Gaia 17bpi: An FU Ori Type Outburst

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this recordWe report on the source Gaia 17bpi and identify it as a new, ongoing FU Ori type outburst, associated with a young stellar object. The optical lightcurve from Gaia exhibited a 3.5 mag rise with the source appearing to plateau in mid/late 2018. Mid-infrared observations from NEOWISE also show a >3 mag rise that occurred in two stages, with the second one coincident with the optical brightening, and the first one preceding the optical brightening by ∼1.5 years. We model the outburst as having started between October and December of 2014. This wavelength-dependent aspect of young star accretion-driven outbursts has never been documented before. Both the mid-infrared and the optical colors of the object become bluer as the outburst proceeds. Optical spectroscopic characteristics in the outburst phase include: a GK-type absorption spectrum, strong wind/outflow in e.g. Mgb, NaD, Hα, K I, O I, and Ca II profiles, and detection of Li I 6707 ˚A. The infrared spectrum in the outburst phase is similar to that of an M-type spectrum, notably exhibiting prominent H2O and 12CO (2-0) bandhead absorption in the K-band, and likely He I wind in the Y-band. The new FU Ori source Gaia 17bpi is associated with a little-studied dark cloud in the galactic plane, located at a distance of 1.27 kpc.Leverhulme TrustScience and Technology Facilities Counci