FUOr-Aur 0544+3330: A New YSO Outburst in the Outskirts of Auriga OB1, Viewed Face-On

Abstract

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 experienced 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 outbursting 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. H$\alpha$, NaD, Ca II) is maximized. Both the optical through infrared spectral 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 parameters are: $\dot{M} = 10^{-5.48}$ $M_\odot$ yr$^{-1}$, $M_* = 0.17 \ M_\odot$, and $R_\mathrm{inner} = 1.04 \ R_\odot$. Other fitted values are disk inclination $i=5.9$ deg and source extinction $A_V=1.83$ mag. These parameters yield accretion luminosity $L_\mathrm{acc} = 8.4\ L_\odot$ and maximum disk temperature $T_{\rm{max}} = 6218$ K.