2020MNRAS.499.1805L

We report the discovery of a mid-infrared outburst in a young stellar object (YSO) with an amplitude close to 8 mag at λ ≃ 4.6 µm. WISEA J142238.82-611553.7 is one of 23 highly variable Wide-field Infrared Survey Explorer (WISE) sources discovered in a search of infrared dark clouds (IRDCs). It lies within the small IRDC G313.671-0.309 (d ≃2.6 kpc), seen by the Herschel/Hi-Gal survey as a compact massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest it is a class I YSO, a view supported by observation of weak 2.12 µm H₂ emission in an otherwise featureless red continuum spectrum in 2019 (6 mag below the peak in K_s). Spitzer, WISE, and VISTA Variables in the Via Lactea (VVV) data show that the outburst began by 2006 and has a duration >13 yr, with a fairly flat peak from 2010 to 2014. The low pre-outburst luminosity implies a low-mass progenitor. The outburst luminosity of a few x 10² L_☉ is consistent with an accretion rate {dot}M ~10^–4 M_☉yr^–1, comparable to a classical FU Orionis event. The 4.6 µm peak in 2010 implies T = 800-1000 K and a disc radial location R ≃ 4.5 au for the emitting region. The colour evolution suggests subsequent progression outwards. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear.