The Origin of Low- and Intermediate-mass Stars in Ob Associations

We present our diagnosis of the role massive stars play in the formation of lowand intermediate-mass stars in OB associations. In the bright-rimmed and comet-shaped clouds in both the OriOB1 and LacOB1 associations, there is compelling evidence of lowand intermediate-mass star formation dominated by the triggering process by the massive stars in the regions. The expanding ionization fronts from the O stars appear to have compressed nearby molecular clouds which are engraved and shaped into bright-rimmed or comet-shaped clouds. Implosive pressure on the surface layers then causes a cloud to collapse and prompts subsequent formation of stars, including lowand intermediate-mass stars. The triggering process may propagate through one cloud after another, and the majority of the stellar population in an entire OB association with a scale of tens of parsec may be formed in the sequence. Subject headings: stars: formation — stars: pre-main-sequence — ISM: clouds — ISM: molecules