The Mid-Infrared Evolution of the FU Orionis Disk

TL;DR

This study compares mid-infrared observations of FU Orionis over 12 years, revealing a slight decrease in brightness and stable dust properties, suggesting a modest decline in accretion rate or disk cooling.

Contribution

First long-term mid-IR spectroscopic comparison of FU Orionis, showing stable dust features and a slight luminosity decrease over 12 years.

Findings

Brightness decreased by ~14% at wavelengths <20 microns.

Silicate emission feature remained unchanged over 12 years.

Water absorption features remained consistent.

Abstract

We present new SOFIA-FORCAST observations obtained in Feburary 2016 of the archetypal outbursting low mass young stellar object FU Orionis, and compare the continuum, solid state, and gas properties with mid-IR data obtained at the same wavelengths in 2004 with Spitzer-IRS. In this study, we conduct the first mid-IR 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 ~ 14%. We find that this decrease in luminosity occurs only at wavelengths < 20 microns. In particular, the continuum short ward of the silicate emission complex at 10 microns exhibits a ~ 12% (~ 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 microns continues to be consistent with unprocessed grains, unchanged over 12 years. We conclude that either the accretion rate in FU Orionis has decreased by ~ 12-14% over this time baseline, or that the inner disk has cooled, but the accretion disk remains in a superheated state outside of the innermost region.