The Mid-infrared Evolution of the FU Orionis Disk

We present new SOFIA-FORCAST observations obtained in 2016 February of the archetypal outbursting low-mass. young stellar object FU Orionis, and we. compare the continuum, solid-state, and gas properties with mid- infrared. data obtained at the same wavelengths in 2004 with Spitzer-IRS. In this study, we conduct the first midinfrared. spectroscopic comparison of an FUor over a long time period. Over a 12-year period, UBVR monitoring indicates that FU Orionis has continued its steady decrease in overall brightness by similar to 14%. We find that this decrease in luminosity occurs only at wavelengths. 20 mu m. In particular, the continuum shortward of the silicate emission complex at 10 mu m exhibits a similar to 12% (similar to 3 sigma) drop in flux density. but no apparent change in slope; both the Spitzer and SOFIA spectra are consistent with a 7200 K blackbody. Additionally, the detection of water absorption is consistent with the Spitzer spectrum. The silicate emission feature at 10 mu m continues to be consistent with unprocessed grains, unchanged over 12 years. We conclude that either the accretion rate in FU Orionis has decreased by similar to 12-14% over this time baseline. or. the inner disk has cooled, but the accretion disk remains in a superheated state outside the innermost region.