Multiwavelength Detection of an Ongoing FUOr-type Outburst on a Low-Mass YSO

During the pre-main-sequence evolution, Young Stellar Objects (YSOs) assemblemost of their mass during the episodic accretion process. The rarely seenFUOr-type events (FUOrs) are valuable laboratories to investigate theoutbursting nature of YSOs. Here, we present multi-wavelength detection of ahigh-amplitude eruptive source in the young open cluster VdBH 221 with anongoing outburst, including optical to mid-infrared time series andnear-infrared spectra. The initial outburst has an exceptional amplitude of>6.3 mag in Gaia and 4.6 mag in K_s, with a peak luminosity up to 16L_⊙ and a peak mass accretion rate of 1.4 × 10^-5 M_⊙yr^-1. The optical to infrared spectral energy distribution (SED) of thisobject is consistent with a low-mass star (0.2M_⊙) with a modestextinction (A_V < 2 mag). A 100-d delay between optical and infrared risingstages is detected, suggesting an outside-in origin of the instability. Thespectroscopic features of this object reveal a self-luminous accretion disc,very similar to FU Orionis, with a low line-of-sight extinction. Most recently,there has been a gradual increase in brightness throughout the wavelengthrange, possibly suggesting an enhancement of the mass accretion rate.