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

Abstract

Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable 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 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 m feature. Our preferred model contains nitrides AlN and Si3N4 in low abundances. Dust masses range from 0.25 to 0.44 M, but M(sub tot) 45 M 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 m. We obtain L = 2.96 x 10(exp 6) Lunar mass, 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 12CO and 13CO J = 5-4 through 9-8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [12CII] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures