The Extensive Substructure and Long Star-Forming Histories of Young Stellar Populations in the Solar Neighborhood

Young stellar associations hold a star formation record that can persist for millions of years, and their study can provide critical insight into the progression of star formation. To identify new nearby young stellar populations, we generate a comprehensive Bayesian framework for the identification of young stars, and use it to identify just over 30000 credible young star candidates within a sample from Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis on all groups with substructure, and in doing so identify patterns in age capable of significantly expanding our knowledge of star formation. These patterns include an apparent primordial star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc, with a star formation propagation speed of ~ 4 km/s We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark a second star-forming event after any initial gas dispersal. This paper represents the first in a series on these young populations, through which we hope to significantly expand our knowledge of the processes that guide star formation in nearby populations.