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 (Eta A) in\nthe massive colliding wind binary Eta Carinae dropped by a factor of 2-3\nbetween 1999 and 2010. We present results from large- (r=1545au) and small-\n(r=155au) domain, 3D smoothed particle hydrodynamic (SPH) simulations of Eta\nCar's colliding winds for 3 Eta A mass-loss rates (2.4, 4.8, and 8.5 x 10^-4\nM_sun/yr), investigating the effects on the dynamics of the binary wind-wind\ncollision (WWC). These simulations include orbital motion, optically thin\nradiative cooling, and radiative forces. We find that Eta A's mass-loss rate\ngreatly affects the time-dependent hydrodynamics at all spatial scales\ninvestigated. The simulations also show that the post-shock wind of the\ncompanion star (Eta B) switches from the adiabatic to the radiative-cooling\nregime during periastron passage. The SPH simulations together with 1D\nradiative transfer models of Eta A's spectra reveal that a factor of 2 or more\ndrop in Eta A's mass-loss rate should lead to substantial changes in numerous\nmultiwavelength observables. Recent observations are not fully consistent with\nthe model predictions, indicating that any drop in Eta A's mass-loss rate was\nlikely by a factor < 2 and occurred after 2004. We speculate that most of the\nrecent observed changes in Eta Car are due to a small increase in the WWC\nopening angle that produces significant effects because our line-of-sight to\nthe system lies close to the dense walls of the WWC zone. A modest decrease in\nEta A's mass-loss rate may be responsible, but changes in the wind/stellar\nparameters of Eta B cannot yet be fully ruled out. We suggest observations\nduring Eta Car's next periastron in 2014 to further test for decreases in Eta\nA's mass-loss rate. If Eta A's mass-loss rate is declining and continues to do\nso, the 2014 X-ray minimum should be even shorter than that of 2009.\n