Simulated analogues I: apparent and physical evolution of young binary protostellar systems

Protostellar binaries harbour complex environment morphologies. Observations represent a snapshot in time, and projection and optical depth effects impair our ability to interpret them. Careful comparison with high-resolution models that include the larger star-forming region can help isolate the driving physical processes and give context in the time domain to the observations. We carry out four zoom-in simulations with au scale resolution that result in three binaries and a single star. For the first time ever, we follow the detailed evolution of a protobinary in a full molecular cloud context until a circumbinary disc forms. We investigate the gas dynamics around the young stars and extract disc sizes. Using radiative transfer, we obtain the evolutionary tracer T-bol of the binary systems. We find that the centrifugal radius in prestellar cores is a poor estimator of the resulting disc size due to angular momentum transport at all scales. For binaries, the disc sizes are regulated periodically by the binary orbit, having larger radii close to the apastron. The bolometric temperature differs systematically between edge-on and face-on views and shows a high-frequency time dependence correlated with the binary orbit and a low-frequency time dependence with larger episodic accretion events. These oscillations can cause the appearance of the system to change rapidly from class 0 to class I and, for short periods, even bring it to class II. The highly complex structure in early stages, as well as the binary orbit itself, affects the classical interpretation of protostellar classes, and the direct translation to evolutionary stages has to be done with caution and include other evolutionary indicators such as the extent of envelope material.