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

Abstract

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.