Revisiting 2D Numerical Models for the 19th century outbursts of $\eta$ Carinae

TL;DR

This study uses advanced 2D hydrodynamical simulations to better understand the formation and evolution of the Homunculus and little Homunculus nebulae around $ta$ Carinae, revealing new insights into their shape, kinematics, and internal structures.

Contribution

It introduces a more realistic wind parametrization in simulations, showing the little Homunculus forms from post-eruption wind impact rather than direct collision, refining previous models.

Findings

Simulations reproduce the shape and expansion speed of the large Homunculus.

The little Homunculus develops Rayleigh-Taylor instabilities and filamentary structures.

The interior cavity contains material expelled after the great eruption, explaining the double-shell structure.

Abstract

We present here new results of two-dimensional hydrodynamical simulations of the eruptive events of the 1840s (the great) and the 1890s (the minor) eruptions suffered by the massive star $\eta$ Car. The two bipolar nebulae commonly known as the Homunculus and the little Homunculus were formed from the interaction of these eruptive events with the underlying stellar wind. As in previous work (Gonzalez et al. 2004a, 2004b), we assume here an interacting, nonspherical multiple-phase wind scenario to explain the shape and the kinematics of both Homunculi, but adopt a more realistic parametrization of the phases of the wind. During the 1890s eruptive event, the outflow speed {\it decreased} for a short period of time. This fact suggests that the little Homunculus is formed when the eruption ends, from the impact of the post-outburst $\eta$ Car wind (that follows the 1890s event) with the eruptive flow (rather than by the collision of the eruptive flow with the pre-outburst wind, as claimed in previous models; Gonzalez et al. 2004a, 2004b). Our simulations reproduce quite well the shape and the observed expansion speed of the large Homunculus. The little Homunculus (which is embedded within the large Homunculus) becomes Rayleigh-Taylor unstable and develop filamentary structures that resembles the spatial features observed in the polar caps. In addition, we find that the interior cavity between the two Homunculi is partially filled by material that is expelled during the decades following the great eruption. This result may be connected with the observed double-shell structure in the polar lobes of the $\eta$ Car nebula. Finally, as in previous work, we find the formation of tenuous, equatorial, high-speed features that seem to be related to the observed equatorial skirt of $\eta$ Car.