Eta Carinae's Dusty Homunculus Nebula from Near-infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity

Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable eta Carinae are crucial to characterize the mass-loss history and help constrain the mechanisms leading to the great eruption. We present the 2.4-670 mu m spectral energy distribution, constructed from legacy Infrared Space Observatory observations and new spectroscopy obtained with the Herschel Space Observatory. Using radiative transfer modeling, we find that the two best-fit dust models yield compositions that are consistent with CNO-processed material, with iron, pyroxene and other metal-rich silicates, corundum, and magnesium-iron sulfide in common. Spherical corundum grains are supported by the good match to a narrow 20.2 mu m feature. Our preferred model contains nitrides AlN and Si3N4 in low abundances. Dust masses range from 0.25 to 0.44 M-circle dot, but M-tot. 45M(circle dot) in both cases, due to an expected high Fe gas-to-dust ratio. The bulk of dust is within a 5 '' x 7 '' central region. An additional compact feature is detected at 390 mu m. We obtain L-IR = 2.96 x 10(6) L-circle dot, a 25% decline from an average of mid-IR photometric levels observed in 1971-1977. This indicates a reduction in circumstellar extinction in conjunction with an increase in visual brightness, allowing 25%-40% of optical and UV radiation to escape from the central source. We also present an analysis of (CO)-C-12 and (CO)-C-13 J = 5-4 through 9-8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [C-12 II] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures