  Gaia19fct is one of the Gaia-alerted eruptive young stars that has undergone
several brightening events. We conducted monitoring observations using
multi-filter optical and near-infrared photometry, as well as near-infrared
spectroscopy, to understand the physical properties of Gaia19fct and
investigate whether it fits into the historically defined two classes. We
present the analyses of light curves, color variations, spectral lines, and CO
modeling. The light curves show at least five brightening events since 2015,
and the multi-filter color evolutions are mostly gray. The gray evolution
indicates that bursts are triggered by mechanisms other than extinction. Our
near-infrared spectra exhibit both absorption and emission lines and show
time-variability throughout our observations. We found lower rotational
velocity and lower temperature from the near-infrared atomic absorption lines
than from the optical lines, suggesting that Gaia19fct has a Keplerian rotating
disk. The CO overtone features show a superposition of absorption and emission
components, which is unlike other young stellar objects. We modeled the CO
lines, and the result suggests that the emission and absorption components are
formed in different regions. We found that although Gaia19fct exhibits
characteristics of both types of eruptive young stars, FU Orionis-type objects
(FUors) and EX Lupi-type objects (EXors), it shows more similarity with EXors
in general.

Solar and Stellar Astrophysics