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This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. ; Observed changes in protostellar brightness can be complicated to interpret. In our James Clerk Maxwell Telescope (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 a 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-IR and NEOWISE W1 and W2 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 H2 emission source at the edge of the outflow, with a tail tracing the outflow back to the source. The W1 emission, likely dominated by scattered light, brightens by 0.7 mag, consistent with expectations based on the submillimeter light curve. The signal of continuum variability in K band and W2 is masked by stable H2 emission, as seen in our Gemini/GNIRS spectrum, and perhaps by CO emission. These differences in emission sources complicate IR searches for variability of the youngest protostars. © 2022. The Author(s). Published by the American Astronomical Society. ; S.Y.Y. and J.E.L. are supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT; grant number 2021R1A2C1011718). G.J.H. is supported by general grants 12173003 and 11773002 awarded by the National Science Foundation of China. D.J. is supported by NRC Canada and by an ...

We have conducted a survey of 328 protostars in the Orion molecular clouds with the Atacama Large Millimeter/submillimeter Array at 0.87 mm at a resolution of similar to 0.'' (40au), including observations with the Very Large Array at 9 mm toward 148 protostars at a resolution of similar to 0.'' 08 (32 au) This is the largest multiwavelength survey of protostars at this resolution by an order of magnitude. We use the dust continuum emission at 0.87 and 9 mm to measure the dust disk radii and masses toward the Class 0, Class I, and flat-spectrum protostars, characterizing the evolution of these disk properties in the protostellar phase. The mean dust disk radii for the Class 0, Class I, and flat-spectrum protostars are 44.9(-3.4)(+5.8), 37.0(-3.0)(+4.9), and 28.5(-2.3)(+3.7) au, respectively, and the mean protostellar dust disk masses are 25.9(-4.0)(+7.7), 14.9(-2.2)(+3.8), 1.6(-1.9)(+3.5) M-circle plus, respectively. The decrease in dust disk masses is expected from disk evolution and accretion, but the decrease in disk radii may point to the initial conditions of star formation not leading to the systematic growth of disk radii or that radial drift is keeping the dust disk sizes small. At least 146 protostellar disks (35% of 379 detected 0.87 mm continuum sources plus 42 nondetections) have disk radii greater than 50 au in our sample. These properties are not found to vary significantly between different regions within Orion. The protostellar dust disk mass distributions are systematically larger than those of Class II disks by a factor of >4, providing evidence that the cores of giant planets may need to at least begin their formation during the protostellar phase. ; National Science Foundation (NSF): AST-1814762, AST-1716259, 1815784, 1910106 Homer L. Dodge Endowed Chair National Aeronautics & Space Administration (NASA): 80NSSC18K1095 European Union (EU): AYA2017-84390-C2-1-R State Agency for Research of the Spanish MCIU through "Center of Excellence Severo Ochoa" award: SEV-20170709 Huygens fellowship from Leiden University