A Lighthouse Effect in Eta Carinae

TL;DR

This paper introduces a 3-D hydrodynamical model explaining asymmetric near-UV variability in Eta Carinae, attributing it to wind-wind collision effects and orbital orientation rather than direct emission from the companion star.

Contribution

The study provides a new 3-D model linking binary orientation and wind interactions to observed UV variability, challenging previous assumptions about the companion's UV contribution.

Findings

Binary orientation angles consistent with observations.

Near-UV variability explained by wind collision cavity effects.

The companion star's UV contribution is minimal.

Abstract

We present a new model for the behavior of scattered time-dependent, asymmetric near-UV emission from the nearby ejecta of {\eta} Car. Using a 3-D hydrodynamical simulation of {\eta} Car's binary colliding winds, we show that the 3-D binary orientation derived by Madura et al. (2012) is capable of explaining the asymmetric near-UV variability observed in the Hubble Space Telescope Advanced Camera for Surveys/High Resolution Camera (HST ACS/HRC) F220W images of Smith et al. (2004b). Models assuming a binary orientation with i ~ 130 to 145 degrees, {\omega} ~ 230 to 315 degrees, PAz ~ 302 to 327 degrees 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 circum-binary material as the companion moves about in its highly elliptical orbit.