Accretion in massive colliding wind binaries and the effect of wind momentum ratio

TL;DR

This study uses numerical simulations to explore how the wind momentum ratio affects accretion onto a secondary star in massive colliding wind binaries, revealing thresholds and power-law behaviors in accretion rates.

Contribution

It provides new insights into the dependence of accretion rates on wind momentum ratio and the transition from sporadic to continuous accretion in such systems.

Findings

Accretion occurs when wind momentum ratio η is below ~0.05.

Accretion rate follows a power-law dependence on η, with different exponents for static and orbital cases.

Accretion transitions from sporadic to continuous as η decreases.

Abstract

We carry out a numerical experiment of ejecting winds in a massive colliding wind binary system, and quantifying the accretion onto the secondary star under different primary mass loss rates. We set a binary system comprising a Luminous Blue Variable (LBV) as the primary and a Wolf-Rayet (WR) star as the secondary, and vary the mass loss rate of the LBV to obtain different values of wind momentum ratio $\eta$. Our simulations include two sets of cases: one where the stars are stationary, and one that includes the orbital motion. As $\eta$ decreases the colliding wind structure moves closer to the secondary. We find that for $\eta \lesssim 0.05$ the accretion threshold is reached and clumps which originate by instabilities are accreted onto the secondary. For each value of $\eta$ we calculate the mass accretion rate and identify different regions in the $\dot{M}_{\rm acc}$ - $\eta$ diagram. For $0.001 \lesssim \eta \lesssim 0.05$ the accretion is sub- Bondi-Hoyle-Lyttleton (BHL) and the average accretion rate satisfies the power-law $\dot{M}_{\rm acc} \propto \eta^{-1.73}$ for static stars. The accretion is not continuous but rather changes from sporadic to a larger duty cycle as $\eta$ decreases. For $\eta\lesssim0.001$ the accretion becomes continuous in time and the accretion rate is BHL, up to a factor of 0.4--0.8. The simulations that include the orbital motion give qualitatively similar results, with the steeper power law $\dot{M}_{\rm acc} \propto \eta^{-1.86}$ for the sub-BHL region and lower $\eta$ as an accretion threshold.