| 摘要: | 我們用2MASS的顏色選取classical T Tauri stars (CTTSs), 這提供了一個研究恆星城區的有效工具. 我們研究了bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN 1616, LDN 1634及Orion East發現大質量恆星如何與分子雲作用, 並進一步引發小質量恆星的形成. 我們的結果符合radiation-driven implosion 模型. 在Lac OB1星協我們發現越靠近BRCs或comet-shaped clouds的CTTSs越年輕,大質量星球形成後向外膨脹的ionization fronts會壓縮附近的分子雲, 這個過程會讓分子雲向內塌縮形成新的恆星. 當ionization fronts向外傳播會引發進一步的恆星形成, 最後一個幾十個pc的OB星協會經由這個方式形成. 我們發現Orion-Monoceros Complex中的年輕星球和Orion-Eridanus Superbubble 有關聯, 震波壓縮分子雲並引發恆星形成, 這個現象可以在Orion A, Orion B, NGC 2149, VdB 64, 及 Crossbones等分子雲中發現. 恆星形成的歷史開始於Ori OB1a, 然後傳到Ori OB1b, Ori OB1c, 然後到Ori OB1d. 這個震波持續向外, 並且將壓縮NGC 2149, VdB 64, 及 Crossbones等分子雲, 而且進一步的在這些分子雲中引發恆星形成. 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. |
