3D radiative transfer simulations of Eta Carinae's inner colliding winds - I. Ionization structure of helium at apastron

The highly eccentric binary system Eta Carinae (η Car) shows numerous time-variable emission and absorption features. These observational signatures are the result of interactions between the complex three-dimensional (3D) wind-wind collision regions and photoionization by the luminous stars. Specifically, helium presents several interesting spectral features that provide important clues on the geometry and physical proprieties of the system and the individual stars. We use the SIMPLEX algorithm to post-process 3D smoothed particle hydrodynamics simulation output of the interacting winds in η Car in order to obtain the fractions of ionized helium assuming three different primary star (η) mass-loss rates. The resultant ionization maps constrain the regions where helium is singly-and doubly-ionized. We find that reducing ηA's mass-loss rate (M˙ η) increases the volume of He. Lowering M˙η produces large variations in the volume of He in the pre-shock η wind on the periastron side of the system. Our results show that binary orientations in which apastron is on our side of the system are more consistent with available observations. We suggest that small variations in M˙η might explain the observed increase in He I absorption in recent decades, although numerous questions regarding this scenario remain open. We also propose that the absence of broad He I lines in the spectra of η Car between its 1890's eruption and ~1944 might be explained by ηB's He-ionizing photons not being able to penetrate the wind-wind interaction region, due to a higher M˙ η at that time (by a factor ≳2, compared to the present value).