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 10 3 -10 4 AU) fraction increases with decreasing orbital periods of the inner binaries. Compared to the field wide binary fraction, the wide tertiary fraction is 2.28 ± 0.10 times higher for eclipsing binaries (a median orbital period of 0.44 day) and 0.65 ± 0.03 times lower for astrometric binaries (a median orbital period of 537 days). The separation distribution of wide tertiaries is similar to wide binaries, with a tentative excess at ∼ 10 4 AU for tertiaries of eclipsing binaries. Using the v-r angle distributions, we show that the wide tertiaries are consistent with isotropic orientations with respect to the inner binaries. The inferred eccentricity distribution of wide tertiaries is close to thermal (f (e) ∝ e), similar to wide binaries at similar separations. The dynamical unfolding scenario is disfavored 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 > 10 3 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.

15 000 ellipsoidal binary candidates in TESS: Orbital periods, binary fraction, and tertiary companions

“…This enhancement factor is significantly lower than the 2.28 ± 0.10 measured by Hwang (2022). However, the comparison is not necessarily valid: note that Hwang (2022) applied corrections for separation-dependent incompleteness which we have not applied.…”

“…Tokovinin et al (2006) estimated that the frequency of tertiary companions to solar-type bi-nary systems with P orb < 3 days may be as high as 96 per cent. Hwang (2022) found that the wide (10 3 -10 4 au) tertiary frequency for targets in the Gaia eclipsing binary sample is 2.28 ± 0.10 times higher than the frequency of wide binary stars. For Gaia eclipsing binaries with periods < 3 days the tertiary frequency was approximately 12-15 per cent.…”

“…Data Release 3 of the Gaia survey presented orbital solutions for approximately 87 000 eclipsing binaries with orbital periods 200 days, 220 000 spectroscopic binaries with orbital periods 2000 days, and 170 000 astrometric binaries with orbital periods 10 000 days, with some overlap between the samples (Gaia Collaboration et al 2022;Halbwachs et al 2022;Bashi et al 2022;Shahaf et al 2022). Hwang (2022) searched for resolved tertiary companions (with separations of 10 3 -10 4 AU) to Gaia binary systems and found that the frequency of companions was significantly enhanced for eclipsing inner binaries, and reduced for astrometric inner binaries, relative to the general stellar population.…”