A Lighthouse Effect in Eta Carinae

Abstract

We present a new model for the behavior of scattered time-dependent, asymmetric near-UV emission from the nearby ejecta of η Car. Using a three-dimensional (3D) hydrodynamical simulation of η Car's binary colliding winds, we show that the 3D binary orientation derived by Madura et al. in 2012 is capable of explaining the asymmetric near-UV variability observed in the Hubble Space Telescope Advanced Camera for Surveys/High Resolution Camera F220W images of Smith et al.. Models assuming a binary orientation with i ≈ 130°-145°, ω ≈ 230°-315°, P.A. _z ≈ 302°-327° are consistent with the observed F220W near-UV images. We find that the hot binary companion does not significantly contribute to the near-UV excess observed in the F220W images. Rather, we suggest that a bore-hole effect and the reduction of Fe II optical depths inside the wind-wind collision cavity carved in the extended photosphere of the primary star lead to the time-dependent directional illumination of circumbinary material as the companion moves about in its highly elliptical orbit.

Based on HST ACS/HRC observations.