On the 3D structure of the nebula around \eta-Carinae

TL;DR

This paper models the 3D structure of the Eta Carinae nebula, explaining its bipolar shape through density variations in the surrounding medium, and compares simulations with observations to understand its evolution since 1840.

Contribution

It introduces a model of nebula shaping via density gradients and provides a detailed simulation approach matching observed radii and velocities of the Homunculus.

Findings

The bipolar shape results from density gradients in the surrounding medium.

Simulated radii and velocities match observations with fine-tuned parameters.

Line-of-sight integrated images help interpret observed nebula features.

Abstract

The asymmetric shape of the nebula around $\eta$-Carinae (Homunculus) can be explained by a spherical expansion in a non-homogeneous medium. Two models are analyzed: an exponential and an inverse power law dependence for the density as a function of distance from the equatorial plane. The presence of a medium with variable density along the polar direction progressively converts the original spherical shell into a bipolar nebula. In the case of the nebula around $\eta$-Carinae, we know the time elapsed since the great outburst in 1840. An exact match between observed radii and velocities can be obtained by fine tuning the parameters involved, such as initial radius, initial velocity and the typical scale that characterizes the gradient in density. The observed radius and velocity of the Homunculus as a function of the polar angle in spherical coordinates can be compared with the corresponding simulated data by introducing the efficiency in a single or multiple directions. Once the 3D spatial structure of the Homunculus is obtained, we can compose the image by integrating along the line of sight. In order to simulate the observed image, we have considered a bipolar nebula with constant thickness and an optically thin emitting layer. Some simulated cuts of the relative intensity are reported and may represent a useful reference for the astronomical cuts.