# Tidal Disruptions of Stars by Binary Black Holes: Modifying the Spin Magnitudes and Directions of LIGO Sources in Dense Stellar Environments

## TL;DR

This paper investigates how tidal disruptions of stars by binary black holes in dense stellar environments can alter the black holes' spins and produce observable electromagnetic signatures, affecting gravitational wave source properties.

## Contribution

It presents hydrodynamic simulations showing how stellar tidal disruptions can modify black hole spins and generate electromagnetic signals, a novel insight into BBH evolution.

## Key findings

- Tidal disruptions can significantly change BH spin magnitudes and directions.
- Stellar debris accretion can temporarily align or anti-align BH spins.
- High accretion rates can launch relativistic jets detectable as electromagnetic signals.

## Abstract

Binary black holes (BBHs) appear to be widespread and are able to merge through the emission of gravitational waves, as recently illustrated by LIGO. The spin of the BBHs is one of the parameters that LIGO can infer from the gravitational wave signal and can be used to constrain their production site. If BBHs are assembled in stellar clusters they are likely to interact with stars, which could occasionally lead to a tidal disruption event (TDE). When a BBH tidally disrupts a star it can accrete a significant fraction of the debris, effectively altering the spins of the BHs. Therefore, although dynamically formed BBHs are expected to have random spin orientations, tidal stellar interactions can significantly alter their birth spins both in direction and magnitude. Here we investigate how TDEs by BBHs can affect the properties of the BH members as well as exploring the characteristics of the resulting electromagnetic signatures. We conduct hydrodynamic simulations with a Lagrangian Smoothed Particle Hydrodynamics code of a wide range of representative tidal interactions. We find that both spin magnitude and orientation can be altered and temporarily aligned or anti-aligned through accretion of stellar debris, with a significant dependence on the mass ratio of the disrupted star and the BBH members. These tidal interactions feed material to the BBH at very high accretion rates, with the potential to launch a relativistic jet. The corresponding beamed emission is a beacon to an otherwise quiescent BBH.