A Quadruple Excess in Wide Binary Systems: Evidence for Correlated Binary Formation

TL;DR

This study uses Gaia DR3 data to show that quadruple systems in wide binaries are more common than expected by chance, indicating correlated binary formation processes.

Contribution

It provides the first observational evidence for correlated binary formation in wide binary systems, challenging the assumption of independent binary formation.

Findings

Quadruple fraction exceeds the product of individual close binary fractions by a factor of about 2.34.

The excess is significant at separations up to 5,000 AU but declines at wider separations.

Dynamical processing may transform quadruples into triples over time, as indicated by velocity dependence.

Abstract

Understanding the multiplicity of stellar systems and the correlations between their hierarchical components provides crucial insights into star formation processes. If binary companions form independently in each component of a wide binary (WB), the fraction of quadruple systems, i.e., 2+2 configurations where both components are themselves close binaries (CBs), should equal the product of individual CB fractions. Using \textit{Gaia} DR3 radial velocity spectroscopy (RVS) data for WB systems, we measure the CB fraction $p$ and quadruple fraction $P_{2+2}$, suggesting an enhancement factor $\kappa = P_{2+2}/p^2 = 2.34_{-0.11}^{+0.12}$, significantly exceeding unity expected under a statistical model of independence. We confirm the significance of this excess by performing two sets of tests: (1) shuffling WB pairings while preserving the overall $\Delta G$ distribution shows no significant enhancement, ruling out selection effects; (2) simulations preserving the spectral type (temperature-dependent) CB fraction also yield the same null excess. When examined as a function of WB separation, the enhancement remains strong at separations $\leq 5\,000$ AU, but shows a decline towards unity at the widest separations ($\geq 10\,000$ AU). An independent proper motion anomaly (PMa) consistency check confirms the enhancement, suggesting a similar value. We further find that the enhancement declines with increasing peculiar velocity, suggesting that dynamical processing in older or dynamically hotter populations may transform 2+2 quadruples into triples over time. Our results provide strong evidence for correlated binary formation processes operating in WB systems.