Simulating the formation of $\eta$ Carinae's surrounding nebula through unstable triple evolution and stellar merger-induced eruption

TL;DR

This paper models the formation of the Homunculus nebula around $ta$ Carinae through unstable triple star evolution and stellar merger, successfully reproducing observed nebula features and supporting the merger scenario.

Contribution

It provides a comprehensive theoretical simulation of the merger-in-a-triple-star system leading to nebula formation, a novel approach to explaining $ta$ Carinae's eruption.

Findings

Simulations reproduce the shape and kinematics of the Homunculus nebula.

The bipolar wind from the merger shapes the surrounding nebula.

The model supports the stellar merger in an unstable triple system as the eruption cause.

Abstract

$\eta$ Carinae is an extraordinary massive star famous for its 19th century Great Eruption and the surrounding Homunculus nebula ejected in that event. The cause of this eruption has been the centre of a long-standing mystery. Recent observations, including light-echo spectra of the eruption, suggest that it most likely resulted from a stellar merger in an unstable triple system. Here we present a detailed set of theoretical calculations for this scenario; from the dynamics of unstable triple systems and the mass ejection from close binary encounters, to the mass outflow from the eruption caused by the stellar merger and the post-merger wind phase. In our model the bipolar post-merger wind is the primary agent for creating the Homunculus, as it sweeps up external eruption ejecta into a thin shell. Our simulations reproduce many of the key aspects of the shape and kinematics of both the Homunculus nebula and its complex surrounding structure, providing strong support for the merger-in-a-triple scenario.