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Abstract We review the use of young low mass stars and protostars, or young stellar objects (YSOs), as tracers of star formation. Observations of molecular clouds at visible, infrared, radio and X-ray wavelengths can identify and characterize the YSOs populating these clouds, with the ability to detect deeply embedded objects at all evolutionary stages. Surveys with the Spitzer, Herschel, XMM-Newton and Chandra space telescopes have measured the spatial distribution of YSOs within a number of nearby (<2.5 kpc) molecular clouds, showing surface densities varying by more than three orders of magnitude. These surveys have been used to measure the spatially varying star formation rates and efficiencies within clouds, and when combined with maps of the molecular gas, have led to the discovery of star-forming relations within clouds. YSO surveys can also characterize the structures, ages, and star formation histories of embedded clusters, and they illuminate the relationship of the clusters to the networks of filaments, hubs and ridges in the molecular clouds from which they form. Measurements of the proper motions and radial velocities of YSOs trace the evolving kinematics of clusters from the deeply embedded phases through gas dispersal, providing insights into the factors that shape the formation of bound clusters. On 100 pc scales that encompass entire star-forming complexes, Gaia is mapping the young associations of stars that have dispersed their natal gas and exist alongside molecular clouds. These surveys reveal the complex structures and motions in associations, and show evidence for supernova driven expansions. Remnants of these associations have now been identified by Gaia, showing that traces of star-forming structures can persist for a few hundred million years. 

Abstract We use hydrodynamical simulations of star-forming gas with stellar feedback and sink particles—proxies for young stellar objects (YSOs)—to produce and analyze synthetic 1.1 mm continuum observations at different distances (150–1000 pc) and ages (0.49–1.27 Myr). We characterize how the inferred core properties, including mass, size, and clustering with respect to diffuse natal gas structure, change with distance, cloud evolution, and the presence of YSOs. We find that atmospheric filtering and core segmentation treatments have distance-dependent impacts on the resulting core properties for d < 300 pc and 500 pc, respectively, which dominate over evolutionary differences. Concentrating on synthetic observations at further distances (650–1000 pc), we find a growing separation between the inferred sizes and masses of cores with and without YSOs in the simulations, which is not seen in recent observations of the Monoceros R2 (Mon R2) cloud at 860 pc. We find that the synthetic cores cluster in smaller groups, and that their mass densities are correlated with gas column density over a much narrower range, than those in the Mon R2 observations. Such differences limit the applicability of the evolutionary predictions we report here, but will motivate our future efforts to adapt our synthetic observation and analysis framework to next generation simulations, such as Star Formation in Gaseous Environments (STARFORGE). These predictions and systematic characterizations will help to guide the analysis of cores on the upcoming TolTEC Clouds to Cores Legacy Survey on the Large Millimeter Telescope Alfonso Serrano.