Dissecting the Homunculus Nebula Around Eta Carinae With Spatially Resolved Near-Infrared Spectroscopy

Near-infrared emission lines provide unique diagnostics of the geometry, structure, kinematics, and excitation of η Carinae’s circumstellar ejecta, and give clues to the nature of its wind. The infrared spectrum is a strong function of position in η Car’s nebula, with a mix of intrinsic and reflected emission. Molecular hydrogen traces cool gas and dust in the polar lobes, while [Fe ii] blankets their inner surfaces. These lines reveal the back wall of the SE polar lobe for the first time, and give the clearest picture yet of the 3-D geometry. Additionally, collisionally-excited [Fe ii] reveals the kinematic structure of a recently discovered ‘Little Homunculus’ expanding inside the larger one. Equatorial gas in the ‘Fan’, on the other hand, shows a spectrum indicating recombination and fluorescent Lyα pumping. Some equatorial ejecta glow in the He i λ10830 line, showing evidence for material ejected in the 1890 outburst of η Car. Closer to the star, the compact ‘Weigelt blobs’ are marginally resolved, allowing their infrared spectrum to be separated from the star for the first time. In general, infrared spectra reveal a coherent, directional dependence of excitation in the Homunculus: polar ejecta are collisionally excited, whereas equatorial ejecta are dominated by fluorescence and normal photoexcitation. These are important clues to the geometry of the central star’s UV radiation field. Reflected near-infrared emission lines also reveal interesting latitudinal dependence in the stellar wind.