The enigma of Gaia18cjb: A possible rare hybrid of FUor and EXor properties
Pál, András; Ogłoza, Waldemar; Ábrahám, Péter; Kóspál, Ágnes; Vida, Krisztián; Park, Sunkyung; Fiorellino, Eleonora; Kun, Mária; Alcalá, Juan M.; Caratti o Garatti, Alessio; Cruz-Sáenz de Miera, Fernando; García-Álvarez, David; Giannini, Teresa; Siwak, Michał; Szilágyi, Máté; Covino, Elvira; Marton, Gabor; Nagy, Zsófia; Nisini, Brunella; Marianna Szabó, Zsófia; Bora, Zsófia; Cseh, Borbála; Kalup, Csilla; Krezinger, Máté; Kriskovics, Levente; Sódor, Ádám; Sonbas, Eda; Szakáts, Róbert; Vinkó, József; Wyrzykowski, Lukasz; Zielinski, Pawel
Italy, Hungary, Austria, Germany, France, Spain, United Kingdom, Poland, Turkey
Abstract
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.