Theory of Triggered Star Formation (Elmegreen, B. G. 1998, ASP Conf. Ser. 148, Origins, 150)

1. Triggered and Induced Star Formation in the Orion and Monoceros Molecular Clouds Hsu-Tai Lee1 and Wen-Ping Chen2 Graduate Institute of Astronomy, National Central University, Taiwan ROC 1eridan@astro.ncu.edu.tw 2wchen@astro.ncu.edu.tw Theory of Triggered Star Formation(Elmegreen, B. G. 1998, ASP Conf. Ser. 148, Origins, 150) Small Scale Triggering: Direct squeezing of pre-existing clouds or globules by high pressure. This includes triggering in bright rims, proplyds, and small cometary globules. Intermediate Scale Triggering: Compression of a nearby pre-existing cloud from one side, leading to a dense ridge of moving gas. Large Scale Triggering: Accumulation of ambient gas into an expanding shell or ring partially surrounding the pressure source, with star formation in the shell or ring presumably triggered by gravitational collapse of swept-up gas. Observations of Triggered Star Formationin the Orion and Monoceros Molecular Clouds (the red dots are CTTS candidates) Small Scale Triggering IC 2118, LDN 1616 and LDN 1634 show strong IR emission, and the Hα filaments follow the surface of 3 bright-rimmed clouds. We find that these 3 shocked bright-rimmed clouds are all associated with the CTTSs. Intermediate Scale Triggering The CTTSs are systematically distributed west side of the ridge of the Orion A & B molecular cloud. This may suggest that the star formation are triggered by supperbubble or Ori OB1 from west side of these 2 giant molecular clouds. Large Scale Triggering LDN 1652 is located at the edge of the Orion-Eridanus superbubble, created by a series of supernovae explosions in Orion region. The morphology of LDN 1652 seems to follow the Hα filaments of the Orion-Eridanus superbubble and CTTSs are found along these interacting regions. This indicates the vital role the Orion-Eridanus superbubble must have played in the star formation activity in LDN 1652. CTTSs and CTTS candidates show close association with molecular clouds. Some of the CTTS candidates have been confirmed by spectroscopic observations. The CTTS candidates are located at the interacting regions between high-mass stars and molecular clouds. This result suggests our observations of the Orion and Monoceros molecular clouds provide convincing evidence of triggered and induced star formation in action from small, intermediate to large scales.