Constraints on decreases in η Carinae's mass-loss from 3D hydrodynamic simulations of its binary colliding winds

Recent work suggests that the mass-loss rate of the primary star η
A
in the massive colliding wind binary η 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 η Car's colliding winds for three η
A
mass-loss rates (
$\dot{M}_{\eta _{{\rm A}}}$
= 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
$\dot{M}_{\eta _{{\rm A}}}$
greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star η
B
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
$\dot{M}_{\eta _{{\rm A}}}$
and is caused by the encroachment of the wind of η
A
into the acceleration zone of η
B
's wind, plus radiative inhibition of η
B
's wind by η
A
. The SPH simulations together with 1D radiative transfer models of η
A
's spectra reveal that a factor of 2 or more drop in
$\dot{M}_{\eta _{{\rm A}}}$
should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop in
$\dot{M}_{\eta _{{\rm A}}}$
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
$\dot{M}_{\eta _{{\rm A}}}$
may be responsible, but changes in the wind/stellar parameters of η
B
, 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
$\dot{M}_{\eta _{{\rm A}}}$
. If
$\dot{M}_{\eta _{{\rm A}}}$
is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.