2013MNRAS.436.3820M

Recent work suggests that the mass-loss rate of the primary star ηA in the massive colliding wind binary η Carinae dropped by a factor of 2-3 between 1999 and 2010. We present results from large- (±1545au) and small- (±155au) domain, 3D smoothed particle hydrodynamics (SPH) simulations of η Car's colliding winds for three ηA mass-loss rates ({dot}M_η_A= 2.4, 4.8 and 8.5x10^–4M_☉/yr), 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 ηB 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 ηA into the acceleration zone of ηB's wind, plus radiative inhibition of ηB's wind by ηA. The SPH simulations together with 1D radiative transfer models of ηA'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 ηB, 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.