What the Sub-mm Variability of Embedded Protostars Tells Us about Accretion: Past, Present, and Future

After four years of monitoring deeply embedded protostars in the sub-mm, the JCMT Transient Survey has uncovered almost two dozen variable sources, corresponding to >30% of the brightest protostars. Period-fitting analyses find that a significant fraction of these protostars are associated with timescales of 3-8 years and fractional sub-mm amplitudes of 5-50%. We compare the strength of variability in the sub-mm with simultaneous observations at near-/mid-IR wavelengths for half our sample, revealing excellent light curve agreement. Expressed in magnitudes, the strength of the variability signal is found to be 4-6 times larger in the IR, confirming our expectation that the sub-mm emission is directly related to the changing dust temperature in the enveloping core, while the IR responds to the changing source luminosity. 

Assuming that the observed changes in brightness are due to time-variable accretion onto the protostar, the measured timescales and amplitudes suggest that dynamic processes taking place within the inner, several AU, protostellar disk play a role in modulating the mass assembly of deeply embedded protostars. Furthermore, combining the observed time-varying SEDs of these sources with radiative transfer models and with high-resolution ALMA imaging yields discriminatory information for the envelope, disk, and outflow cavity, breaking some of the degeneracy found when fitting static SEDs. Finally, we use ancillary information on the evolutionary stages of protostars in Orion to compile a census of which sources are most likely to be varying.