Dissecting the different components of the modest accretion bursts of the very young protostar HOPS 373

Abstract

Observed changes in protostellar brightness can be complicated to interpret. In our JCMT Transient monitoring survey, we discovered that a young binary protostar, HOPS 373, is undergoing a modest 30% brightness increase at 850 µm, caused by a factor of 1.8-3.3 enhancement in the accretion rate. The initial burst occurred over a few months, with a sharp rise and then shallower decay. A second rise occurred soon after the decay, and the source is still bright one year later. The mid-IR emission, the small-scale CO outflow mapped with ALMA, and the location of variable maser emission indicate that the variability is associated with the SW component. The near-infrared and NEOWISE W 1 and W 2 emission is located along the blueshifted CO outflow, spatially offset by ∼ 3 to 4 from the SW component. The K-band emission imaged by UKIRT shows a compact H 2 emission source at the edge of the outflow, with a tail tracing the outflow back to the source. The W 1 emission, likely dominated by scattered light, brightens by 0.7 mag, consistent with expectations based on the sub-mm lightcurve. The signal of continuum variability in K-band and W 2 is masked by stable H 2 emission, as seen in our Gemini/GNIRS spectrum, and perhaps by CO emission. These differences in emission sources complicate infrared searches for variability of the youngest protostars.