η 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 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 3 N 4 in low abundances. Dust masses range from 0.25 to 0.44tot in both cases, due to an expected high Fe gas-to-dust ratio. The bulk of dust is within a ´ 5 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.

The peculiar chemistry of the inner ejecta of Eta Carina

“…We note that our column densities are consistently lower than those reported by Loinard et al (2012) by a factor of 3-5. While this may obey to beam filling issues, our estimates are more in line with the column densities derived by Morris et al (2017) from large beam CO J = 5 → 4 to 9 → 8 observations of the Homunculus.…”

“…Davidson et al 1995, Smith et al 2003b, Smith 2008). E-mail: cbordiu@cab.inta-csic.es η Car underwent a devastating outburst in the 1840s, an event nicknamed the Great Eruption that expelled more than 40 M of processed material (Gomez et al 2010;Morris et al 2017), forming the bipolar Homunculus Nebula (Davidson 1989). This outburst was followed by a second, lesser eruption in the 1890s that gave birth to a smaller structure known as the Little Homunculus (Ishibashi et al 2003).…”

Distinguishing circumstellar from stellar photometric variability in Eta Carinae

“…Since the orbital modulation is colour-invariant, the WWC is not creating dust. As a matter of fact, the dust content of η Car is decreasing because of the drop in the infrared luminosity reported by Morris et al (2017) and by Gaczkowski et al (2013).…”

“…Such a small change in the period would occur only if the mass lost by the primary star was replaced by a merger with a third star in the system, as discussed by Portegies Zwart & van den Heuvel (2016) and by Smith et al (2018b). Since the mass lost in the great eruption was large (> 12 M following Smith et al 2003 or > 45 M following Morris et al 2017), the putative merger star should have been very massive. Another possibility is that those peaks have no connection with the present-day P-peak and the closeness between them in orbital phase is merely a coincidence.…”

Maximally Dusty Star-forming Galaxies: Supernova Dust Production and Recycling in Local Group and High-redshift Galaxies

“…Finally, massive progenitors of SNe, such as luminous blue variable stars, also appear to produce dust with a standard dust-to-gas ratio (Smith 2012). For example, about 0.4 M of dust within 2 × 10 17 cm has been inferred in the massive erupting star, η Car (Gomez et al 2010;Morris et al 2017). Such dust is usually assumed to be evaporated by the shock breakout radiation from the ultimate supernova explosion, but dust at sufficiently large radius may possibly survive, depending on the details of the shock breakout radiation.…”