The formation of stars by gravitational collapse rather than competitive accretion

There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (M(o)), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (approximately 0.5M(o)), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump. Competitive accretion models interpret brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before they have accreted much mass; key predictions of this model are that such objects should lack disks, have high velocity dispersions, form more frequently in denser clumps, and that the mean stellar mass should vary within the Galaxy. Here we derive the rate of competitive accretion as a function of the star-forming environment, based partly on simulation, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region can undergo significant competitive accretion, and that the simulations that show competitive accretion do so because the assumed properties differ from those determined by observation. Our result shows that stars form by gravitational collapse, and explains why observations have failed to confirm predictions of the competitive accretion model.     

The formation of a massive protostar through the disk accretion of gas

The formation of low-mass stars like our Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above ten solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via accretion through a disk. Given this uncertainty and the fact that most massive stars are born in dense clusters, it was suggested that high-mass stars are the result of the runaway merging of intermediate-mass stars. Here we report observations that clearly show a massive star being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and the accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.     

Imprints of fast-rotating massive stars in the Galactic Bulge

The first stars that formed after the Big Bang were probably massive, and they provided the Universe with the first elements heavier than helium ('metals'), which were incorporated into low-mass stars that have survived to the present. Eight stars in the oldest globular cluster in the Galaxy, NGC?6522, were found to have surface abundances consistent with the gas from which they formed being enriched by massive stars (that is, with higher α-element/Fe and Eu/Fe ratios than those of the Sun). However, the same stars have anomalously high abundances of Ba and La with respect to Fe, which usually arises through nucleosynthesis in low-mass stars (via the slow-neutron-capture process, or s-process). Recent theory suggests that metal-poor fast-rotating massive stars are able to boost the s-process yields by up to four orders of magnitude, which might provide a solution to this contradiction. Here we report a reanalysis of the earlier spectra, which reveals that Y and Sr are also overabundant with respect to Fe, showing a large scatter similar to that observed in extremely metal-poor stars, whereas C abundances are not enhanced. This pattern is best explained as originating in metal-poor fast-rotating massive stars, which might point to a common property of the first stellar generations and even of the 'first stars'.     

The formation of the first low-mass stars from gas with low carbon and oxygen abundances

The first stars in the Universe are predicted to have been much more massive than the Sun. Gravitational condensation, accompanied by cooling of the primordial gas via molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass it undergoes a slow, quasi-hydrostatic contraction without further fragmentation; lower-mass stars cannot form. Here we show that as soon as the primordial gas--left over from the Big Bang--is enriched by elements ejected from supernovae to a carbon or oxygen abundance as small as approximately 0.01-0.1 per cent of that found in the Sun, cooling by singly ionized carbon or neutral oxygen can lead to the formation of low-mass stars by allowing cloud fragmentation to smaller clumps. This mechanism naturally accommodates the recent discovery of solar-mass stars with unusually low iron abundances (10(-5.3) solar) but with relatively high (10(-1.3) solar) carbon abundance. The critical abundances that we derive can be used to identify those metal-poor stars in our Galaxy with elemental patterns imprinted by the first supernovae. We also find that the minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background.     

英吉苏凹陷天然气藏成岩演化模式及成藏机理

在对英吉苏凹陷基本地质条件、次生孔隙的形成、成岩序列分析及成岩演化阶段划分的基础上,建立了英吉苏凹陷天然气藏的成岩演化模式,研究了英吉苏凹陷天然气的成藏机理,建立了成藏模式.结果表明:英吉苏凹陷具备了形成大中型气藏的地质条件和成藏要素.研究结论对塔里木盆地气藏分布规律的研究以及寻找新的油气资源提供必要的依据.     

大康留日月星辰汉画像石考

山东滕州大康留日月星辰汉画像石为东汉中晚期画像石鼎盛时期之佳作,其上标绘日月星辰等天文图像,反映了汉代劳动人民对天文知识的了解及记录,其雕刻技法娴熟多样,画面舒展流畅,形象逼真.     

The signature of the first stars in atomic hydrogen at redshift 20

Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions. This was estimated to imprint a large-scale fluctuation signal of about two millikelvin in the 21-centimetre spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays and major enhancements of the suppression. A large velocity difference reduces the abundance of haloes and requires the first stars to form in haloes of about a million solar masses, substantially greater than previously expected. Here we report a simulation of the distribution of the first stars at redshift 20 (cosmic age of around 180 million years), incorporating all these ingredients within a 400-megaparsec box. We find that the 21-centimetre hydrogen signature of these stars is an enhanced (ten millikelvin) fluctuation signal on the hundred-megaparsec scale, characterized by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array or the Low Frequency Array but designed to operate in the range of 50-100 megahertz.     

建国以来孙中山经济思想研究综述

经济思想是孙中山思想最具特色的部分。建国以来，研究研究的主要内容包括：“平均地权”与“耕有田”、国民经济建设思想、与近现代重要人物经济思想的比较等方面。     

The critical role of disks in the formation of high-mass stars

Although massive stars (commonly defined as those in excess of about eight solar masses, or with initial luminosities of a thousand times the solar luminosity or more) have an enormous impact on the galactic environment, how they form has been a mystery. The solution probably involves the existence of accretion disks. Rotational motions have been found in the gas surrounding young high-mass stars, which suggests that non-spherical accretion could be the fundamental ingredient of the massive-star formation recipe.     

"人少任务多"最小分派问题的一种解法

通过对“人少任务多”最小分派问题的解法探析，指出了“加边补零法”的局限性，并得到了一种新的解法——“加边补最小值”法，算例显示此法在n-m=1时优于其它算法。     

应用激光研究瓦斯爆炸的机理

依据气态反应的自由基机制和多原子分子光谱理论,提出用激光研究瓦斯爆炸的机理。利用自行设计的实验装置,在4种激光波长和8种瓦斯浓度下进行实验。初步实验表明,这种方法是可行的,可望获得矿井下激光安全应用的条件。     

准噶尔盆地车排子凸起隐蔽油气藏成藏机理

综合运用地质、地球物理、油气藏地球化学及盆地模拟领域中先进的技术与研究方法,以盆地演化和流体输导格架为成藏背景,从有效烃源岩及其生排烃历史出发,以能量场演化及其控制的化学动力学、流体动力学和运动学过程为核心,探讨了准噶尔盆地车排子凸起隐蔽油气藏的成藏机理.研究表明,油气藏主要以昌吉凹陷二叠系和四棵树凹陷侏罗系油气2期充注混源而成,油气分别以早期净浮力、晚期强超压为驱动机制,以不整合/区域性砂体、构造断裂为优势运移通道构成2源2期充注、阶梯式输导、地层/岩性型聚集的成藏模式.     

清洁发展机制对温室气体减排的贡献

为判断碳市场的规模和作用,应用一个全球碳排放贸易局部均衡模型研究清洁发展机制(CDM)的市场潜力和结构,讨论它对全球温室气体减排的贡献.计算显示,京都议定书第一承诺期中国CDM的市场规模(CO2当量)为2.2亿t, 占CDM市场总量的40%.如果只有联合履行和排放贸易机制而没有CDM, 附件1国家的总履约成本将比目前高67%;中国CDM市场使附件1国家在第一承诺期的履约成本降低23%.热空气比例和CDM交易成本是影响CDM市场结构的2个主要敏感参数.     

异常高压气藏储层应力敏感性研究

异常高压气藏开采过程中,由于流体的产出,使储层岩石受力发生改变并使储层岩石发生弹塑性变形;而弹塑性变形反过来又影响到储层的孔隙度和渗透率,因此,研究储层孔隙度和渗透率应力敏感性具有极其重要的意义.本文基于岩石力学的基本理论,推导出异常高压气藏岩石变形规律及变形方程,以此理论推导指导试验,将理论研究与实验规律相结合,在模拟地层条件下,对实际岩心样品进行了储层应力敏感性实验研究.实验研究表明,该方法能精确的描述储层孔隙度和渗透率应力敏感性,实验结果与理论推导结果完全吻合,进一步证明了理论推导的正确.进而探讨了异常高压气藏储层应力敏感性对气藏开发的影响.     

Microtubules with more than 13 protofilaments in the dividing nuclei of ciliates

Microtubules are largely composed of proteins called tubulins. These are stacked in linear arrays called protofilaments (p). Most microtubules have precisely 13p (ref. 1). The 'incomplete' B and C microtubules (10 or 11p) of cilia, flagella, basal bodies and centrioles are widespread exceptions. Very few examples of 'complete' microtubules with more, or less, than 13p have been found. However, the 'ciliate cell' includes a larger number of highly differentiated types of microtubule arrays than most other cell types. The present study was undertaken to ascertain whether there is variation in p number in two ciliates. In both, all complete cytoplasmic microtubules examined have 13p but microtubules with 13-16p are present in the nucleoplasm of dividing nuclei. These features are probably common to ciliates in general because the free-living hymenostone Paramecium tetraurelia and the parasitic heterotrich Nycotherus ovalis are not closely related in terms of taxonomic criteria or life-style.     

精细气藏描述中的沉积微相研究——以大牛地气田山一段地层为例

以大牛地气田山一段地层为例 ,从岩心、测井等资料入手 ,通过各种相标志的识别 ,并结合区域地质背景 ,认为该区主要发育一套辫状河三角洲平原沉积 ,其中辫状河河道、分流河道、废弃充填河道 (河道边缘 )、沼泽、泛滥平原 5种微相较为发育 .在此基础上 ,通过横向对比 ,勾绘了不同小层的沉积微相平面分布图 ,从而在宏观上勾勒出该区沉积微相的展布及其演化特征 ,为气藏的勘探开发提供了重要的地质依据     

孙中山与20世纪中华文化复兴思潮

复兴中华文化是 2 0世纪中华儿女为之奋斗不息的标的。孙中山在 1 9世纪末至 2 0世纪 2 0年代 ,提出对传统文化要“翻陈出新” ,主张融合中西 ,“创获”一种民族的、民主的、为人民为社会服务的、科学的、文明的新文化 ,科学地将文化的民族性和时代性结合起来 ,成为中华文化复兴思潮中的一面旗帜