Low- and Intermediate-mass Star Formation in OB Associations

• Main Authors: Hsu-Tai Lee, 李昫岱
• Other Authors: Wen-Ping Chen
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/hw67gr

博士 === 國立中央大學 === 天文研究所 === 94 === We have developed an empirical and effective set of criteria, based on the 2MASS colors, to select candidate classical T Tauri stars (CTTSs). 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) B 35, B 30, IC 2118, LDN 1616, LDN 1634, and Orion East to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiation-driven 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 micron emission (tracer of high density) and H-alpha emission (tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding HII regions, and the ages of CTTSs, are consistent with sequential star formation. We also find that CTTSs are only seen between the OB stars and the BRCs, with those closer to the BRCs being progressively younger. There is no CTTS leading the ionization fronts, i.e., within the molecular clouds. All these provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the star-forming activities in the region. We present our diagnosis of the role massive stars play in the formation of low- and intermediate-mass stars in OB associations. In Lacerta OB1 we find that CTTSs and Herbig Ae/Be (HAeBe) stars tend to line up between luminous O stars and bright-rimmed or comet-shaped clouds, with those closer to a cloud progressively younger, just like BRCs in the Orion. A luminous O star formed in a giant molecular cloud would create expanding ionization fronts to evaporate and compress nearby clouds into bright-rimmed or comet-shaped clouds. The implosive pressure then causes dense clumps to collapse and prompts subsequent formation of low-mass stars on the cloud surface (i.e., the bright rim) and intermediate-mass stars somewhat deeper into the cloud. These stars represent the current star formation and no young stars are seen leading the implosive shocks further into the cloud. The 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. We find that young stars in the Orion-Monoceros Complex are tightly related to the Orion-Eridanus Superbubble, created by Wolf-Rayet winds and supernovae from in the Ori OB1 association. The shock fronts would compress molecular clouds and then trigger star formation. This phenomena can be found in the Orion A, Orion B, NGC 2149, VdB 64, Crossbones. We propose that the star formation in the Orion-Monoceros Complex starts from Ori OB1a, then propagates to 1b, 1c related to the Orion A and B molecular clouds, respectively, and eventually to 1d. Star formation is spread out further by the expanding shock front, i.e., the Orion-Eridanus Superbubble, to NGC 2149, VdB 64, and Crossbones, and probably Mon R2. As the shock fronts expand, they do not only induce star formation but also inject short-lived nuclides, synthesized by Wolf-Rayet stars and supernovae, into protostellar nebulae. These now extinct short-lived nuclides have been found in meteorites. Our solar system was likely formed in an environment similar to that of the Ori OB1 association.