Triggered Star Formation in the Orion Bright-rimmed Clouds

We have developed an empirical and effective set of criteria, based on the 2MASS colors, to select candidate classical T Tauri stars (CTTS). This provides a useful tool to study the young stellar population in star-forming regions. Here we present our analysis of the bright-rimmed clouds (BRCs) B35, B 30, IC 2118, LDN1616, LDN1634, and OrionEast to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiationdriven implosion model in which the ionization fronts from OB stars compress a nearby cloud until the local density exceeds the critical value, thereby inducing the cloud to collapse to form stars. We find that only BRCs associated with strong IRAS 100 μm emission (tracer of high density) and Hα emission (tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding H II regions, 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, with those closer to the BRCs being progressively younger. There are no CTTS leading the ionization fronts, i.e., Institute of Astronomy, National Central University, Taiwan 320, R.O.C. Department of Physics, National Central University, Taiwan, 320, R.O.C. National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, People’s Republic of China.