Mutual Orbit Alignment in Resolved Triple Systems

Abstract A sample of 278 triple systems with outer separations under 300 au and resolved inner pairs is studied, focusing on the mutual alignment between the inner and outer orbits. The degree of alignment increases with (i) decreasing outer separation, (ii) decreasing ratio of outer and inner separations, (iii) decreasing mass of the inner primary component, and (iv) increasing inner mass ratio. There is no dependence on the outer mass ratio. The average mutual inclination is ∼40° for the full sample and ∼10° for 38 triples with primary components less massive than 1 M ⊙ and outer separations below 50 au. Inner eccentricities in aligned triples are smaller compared to misaligned ones. In another sample of 371 hierarchies with known outer orbits and inner eclipsing subsystems, only 22% show mutual alignment within 20°, while the rest are aligned randomly. These findings match qualitatively the current understanding of the formation of hierarchical systems, where the N -body dynamics dominates at large scales, while the accretion and migration shape systems closer than ∼100 au. Fragmentation of isolated cores apparently produces approximately aligned low-mass hierarchies.