The mystery in Gaia DR3 triples: occurrence rates, orientations, and eccentricities of wide tertiaries around close binaries

Abstract

The formation of close binaries has been an open question for decades. A large fraction of close binaries are in triple systems, suggesting that their formation may be associated with the Kozai-Lidov mechanism. However, this picture remains under debate because the configurations of many observed triples are unlikely to trigger the Kozai-Lidov mechanism. In this paper, we use the close binary samples, including eclipsing, spectroscopic, and astrometric binaries, from Gaia Data Release 3 to investigate the mysterious connection between inner binaries and their wide tertiaries. We show that the wide tertiary (at 10³-10⁴ au) fraction increases with decreasing orbital periods of the inner binaries. The wide tertiary fraction of eclipsing binaries (a median orbital period of 0.41 d) is 2.33 ± 0.11 times higher than the field wide binary fraction. Furthermore, there is a tentative excess at ~10⁴ au for tertiaries of eclipsing binaries. Using the v - r angle distributions, we show that wide tertiaries have isotropic orientations with respect to the inner binaries, and the co-planar orbits can be ruled out. The inferred eccentricity distribution of wide tertiaries is consistent (<1σ) with being thermal (f(e) ∝ e), similar to wide binaries at similar separations. The dynamical unfolding scenario is disfavoured because it predicts highly eccentric wide tertiaries, which is inconsistent with our findings. For the Kozai-Lidov mechanism to be effective for wide tertiaries at >10³ au, the initial separations of the inner binaries need to be >3 au. Future theoretical investigations are needed to explore the parameter space at these large initial separations and large tertiary separations.