The Young Stellar Population, Distance, and Cloud-Cloud Collision
  Induced Star Formation Scenario of the Trifid Nebula
  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
riH$\alpha$ 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 $\sim$1250 pc,
significantly closer than previously reported. The Class II YSOs and PMS stars
($\sim$1.5 Myr old) are spread toward the edge of the molecular clouds. They
are slightly younger than the estimated crossing time of $\sim$2.7Myr and
closer to the estimated dynamical age $\sim$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 $\sim$1 Myr ago, and this result
can be corroborated using future spectroscopic studies.
