Kavli Affiliate: Gregory Herczeg
| Summary:
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 $7times10^-9$ 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 $sim 1.45pm0.3$~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.
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