Mass and Kinetic Energy of the Homunculus Nebula around η Carinae

We examine thermal infrared images of η Car at wavelengths from 4.8 to 25 μm obtained with the MIRAC3 camera system at the Magellan Observatory, and we conclude that η Car's circumstellar Homunculus Nebula contains much more mass (12 M⊙) and kinetic energy (1049.6–1050 ergs) than had previously been recognized. Excess far-infrared emission from a large mass of cold dust had been discovered recently by Morris et al., which they attributed to 110 K dust in a compact equatorial torus. Our images suggest, however, that roughly 10 M⊙ or more resides instead in the rapidly expanding polar lobes of the Homunculus, known to be ejected during η Car's Great Eruption in the 19th century. Combining this large mass with reliable expansion speeds for the polar lobes allows us to estimate the large kinetic energy and efficient momentum transfer associated with the eruption. Most mass resides in a cool outer layer of the lobes, with dust at ~140 K. This material is only revealed at 18–25 μm with high spatial resolution images that can separate adjacent dust components. Warmer dust at ~200 K that has been recognized for some time (responsible for previous mass estimates of 2–3 M⊙) blankets the inner surfaces of the mostly hollow polar lobes as they are irradiated more directly by the central engine.