ALBERT

Description: Recent work suggests that the mass-loss rate of the primary star A in the massive colliding wind binary 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 Car's colliding winds for three A mass-loss rates ( $\dot{M}_{\eta _{\mathrm{A}}}$ = 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 $\dot{M}_{\eta _{\mathrm{A}}}$ greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star B switches from the adiabatic to the radiative-cooling regime during periastron passage (  0.985–1.02). This switchover starts later and ends earlier the lower the value of $\dot{M}_{\eta _{\mathrm{A}}}$ and is caused by the encroachment of the wind of A into the acceleration zone of B 's wind, plus radiative inhibition of B 's wind by A . The SPH simulations together with 1D radiative transfer models of A 's spectra reveal that a factor of 2 or more drop in $\dot{M}_{\eta _{\mathrm{A}}}$ should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop in $\dot{M}_{\eta _{\mathrm{A}}}$ was likely by a factor of 2 and occurred after 2004. We speculate that most of the recent observed changes in 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 $\dot{M}_{\eta _{\mathrm{A}}}$ may be responsible, but changes in the wind/stellar parameters of B , while less likely, cannot yet be fully ruled out. We suggest observations during Car's next periastron in 2014 to further test for decreases in $\dot{M}_{\eta _{\mathrm{A}}}$ . If $\dot{M}_{\eta _{\mathrm{A}}}$ is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.