The enigma of Gaia18cjb: A possible rare hybrid of FUor and EXor properties

Context. Gaia18cjb is one of the Gaia-alerted eruptive young star candidates that has been experiencing a slow and strong brightening during the last 13 years, similarly to some FU Orionis-type objects.
Aims: The aim of this work is to derive the young stellar nature of Gaia18cjb and determine its physical and accretion properties to classify its variability.
Methods: We conducted monitoring observations using multi-filter optical and near-infrared (NIR) photometry, as well as NIR spectroscopy. We present an analysis of pre-outburst and outburst optical and IR light curves, color-magnitude diagrams in different bands, the detection of NIR spectral lines, and estimates of both stellar and accretion parameters during the burst.
Results: The optical light curve shows an unusually long (over 8 years) brightening event of 5 mag in the last 13 years, before reaching a plateau indicating that the burst is still ongoing, suggesting a FU Orionis-like (FUor-like) nature. The same outburst is less strong in the IR light curves. The NIR spectra, obtained during the outburst, exhibit emission lines typical of highly accreting low-intermediate mass young stars with typical EX Lupi-type (EXor) features. The spectral index of Gaia18cjb SED classifies it as a Class I in the pre-burst stage and a flat-spectrum young stellar object (YSO) during the burst.
Conclusions: Gaia18cjb is an eruptive YSO that exhibits FUor-like photometric features (in terms of brightening amplitude and length of the burst) as well as EXor-like spectroscopic features and accretion rate. Its nature appears similar to that of V350 Cep and V1647 Ori, which have been classified as objects in between FUors and EXors.

Spectra from various sources (LT, LBT, GTC) are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/686/A160

Based on observations collected at the Large Binocular Telescope under LBT programme IT-2019B-008, the European Southern Observatory under ESO/NTT programmes 105.203T.001 and 105.203T.003, and the Gran Telescopio Canarias under GTC program GTC29-22B.