SPYGLASS. VII-B. Tracing the Fragments of Massive Star Formation Using Low-mass Associations

Journal article, 2026

New observations from the Gaia spacecraft have traced an emerging demographic of low-mass associations disconnected from larger associations or giant molecular clouds. The first of these associations were recently characterized, but the star-forming environments they trace remain unknown. Using new velocities and ages alongside literature catalogs, we uncover the origins of 16 low-mass associations (M ≲ 100 M⊙, τ ≲ 50 Myr) using dynamical trace back. We reveal that three groups of currently disparate populations share common formation sites, comprising the Leo, Canis Major North–Monoceros South (CaNMoS), and Aquila East–Northern Scutum Associations. Twelve of 16 associations have plausible connections to larger complexes, six of which formed while moving outward from well-established multigenerational star-forming events that drive known or suspected bubbles. We find that feedback from the oldest cospatial and comoving relatives of these associations can explain the current morphologies of the Local and Orion–Eridanus Bubbles, along with the formation of related associations like Sco–Cen and Orion OB1. Most remaining populations show evidence for triggered star formation. In the Leo Association, high vertical velocities and a deceleration signature suggest that it formed out of an intermediate-velocity cloud colliding with gas in Orion, which would make it the first known case of star formation in one of these clouds. The other newly defined associations show similar asymmetric velocity signatures, such as CaNMoS, which may trace bubble-driven acceleration or a cloud collision. We conclude that the lowest-mass young associations remain undiscovered, and that these populations may have a critical role revealing the small gas overdensities that trace the processes sculpting Galactic star formation.