Is the process of star and planet formation a slow and steady one, or are there bumps in the road to planet construction? In 1936, the young star FU Orionis (FU Ori) became 100 times brighter in only a few short months. FU Ori was undergoing a "burst" of accretion -- and nearly 20 Jupiter masses of gas have accreted in during the past 80 years. This sustained flow is a large fraction of the entire measurable disk mass (both gas and dust) surrounding FU Ori. What changes did this increased brightness wreak upon FU Ori's disk, and what implications would it have for any planets that might have formed or form later? Unlike most observed young stars, FU Ori and its (~10) brethren with similar behavior show no evidence of crystalline dust grains like forsterite (peridot), and the temperatures at ~1 AU distance would have risen to a scalding 1000 degrees Kelvin. Thus we would expect significant chemical changes that could lead to long term grain segregation, grain processing and circulation. By comparing with a 12 year previous Spitzer/IRS study, we used SOFIA/FORCAST to investigate both the instantaneous changes in the disk of FU Ori, and observe the evolution of the disk through the burst. We present the first multi-epoch infrared spectroscopic study of an FUor, as it appeared in 2004 and 2016. First, the continuum (the energy emitted by viscous heating in the disk) has decreased by ~13%. The silicate dust remains unchanged, and evidence of a hot disk photosphere remains. We conclude that the accretion rate of material in the innermost portion of the disk has been slowly reducing, perhaps combined with overall cooling, and is not being resupplied on this timescale. We further conclude that the outer disk reacts more slowly than the inner disk to changes in the central region, and plan to continue further monitoring with JWST.
Altering the Seeds of Planet Formation