Accretion rates in hierarchical triple systems with discs

TL;DR

This study uses 3D simulations to explore how accretion processes in hierarchical triple star systems differ from binary systems, revealing unique mass accretion trends that influence their evolution.

Contribution

It introduces the first analysis of accretion trends in hierarchical triples and provides a semi-analytical model for their deviations from binary accretion behavior.

Findings

Small binaries in HTs tend to accrete more mass than single stars of the same mass.

Accretion patterns depend on the mass hierarchy between the binary and the third body.

Differential accretion in HTs can be enhanced or hampered based on mass relations.

Abstract

Young multiple systems accrete most of their final mass in the first few Myr of their lifetime, during the protostellar and protoplanetary phases. Previous studies showed that in binary systems the majority of the accreted mass falls onto the lighter star, thus evolving to mass equalisation. However, young stellar systems often comprise more than two stars, which are expected to be in hierarchical configurations. Despite its astrophysical relevance, differential accretion in hierarchical systems remains to be understood. In this work, we investigate whether the accretion trends expected in binaries are valid for higher order multiples. We performed a set of 3D Smoothed Particle Hydrodynamics simulations of binaries and of hierarchical triples (HTs) embedded in an accretion disc, with the code Phantom. We identify for the first time accretion trends in HTs and their deviations compared to binaries. These deviations, due to the interaction of the small binary with the infalling material from the circum-triple disc, can be described with a semi-analytical prescription. Generally, the smaller binary of a HT accretes more mass than a single star of the same mass as the smaller binary. We found that in a HT, if the small binary is heavier than the third body, the standard differential accretion scenario (whereby the secondary accretes more of the mass) is hampered. Reciprocally, if the small binary is lighter than the third body, the standard differential accretion scenario is enhanced. The peculiar differential accretion mechanism we find in HTs is expected to affect their mass ratio distribution.