Formation of Tidal Captures and Gravitational Wave Inspirals in Binary-Single Interactions

TL;DR

This study systematically investigates how stellar tides and relativistic effects influence binary-single star interactions, revealing that tides significantly increase stellar coalescence rates and lead to diverse transient phenomena.

Contribution

It introduces a novel N-body simulation incorporating tides and GR effects, and develops an analytical framework for tidal inspiral cross sections in stellar interactions.

Findings

Tides cause the formation of tidal captures leading to stellar mergers.

Tidal effects increase the rate of stellar coalescence in dynamical interactions.

Tidal inspirals are more common in systems with smaller stellar radii and larger initial binary separations.

Abstract

We perform the first systematic study on how dynamical stellar tides and general relativistic (GR) effects affect the dynamics and outcomes of binary-single interactions. For this, we have constructed an N-body code that includes tides in the affine approximation, where stars are modeled as self-similar ellipsoidal polytropes, and GR corrections using the commonly-used post-Newtonian formalism. Using this numerical formalism, we are able resolve the leading effect from tides and GR across several orders of magnitude in both stellar radius and initial target binary separation. We find that the main effect from tides is the formation of two-body tidal captures that form during the chaotic and resonant evolution of the triple system. The two stars undergoing the capture spiral in and merge. The inclusion of tides can thus lead to an increase on the stellar coalescence rate. We also develop an analytical framework for calculating the cross section of tidal inspirals between any pair of objects with similar mass. From our analytical and numerical estimates we find that the rate of tidal inspirals relative to collisions increases as the initial semi-major axis of the target binary increases and the radius of the interacting tidal objects decreases. The largest effect is therefore found for triple systems hosting white dwarfs and neutron stars. In this case, we find the rate of highly eccentric white dwarf - neutron star mergers to likely be dominated by tidal inspirals. While tidal inspirals occur rarely, we note that they can give rise to a plethora of thermonuclear transients such as Ca-rich transients.