# Orbit alignment in triple stars

## TL;DR

This study analyzes the angles between orbital angular momenta in hierarchical triple star systems, revealing correlations, dependencies on separation and mass, and suggesting different formation mechanisms for low-mass and high-mass triples.

## Contribution

It provides new statistical insights into orbit alignment patterns in triple stars, highlighting the influence of gas interactions and chaotic dynamics on their orbital architecture.

## Key findings

- Strong orbit alignment in triples with outer separation <50 AU
- No alignment observed in triples with outer separation >1000 AU
- Alignment decreases with increasing primary star mass

## Abstract

Statistics of the angle \Phi between orbital angular momenta in hierarchical triple systems with known inner visual or astrometric orbits are studied. Correlation between apparent revolution directions proves partial orbit alignment known from earlier works. The alignment is strong in triples with outer projected separation less than ~50 AU, where the average \Phi is about 20 degrees. In contrast, outer orbits wider than 1000 AU are not aligned with the inner orbits. It is established that the orbit alignment decreases with increasing mass of the primary component. Average eccentricity of inner orbits in well-aligned triples is smaller than in randomly aligned ones. These findings highlight the role of dissipative interactions with gas in defining the orbital architecture of low-mass triple systems. On the other hand, chaotic dynamics apparently played a role in shaping more massive hierarchies. Analysis of projected configurations and triples with known inner and outer orbits indicates that the distribution of \Phi is likely bimodal, where 80% of triples have \Phi<70 degrees and the remaining ones are randomly aligned.