Accretion Disk Modeling of FU Ori Stars

FU Ori stars are a class of young stellar object (YSO) currently seen in an outburst state. Approximately half of the known FU Ori stars were detected during their transition from quiescence to outburst via optical/infrared brightness increases on the order of four to five magnitudes. It is generally accepted that the outbursts are caused by the onset of rapid accretion at rates three to four orders of magnitude larger than those of quiescent-state young T Tauri stars. During the ensuing decades- to century-long outbursts, the disks outshine their central stars by factors of 100-1000, leading to models of the outburst radiation as being purely due to a rotating accretion disk. We present new optical and near-infrared observations of the newest confirmed FU Ori stars (Gaia 17bpi and HBC 722), and discuss the successes and limitations of the conventional FU Ori model. Our model is a Keplerian disk featuring a modified Shakura-Sunyaev temperature profile, with each annulus radiating as an area-weighted spectrum given by a NextGen atmosphere at the appropriate temperature. We consider the overall SED as well as medium-resolution spectra in considering best-fit models to the data. Both sources have lower luminosity than the conventionally studied FU Ori outbursts, and correspondingly are fit by lower accretion rate disks with lower maximum temperatures: 5600K and 5000K.