The Structure of the Homunculus. I. Shape and Latitude Dependence from H2and [Feii] Velocity Maps of η Carinae

High resolution long-slit spectra obtained with the Phoenix spectrograph onGemini South provide our most accurate probe of the three dimensional structureof the Homunculus around eta Car. The new near-infrared spectra dramaticallyconfirm the double-shell structure inferred previously from thermal dustemission, resolving the nebula into a very thin outer shell seen in H2 21218,and a warmer, thicker inner layer seen in [Fe II] 16435. The thin H2 skin hintsthat the most important mass loss during the 19th century eruption had a veryshort duration of less than 5 yr. H2 emission traces the majority of the massin the nebula, and has an average density of order 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 2350pm50 pc and an inclination angle of 41deg (the polaraxis is tilted 49deg from the plane of the sky). The distribution of the H2emission provides the first measure of the latitude dependence of the speed,mass loss, and kinetic energy associated with eta Car's 19th century explosion.Almost 75 percent of the total mass and more than 90 percent of the kineticenergy in the ejecta were released at high latitudes. This rules out a modelfor the bipolar shape wherein an otherwise spherical explosion was pinched atthe waist by a circumstellar torus. Also, the ejecta could not have beendeflected toward polar trajectories by a companion star, since the kineticenergy of the polar ejecta is greater than the binding energy of the putativebinary system. Instead, most of the mass appears to have been directed polewardby the explosion itself. [abridged]Comment: 25 pages, figs 2 and 3 in color. Accepted by Ap