Spiralling out of control: 3D hydrodynamical modelling of the colliding winds in $\eta\thinspace$Carinae

TL;DR

This study uses 3D hydrodynamical simulations to model the wind collision in Eta Carinae, revealing how orbital motion influences shock stability and improves X-ray spectral predictions, but also highlighting the importance of cooling rates.

Contribution

It introduces comprehensive 3D simulations including radiative driving, gravity, cooling, and orbital motion, providing new insights into wind collision dynamics and X-ray emission in Eta Carinae.

Findings

Orbital motion stabilizes the wind collision region against catastrophic instabilities.

Simulated X-ray spectra align better with observations prior to periastron.

Cooling rates of the postshock wind are crucial for explaining X-ray minima.

Abstract

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