The Pre-Outburst Properties of the FU Ori Object HBC 722

Abstract

FU Ori outbursts are thought to play an important role in stellar assembly and the evolution of protoplanetary disks. However, the progenitor young stellar objects are largely uncharacterized. We obtained a low-resolution optical spectrum of HBC 722 before its FU Ori outburst as part of a survey of young stellar objects in the North America Nebula. The spectrum yields a spectral type of M3.3$\pm$0.4, which when combined with archival photometry allows us to measure the stellar and accretion properties of a young star prior to its FU Ori outburst. The pre-outburst accretion rate of $7\times10^{-9}$ M$_\odot$ yr$^{-1}$ is high for a protoplanetary disk around an M3-M3.5 star, though about 15,000 times weaker than the accretion rate during the outburst. The pre-outburst variability, inferred from archival B-band photometry, is about a factor 5 with a standard deviation of 0.16 dex and is consistent with variable accretion onto young low-mass stars. The stellar radius is larger than the radius of accreting young stars of similar spectral type by a factor of two. The extinction to HBC 722 is $\sim 1.45\pm0.3$~mag, lower than the 2.5--3.7~mag extinction values measured during the outburst. The u-band photometry plays an especially important role in constraining the veiling at longer wavelengths and therefore also the extinction and photospheric luminosity.