FUOr-Aur 0544+3330: A New YSO Outburst in the Outskirts of Auriga, Viewed Face-On

We present a newly appreciated FU Ori outburst event that began in 2019 and reached a peak in early 2021. Suspected young stellar object WISE J054452.25+333009.6 expe- rienced substantial brightening, in excess of −5 mag at optical wavelengths and −2.5 mag at mid-infrared wavelengths. The time from near-quiescence to peak brightness was approximately 24 months. Optical and near-infrared spectra confirm that the out- bursting source (hereby designated FUOr-Aur 0544+3330) shows all the hallmarks of the FU Ori class, including the Li I indicator of stellar youth. The mix of ionized and neutral atomic lines, alongside prominent molecular absorption features, is consistent with the expected change in spectral type from earlier in the optical to later-type in the near-infrared. The closest analog among well-studied FU Ori objects is V1515 Cyg. Both sources have unusually narrow-lined absorption spectra that can be explained by a face-on disk orientation, such that disk-broadening is minimized and wind-induced blueshift (in e.g. Hα, NaD, Ca II) is maximized. Both the optical through infrared spec- tral energy distribution and high-resolution spectrum are well-fit by a pure-accretion disk model. Adopting a distance of d = 1.5 kpc, the accretion and central star param- eters are: M ̇ = 10−5.48 M⊙ yr−1, M∗ = 0.17 M⊙, and Rinner = 1.04 R⊙. Other fitted values are disk inclination i = 5.9 deg and source extinction AV = 1.83 mag. These parameters yield accretion luminosity Lacc = 8.4 L⊙ and maximum disk temperature Tmax = 6218 K. In order to rectify the post-outburst disk with the pre-outburst spec- tral energy distribution, we hypothesize that the outburst originated from the secondary star in a young Class I binary system.