Gaia21bty: An EXor light curve exhibiting a FUor spectrum
Udalski, Andrzej; Moór, Attila; Ábrahám, Péter; Kóspál, Ágnes; Park, Sunkyung; Fiorellino, Eleonora; Kun, Mária; Cruz-Sáenz de Miera, Fernando; Giannini, Teresa; Siwak, Michał; Szilágyi, Máté; Nagy, Zsófia; De, Kishalay; Hillenbrand, Lynne A.; Koen, Chris; Szabó, Zsófia Marianna; Marton, Gábor; Janík, Jan; Lucas, Philip W.
Hungary, United States, Germany, Italy, Czech Republic, South Africa, United Kingdom, Poland
Abstract
Gaia21bty, a pre-main-sequence star that previously had shown aperiodic dips in its light curve, underwent a considerable ΔG ≈ 2.9 mag brightening that occurred over a few months between 2020 October and 2021 February. The Gaia light curve shows that the star remained near maximum brightness for about 4-6 months, and then started slowly fading over the next 2 yr, with at least three superimposed ~1 mag sudden rebrightening events. Whereas the amplitude and duration of the maximum is typical for EX Lupi-type stars, optical and near-infrared spectra obtained at the maximum are dominated by features which are typical for FU Ori-type stars (FUors). Modelling of the accretion disc at the maximum indicates that the disc bolometric luminosity is 43 L⊙ and the mass accretion rate is 2.5 × 10-5 M⊙ yr-1, which are typical values for FUors even considering the large uncertainty in the distance ($1.7_{-0.4}^{+0.8}$ kpc). Further monitoring is necessary to understand the cause of the quick brightness decline, the rebrightening, and the other post-outburst light changes, as our multicolour photometric data suggest that they could be caused by a long and discontinuous obscuration event. We speculate that the outburst might have induced large-scale inhomogeneous dust condensations in the line of sight leading to such phenomena, whilst the FUor outburst continues behind the opaque screen.