2021ApJ...921...81K

The Trifid Nebula is a young, nearby star-forming region where star formation is proposed to have been triggered by cloud-cloud collision (CCC), based on observations of molecular clouds. It offers a unique opportunity to test whether the CCC hypothesis is supported by the spatial distribution and star formation chronology of young stars. We present the first study of the optically visible pre-main sequence (PMS) population of the region using riHα imaging and Gaia astrometry. Combined with an analysis of young stellar objects (YSOs) using infrared imaging, we capture the spatial distribution and star formation chronology of the young stellar population. From the analysis, 15 Flat/Class I YSOs, 46 Class II YSOs, and 41 accreting PMS stars are identified (diskless/non-accreting sources are not included in the analysis). The distance based on Gaia parallaxes is ∼1250 pc, significantly closer than previously reported. The Class II YSOs and PMS stars (∼1.5 Myr old) are spread toward the edge of the molecular clouds. They are slightly younger than the estimated crossing time of ∼2.7 Myr and closer to the estimated dynamical age ∼0.85 Myr. Younger Class I YSOs are more concentrated spatially. There exists a cavity devoid of young stars where the two clouds overlap. This evidence suggests that the current generation of stars formed after the collision of two clouds ∼1 Myr ago, and this result can be corroborated using future spectroscopic studies.