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

Morris, Patrick W. and Gull, Theodore R. and Hillier, D. John and Barlow, M. J. and Royer, Pierre and Nielsen, Krister and Black, John and Swinyard, Bruce (2017) η Carinae's Dusty Homunculus Nebula from Near-infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity. Astrophysical Journal, 842 (2). Art. No. 79. ISSN 1538-4357.

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Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable η 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 μm 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 μm feature. Our preferred model contains nitrides AlN and Si_3N_4 in low abundances. Dust masses range from 0.25 to 0.44 M_⊙, but M_(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 μm. We obtain L_(IR) = 2.96 × 10^6 L_⊙, 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 ^(12)CO and ^(13)CO J = 5–4 through 9–8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [^(12)C ii] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures.

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Additional Information:© 2017 American Astronomical Society. Received 2017 March 28. Accepted 2017 May 4. Published 2017 June 15. We appreciate the comments from our anonymous referee, leading to several important clarifications, and discussions with A. Mehner, P. Goldsmith, V. Ossenkopf, and D. Teyssier. We also extend thanks to L. Loinard for helpful comments on the manuscript, K. Pitman for pointers on laboratory nitride reflectance spectra that aided in our interpretations, and M. Teodoro for kindly granting figure adaptation permission. P.M. also thanks E. Caux for advice on the CASSIS package. P.M., T.R.G., and K.N. acknowledge partial financial support from NASA grant SCEX22012D for the Herschel program OT1_tgull_3.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:circumstellar matter; dust, extinction; stars: individual (η Carinae) ; stars: massive; stars: mass-loss
Issue or Number:2
Record Number:CaltechAUTHORS:20170615-152452348
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Official Citation:Patrick W. Morris et al 2017 ApJ 842 79
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:78255
Deposited By: Ruth Sustaita
Deposited On:16 Jun 2017 15:45
Last Modified:03 Oct 2019 18:06

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