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

Abstract

ABSTRACTWe carry out a numerical experiment for ejecting winds in a massive colliding-wind binary system and quantify the accretion on to 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 the wind momentum ratio η. Our simulations include two sets of cases: one in which the stars are stationary; and one that includes the orbital motion. As η decreases, the colliding-wind structure moves closer to the secondary. We find that for η ≲ 0.05, the accretion threshold is reached and clumps that originate from instabilities are accreted on to the secondary. For each value of η, we calculate the mass accretion rate and identify different regions in the $\dot{M} _{\rm acc}$ – η diagram. For 0.001 ≲ η ≲ 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 η decreases. For η ≲ 0.001, 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 η as an accretion threshold.