0.2\,M_\odot$ extremely low-mass white dwarf (hereafter, EW) in a wide orbit ($P_{\rm orb}\approx450$ days) with a $1.1\,M_\odot$ main-sequence (MS) companion KIC 8145411 challenges our current understanding of how EWs form. The traditional channel for EW formation via mass transfer from the WD progenitor is expected to form EW binaries in tight orbits. Indeed, majority of known EWs are found in tight binaries with a median $P_{\rm orb}\approx 5.4$ hrs. 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 $P_{\rm orb}$ 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 $P\sim$ 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 clusters mass of $\sim10^5\,M_\odot$ with a rate of $\sim10^{-3}\,\rm{Gyr^{-1}}$. Assuming that $50\%$ of all stars form in star clusters and an initial cluster mass function $\propto m^{-2}$, we estimate a galactic formation rate of $\sim3.64\times10^3\,\rm{Gyr^{-1}}$ for wide EW binaries.
A. Khurana, C. Chawla and S. Chatterjee
Thu, 15 Sep 22
34/67
Comments: 17 pages, 14 figures, and 2 tables; submitted to the Astrophysical Journal; comments welcome
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