Low Mass Stars as Tracers of Star and Cluster Formation
  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 and 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.
