The Shocking Near-Infrared Spectrum of the Homunculus Nebula Surrounding η Carinae

Near-infrared long-slit spectra of the Homunculus Nebula surrounding η Carinae reveal emission from molecular hydrogen for the first time in this object, as well as strong intrinsic [Fe II] emission. H₂ emission is seen only in the polar lobes of the Homunculus, which show v = 1-0 emission in the K band, but no v = 2-1 emission. The strengths of certain [Fe II] lines are enhanced in the same regions of the polar lobes as the H₂ emission, where [Fe II] 1.6435 μm is by far the brightest near-infrared line emitted in the Homunculus Nebula. The intrinsic spectrum of the northwest polar lobe resembles the near-infrared spectra of some supernova remnants, and shock excitation is required to explain the strengths of the [Fe II] and H₂ lines. Shock-excited emission in the polar lobes suggests that the central star has a fast bipolar wind and a slower equatorial wind. The H₂ lines observed in these spectra represent the first detection of any molecular gas in the Homunculus Nebula, which has important implications for the density structure in the polar lobes and in the stellar wind during and after the Great Eruption.