Triggered Star Formation by Massive Stars

We present our diagnosis of the role that massive stars play in the formation of lowand intermediate-mass stars in OB associations (the λ Ori region, the OriOB1, and LacOB1 associations). We find that the classical T Tauri stars and Herbig Ae/Be stars tend to line up between luminous O stars and brightrimmed or comet-shaped clouds; the closer to a cloud the progressively younger they are. Our positional and chronological study lends support to the validity of the radiation-driven implosion mechanism, where the Lyman continuum photons from a luminous O star create expanding ionization fronts to evaporate and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive pressure then causes dense clumps to collapse, prompting the formation of lowmass stars on the cloud surface (i.e., the bright rim) and intermediate-mass stars somewhat deeper into the cloud. These stars are a signpost of current star formation; no young stars are seen leading the ionization fronts further into the cloud. Young stars in bright-rimmed or comet-shaped clouds are likely to have been formed by triggering, which would result in an age spread of several Myrs between the member stars or star groups formed in the sequence. Subject headings: stars: formation — stars: pre-main-sequence — ISM: clouds — ISM: molecules