Constraints on decreases in η Carinae's mass-loss from 3D hydrodynamic simulations of its binary colliding winds

Abstract

Recent work suggests that the mass-loss rate of the primary star η_A in the massive colliding wind binary η Carinae dropped by a factor of 2-3 between 1999 and 2010. We present results from large- (±1545 au) and small- (±155 au) domain, 3D smoothed particle hydrodynamics (SPH) simulations of η Car's colliding winds for three η_A mass-loss rates (dot{M}_{η _A}= 2.4, 4.8 and 8.5 × 10^-4 M_⊙ yr^-1), investigating the effects on the dynamics of the binary wind-wind collision (WWC). These simulations include orbital motion, optically thin radiative cooling and radiative forces. We find that dot{M}_{η _A} greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star η_B switches from the adiabatic to the radiative-cooling regime during periastron passage (φ ≈ 0.985-1.02). This switchover starts later and ends earlier the lower the value of dot{M}_{η _A} and is caused by the encroachment of the wind of η_A into the acceleration zone of η_B's wind, plus radiative inhibition of η_B's wind by η_A. The SPH simulations together with 1D radiative transfer models of η_A's spectra reveal that a factor of 2 or more drop in dot{M}_{η _A} should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop in dot{M}_{η _A} was likely by a factor of ≲2 and occurred after 2004. We speculate that most of the recent observed changes in η Car are due to a small increase in the WWC opening angle that produces significant effects because our line of sight to the system lies close to the dense walls of the WWC zone. A modest decrease in dot{M}_{η _A} may be responsible, but changes in the wind/stellar parameters of η_B, while less likely, cannot yet be fully ruled out. We suggest observations during η Car's next periastron in 2014 to further test for decreases in dot{M}_{η _A}. If dot{M}_{η _A} is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.