Discovery of a mid-infrared protostellar outburst of exceptional amplitude

Abstract

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 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 H 2 emission in an otherwise featureless red continuum spectrum in 2019 (6 mag below the peak in K s ). Spitzer, WISE and VVV data show that the outburst began by 2006 and has a duration > 13 yr, with a fairly flat peak from 2010-2014. The low pre-outburst luminosity implies a low mass progenitor. The outburst luminosity of a few ×10 2 L is consistent with an accretion rate close to 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 outward. 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.