RNO 54: A Previously Unappreciated FU Ori Star

We present evidence in support of the hypothesis that the young stellar object RNO 54 is a mature-stage FU Ori type source. The star was first cataloged as a ``red nebulous object" in the 1980s but appears to have undergone its outburst prior to the 1890s. Present-day optical and near-infrared spectra are consistent with those of other FU Ori type stars, both in the details of spectral line presence and shape, and in the overall change in spectral type from an FGK-type in the optical, to the M-type presented in the near-infrared. In addition, the spectral energy distribution of RNO 54 is well-fit by a pure-accretion disk model with parameters: $\dot{M} = 10^{-3.45\pm0.06}$ $M_\odot$ yr$^{-1}$, $M_* = 0.23\pm0.06 \ M_\odot$, and $R_\mathrm{inner} = 3.68\pm0.76 \ R_\odot$, though we believe $R_\mathrm{inner}$ is likely close to its upper range of $4.5 R_\odot$ in order to produce a $T_\mathrm{max} = 7000$ K that is consistent with the optical to near-infrared spectra. The resulting $L_\mathrm{acc}$ is $\sim 265 \ L_\odot$. To find these values, we adopted a source distance $d=1400$ pc and extinction $A_V=3.9$ mag, along with disk inclination $i=50$ deg based on consideration of confidence intervals from our initial disk model, and in agreement with observational constraints. The new appreciation of a well-known source as an FU Ori type object suggests that other such examples may be lurking in extant samples.