SPIRALING OUT OF CONTROL: THREE-DIMENSIONAL HYDRODYNAMICAL MODELING OF THE COLLIDING WINDS IN η CARINAE

Abstract

Three dimensional (3D) adaptive-mesh refinement (AMR) hydrodynamical\nsimulations of the wind-wind collision between the enigmatic super-massive star\n\\etacar and its mysterious companion star are presented which include radiative\ndriving of the stellar winds, gravity, optically-thin radiative cooling, and\norbital motion. Simulations with static stars with a periastron passage\nseparation reveal that the preshock companion star's wind speed is sufficiently\nreduced that radiative cooling in the postshock gas becomes important,\npermitting the runaway growth of non-linear thin shell (NTSI) instabilities\nwhich massively distort the WCR. However, large-scale simulations which include\nthe orbital motion of the stars, show that orbital motion reduces the impact of\nradiative inhibition, and thus increases the acquired preshock velocities. As\nsuch, the postshock gas temperature and cooling time see a commensurate\nincrease, and sufficient gas pressure is preserved to stabilize the WCR against\ncatastrophic instability growth. We then compute synthetic X-ray spectra and\nlightcurves and find that, compared to previous models, the X-ray spectra agree\nmuch better with {\\it XMM-Newton} observations just prior to periastron. The\nnarrow width of the 2009 X-ray minimum can also be reproduced. However, the\nmodels fail to reproduce the extended X-ray mimimum from previous cycles. We\nconclude that the key to explaining the extended X-ray minimum is the rate of\ncooling of the companion star's postshock wind. If cooling is rapid then\npowerful NTSIs will heavily disrupt the WCR. Radiative inhibition of the\ncompanion star's preshock wind, albeit with a stronger radiation-wind coupling\nthan explored in this work, could be an effective trigger.\n