Mutual Orbit Alignment in Resolved Triple Systems

TL;DR

This study analyzes 278 triple star systems to understand how their inner and outer orbits align, revealing that alignment depends on separation, mass ratios, and primary mass, with implications for their formation mechanisms.

Contribution

It provides the first comprehensive analysis of mutual orbit alignment in a large sample of resolved triple systems, linking alignment to system properties and formation processes.

Findings

Alignment increases with decreasing separation and primary mass.

Average mutual inclination is ~40 degrees, lower in low-mass, close systems.

Only 22% of systems show strong mutual alignment within 20 degrees.

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 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 ~40deg for the full sample and ~10deg for 38 triples with primary components less massive than 1 solar 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 20deg, while the rest are aligned randomly. These findings match qualitatively 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 $\sim$100 au. Fragmentation of isolated cores apparently produces approximately aligned low-mass hierarchies.