Doppler tomography of the Little Homunculus: High-resolution spectra of [Fe II] λ16 435 around Eta Carinae

High-resolution spectra of [Fe II] λ16435 around η Carinae provide powerful diagnostics of the geometry and kinematics of the 'Little Homunculus' (LH) growing inside the larger Homunculus nebula. The LH expansion is not perfectly homologous: while low latitudes are consistent with linear expansion since 1910, the polar caps imply ejection dates around 1920-1930. However, the expansion speed of the LH is much slower than the post-eruption wind, so the star's powerful wind may accelerate the LH. With an initial ejection speed of 200 km s^-1 in 1890, the LH would have been accelerated to its present speed if the mass is roughly 0.1 M_⊙. This agrees with an independent estimate of the LH mass based on its density and volume. In any case, an ejection after 1930 is ruled out. Using the LH as a probe of the 1890 event then, it is evident that its most basic physical parameters (total mass and kinetic energy, which are 0.1 M_⊙ and 10^46.9 erg, respectively) are orders of magnitude less than during the giant eruption in the 1840s. Thus, the ultimate energy sources were different for these two events - yet their ejecta have the same bipolar geometry. This clue may point toward a collimation mechanism separate from the underlying causes of the outbursts.