a study on rotational and thermal properties

We investigate the rotational and thermal properties of star-forming molecular clouds using hydrodynamic simulations. Stars form from molecular cloud cores by gravoturbulent fragmentation. Understanding the angular momentum and the thermal evolution of cloud cores thus plays a fundamental role in completing the theoretical picture of star formation. This is true not only for current star formation as observed in regions like the Orion nebula or the ?-Ophiuchi molecular cloud but also for the formation of stars of the first or second generation in the universe...thesi

A Study on the System and Mechanism Construction of Social Management with Chinese Characteristics

社会管理是一个相当有中国特色的词语。改革开放以来，随着经济体制的转轨和社会的转型，社会流动性大大增加，社会问题与事务与日增多，传统的社会管理理念和方式都难以实现有效的管理，社会问题造成的影响在不断的增大。在这样的情况下迫切需要建立起一种与社会发展阶段相适应社会管理模式，这已经成为广泛的共识，政府、学者、社会人士都在积极呼吁与不断实践。 在构建具有中国特色的社会管理模式时，我们首先要明确的是社会管理是一个涵盖社会生活方方面面的完整体系，是由各个社会管理体制与社会管理机制有机结合组成的。不断完善社会管理体制与机制建设是当前社会管理水平不断提高的必经之路。创新与完善需要建立在对历史的清醒认识和对现...Social management is a word with Chinese characteristics. Since the Chinese reform and opening, especially since the economic reform and social transformation, social mobility is greatly increased. At the same time, social problems and social affairs are also increasing quickly. Traditional social management ideas and methods are difficult to provide effective management; the impact of social prob...学位：管理学硕士院系专业：公共事务学院_行政管理学号：1392010115049

Star Formation at Very Low Metallicity. V. The Greater Importance of Initial Conditions Compared to Metallicity Thresholds

The formation of the first stars out of metal-free gas appears to result in stars at least an order of magnitude more massive than in the present-day case. We here consider what controls the transition from a primordial to a modern initial mass function. It has been proposed that this occurs when effective metal line cooling occurs at a metallicity threshold of Z/Z_sun > 10^{-3.5}. We study the influence of low levels of metal enrichment on the cooling and collapse of initially ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations with particle splitting. Our initial conditions represent protogalaxies forming within a previously ionized H ii region that has not yet had time to cool and recombine. These differ considerably from those used in simulations predicting a metallicity threshold, where the gas was initially cold and only partially ionized. In the centrally condensed potential that we study here, a wide variety of initial conditions for the gas yield a monolithic central collapse. Our models show no fragmentation during collapse to number densities as high as 10^5 cm^{-3}, for metallicities reaching as high as 10^{-1} Z_sun in one rotating case, far above the threshold suggested by previous work. Rotation allows for the formation of gravitationally stable gas disks over large fractions of the local Hubble time. Turbulence slows the growth of the central density slightly, but both spherically symmetric and turbulent initial conditions collapse and form a single sink particle. We therefore argue that fragmentation at moderate density depends on the initial conditions for star formation more than on the metal abundances present.Comment: 29 pages, 8 figures, 3 tables, accepted for publication by Ap