ABSTRACT. High-resolution long-slit spectra obtained with the Phoenix spectrograph on Gemini South provide our most accurate probe of the three-dimensional structure of the Homunculus Nebula around η Carinae. The new near-infrared spectra dramatically confirm the double-shell structure inferred previously from thermal dust emission, resolving the nebula into a very thin outer shell seen in H₂ and a warmer, thicker inner layer seen in [Fe II]. The remarkably thin and uniform H ₂ skin hints that the most important mass loss during the 19th century eruption had a very short duration of 5 yr. H₂ emission traces the majority of the more than 10 M of material in the nebula and has an average density of order n_H 10^6.5 cm^-3. This emission, in turn, yields our first definitive picture of the exact shape of the nebula, plus a distance of 2350 ± 50 pc and an inclination angle of ~41°. The distribution of the H₂ emission provides the first measure of the latitude dependence of the speed, mass loss, and kinetic energy associated with η Car's 19th century explosion. Almost 75% of the total mass and more than 90% of the kinetic energy in the ejecta were released at high latitudes between 45° and the polar axis. This rules out a model for the bipolar shape in which an otherwise spherical explosion was pinched at the waist by a circumstellar torus. Instead, most of the mass appears to have been directed poleward by the explosion itself. H₂ emission also provides our first reliable picture of the critical innermost waist of the Homunculus, yielding clues to the observed morphology of the core and the more extended equatorial debris.

Subject headings: circumstellar matter; ISM: individual (Homunculus Nebula); stars: individual (η Carinae); stars: mass loss; stars: winds, outflows

^* Based on observations obtained at the Gemini Observatory, which is operated by AURA, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (USA), the Particle Physics and Astronomy Research Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).

Print publication: Issue 2 (2006 June 20)
Received 2006 January 31, accepted for publication 2006 February 17