Resolved Gaia Triples

Abstract

Abstract A sample of 392 low-mass hierarchical triple stellar systems within 100 pc resolved by Gaia as distinct sources is defined. Owing to the uniform selection, the sample is ideally suited to study unbiased statistics of wide triples. The median projected separations in their inner and outer pairs are 151 and 2569 au, respectively, and the median separation ratio is close to 15. Some triples appear in nonhierarchical configurations, and many are just above the dynamical stability limit. Internal motions in these systems are known with sufficient accuracy to determine the orbital motion sense of the outer and inner pairs and to reconstruct the eccentricity distributions. The mean inner and outer eccentricities are 0.66 ± 0.02 and 0.54 ± 0.02, respectively; the less eccentric outer orbits are explained by dynamical stability. The motion sense of the inner and outer pairs is almost uncorrelated, implying a mean mutual inclination of 83.°1 ± 4.°5. The median mass of the most massive component is 0.71 M ⊙, and the median system mass is 1.53 M ⊙. In a 0.69 fraction of the sample the primary belongs to the inner binary, while in the remaining systems it is the tertiary. A 0.21 fraction of the inner subsystems are twins with mass ratios >0.95. The median outer mass ratio is 0.41; it decreases mildly with increasing outer separation. Presumably, these wide hierarchies were formed by collapse and fragmentation of isolated cores in low-density environments and represent a small fraction of initial systems that avoided dynamical decay. Wide pre-main-sequence multiples in Taurus could be their progenitors.