Dancing twins: stellar hierarchies that formed sequentially?

TL;DR

This paper investigates a specific class of triple star systems with nearly circular, aligned orbits and twin inner stars, exploring their possible formation mechanisms through orbital analysis of four hierarchical systems.

Contribution

It provides detailed orbital computations for four hierarchical triple star systems, supporting theories of sequential or cascade formation of such stellar hierarchies.

Findings

Inner star pairs are nearly identical in mass.

Outer and inner orbits may be in mean motion resonance.

Systems could form via disk fragmentation or cloud fragmentation.

Abstract

This paper attracts attention to the class of resolved triple stars with moderate ratios of inner and outer periods (possibly in a mean motion resonance) and nearly circular, mutually aligned orbits. Moreover, stars in the inner pair are twins with almost identical masses, while the mass sum of the inner pair is comparable to the mass of the outer component. Such systems could be formed either sequentially (inside-out) by disk fragmentation with subsequent accretion and migration or by a cascade hierarchical fragmentation of a rotating cloud. Orbits of the outer and inner subsystems are computed or updated in four such hierarchies: LHS 1070 (GJ 2005, periods 77.6 and 17.25 years), HIP 9497 (80 and 14.4 years), HIP 25240 (1200 and 47.0 years), and HIP 78842 (131 and 10.5 years).