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) B 35, B 30, IC 2118, LDN 1616, LDN 1634 East 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., -2within the molecular clouds. All these provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the starforming activities in the region.Subject headings: stars: formation -stars: pre-main sequence -ISM: clouds -ISM: molecules INTRODUCTIONYoung stars tend to form in clusters or groups. A giant molecular cloud may collapse and fragment to form stars of different masses. The formation of massive stars has an immense impact on the environments. On one hand stellar winds and shock waves from a supernova explosion may squeeze molecular clouds and induce subsequent birth of stars which otherwise may not have occurred. On the other hand the agitation may be so violent as to disperse the material, hindering further star-forming activity. Triggered star formation (see Elmegreen 1998, for a comprehensive review) -as opposed to spontaneous cloud collapse -would have profound consequences in stellar population, chemical homogeneity, star formation efficiency, and cloud energetics.While the OB and young low-mass member stars are often distributed cospatially, in some cases low-mass pre-main-sequence (PMS) stars are seen to be located between the molecular cloud and the cluster. Such a configuration can result from either triggered or spontaneous star formation. In the former case, high-mass stars form in dense molecular cores, and then their UV photons create an expanding Strömgren sphere. The expanding ionization fronts (I-fronts) then compress nearby molecular clouds and trigger the formation of next-generation PMS stars at the edge of clouds. Alternatively, star formation may proceed in a spontaneous way; that is, all stars form coevally but high-mass stars photoionize the surrounding clouds to expose the low-mass stars.The Orion complex is an active star-forming region with a wealth of star-forming signatures, such as high-mas...

We present our diagnosis of the role that massive stars play in the formation of low- and intermediate-mass stars in OB associations (the Lambda Ori region, Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars and Herbig Ae/Be stars tend to line up between luminous O stars and bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively younger they are. Our positional and chronological study lends support to the validity of the radiation-driven implosion mechanism, where the Lyman continuum photons from a luminous O star create expanding ionization fronts to evaporate and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive pressure then causes dense clumps to collapse, prompting the formation of low-mass stars on the cloud surface (i.e., the bright rim) and intermediate-mass stars somewhat deeper in the cloud. These stars are a signpost of current star formation; no young stars are seen leading the ionization fronts further into the cloud. Young stars in bright-rimmed or comet-shaped clouds are likely to have been formed by triggering, which would result in an age spread of several megayears between the member stars or star groups formed in the sequence.Comment: 2007, ApJ, 657, 88

We describe a general target selection algorithm that is applicable to any survey in which the number of available candidates is much larger than the number of objects to be observed. This routine aims to achieve a balance between a smoothly-varying, well-understood selection function and the desire to preferentially select certain types of targets. Some target-selection examples are shown that illustrate different possibilities of emphasis functions. Although it is generally applicable, the algorithm was developed specifically for the LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) survey that will be carried out using the Chinese Guo Shou Jing Telescope. In particular, this algorithm was designed for the portion of LEGUE targeting the Galactic halo, in which we attempt to balance a variety of science goals that require stars at fainter magnitudes than can be completely sampled by LAMOST. This algorithm has been implemented for the halo portion of the LAMOST pilot survey, which began in October 2011.

The Spitzer GLIMPSE survey has revealed a number of "Extended Green Objects" (EGOs) which display extended emission at 4.5 µm. These EGOs are potential candidates for high mass protostellar outflows. We have used high resolution (< 1 ′′ ) H 2 1-0 S(1) line, K, and H-band images from the United Kingdom Infrared Telescope to study 34 EGOs to investigate their nature. We found that 12 EGOs exhibit H 2 outflows (two with chains of H 2 knotty structures; five with extended H 2 bipolar structures; three with extended H 2 lobes; two with pairs of H 2 knots). In the 12 EGOs with H 2 outflows, three of them exhibit similar morphologies between the 4.5 µm and H 2 emission. However, the remaining 9 EGOs show that the H 2 features are more extended than the continuum features, and the H 2 emission is seldom associated with continuum emission. Furthermore, -2the morphologies of the near-infrared continuum and 4.5 µm emission are similar to each other for those EGOs with K-band emission, implying that at least a part of the IRAC-band continuum emission of EGOs comes from scattered light from the embedded YSOs.

The weather at Xinglong Observing Station, where the Guo Shou Jing Telescope (GSJT) is located, is strongly affected by the monsoon climate in north-east China. The LAMOST survey strategy is constrained by these weather patterns. In this paper, we present a statistics on observing hours from 2004 to 2007, and the sky brightness, seeing, and sky transparency from 1995 to 2011 at the site. We investigate effects of the site conditions on the survey plan. Operable hours each month shows strong correlation with season: on average there are 8 operable hours per night available in December, but only 1-2 hours in July and August. The seeing and the sky transparency also vary with seasons. Although the seeing is worse in windy winters, and the atmospheric extinction is worse in the spring and summer, the site is adequate for the proposed scientific program of LAMOST survey. With a Monte Carlo simulation using historical data on the site condition, we find that the available observation hours constrain the survey footprint from 22 h to 16 h in right ascension; the sky brightness allows LAMOST to obtain the limit magnitude of V = 19.5 mag with S/N = 10.