Triggered Star Formation in the Orion Bright‐rimmed Clouds

We have developed an empirical and effective set of criteria, based on the2MASS colors, to select candidate classical T Tauri stars (CTTS). This providesa useful tool to study the young stellar population in star-forming regions.Here we present our analysis of the bright-rimmed clouds (BRCs) B 35, B 30, IC2118, LDN 1616, LDN 1634, and Orion East to show how massive stars interactwith molecular clouds to trigger star formation. Our results support theradiation-driven implosion model in which the ionization fronts from OB starscompress a nearby cloud until the local density exceeds the critical value,thereby inducing the cloud to collapse to form stars. We find that only BRCsassociated with strong IRAS 100 micron emission (tracer of high density) andH-alpha emission (tracer of ionization fronts) show signs of ongoing starformation. Relevant timescales, including the ages of O stars, expanding HIIregions, and the ages of CTTS, are consistent with sequential star formation.We also find that CTTS are only seen between the OB stars and the BRCs, withthose closer to the BRCs being progressively younger. There are no CTTS leadingthe ionization fronts, i.e., within the molecular clouds. All these providestrong evidence of triggered star formation and show the major roles massivestars play in sustaining the star-forming activities in the region.Comment: To appear in Ap