The brown dwarf desert as a consequence of orbital migration

We show that the dearth of brown dwarfs in short-period orbits around Solar-mass stars – the brown dwarf desert – can be understood as a consequence of inward migration within an evolving protoplanetary disc. Brown dwarf secondaries forming at the same time as the primary star have masses which are comparable to the initial mass of the protoplanetary disc. Subsequent disc evolution leads to inward migration, and destruction of the brown dwarf, via merger with the star. This is in contrast with massive planets, which avoid this fate by forming at a later epoch when the disc is close to being dispersed. Within this model, a brown dwarf desert arises because the mass at the hydrogen-burning limit is coincidentally comparable to the initial disc mass for a Solar mass star. Brown dwarfs should be found in close binaries around very low mass stars, around other brown dwarfs, and around Solar-type stars during the earliest phases of star formation.     

基于重整化群方法的红层软岩损伤破坏逾渗阈值研究

逾渗现象广泛存在于大自然中,当物体中起决定性作用的关键变量变化至某一阈值时,其状态发生突变,此时关键变量的临界值称为逾渗阈值。岩石在水的作用下的损伤破坏过程中存在着逾渗现象,可通过重整化群的方法来研究。本文针对华南地区广泛分布的红层软岩,探讨了其在损伤过程中的逾渗现象与破坏的本质关联。通过对典型红层软岩的微观结构分析,概化出红层软岩的骨架构件和胶结物构件,建立红层软岩的重整化群模型的基本单元,将基本单元进行组合,建立出红层软岩的重整化群模型。根据重整化变换规则,推导计算出其逾渗阈值。研究表明:当红层软岩在荷载条件下的破坏临近逾渗阈值时,其内部结构发生大量破坏,软岩内部从无序损伤突变为有序损伤,继之损伤破坏加剧,由稳定破坏向非稳定破坏突变,最终导致其失稳破坏。研究对揭示红层软岩损伤破坏机理具有重要意义。     

Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter halo

We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and   r ⁻¹  profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully–Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low.     

新疆柯坪新生代推覆岩席的走向连接与成因分析

柯坪断隆新生代推覆构造是南天山南缘冲断系统的一部分,断隆上多排逆冲岩席被数条横向断裂复杂化,致使不同地段岩席排数不等,各排岩席在走向上的对应关系不明,形成了十分奇特的山弯构造.逆冲岩席的变形特点是南天山新生代造山运动的重要表征.笔者通过卫星影像分析和野外路线地质调查,对横向断裂切割各排逆冲岩席之切点部位的构造特征及岩席内部中小型构造所显示的成因信息进行了详细分析,据此,对复杂岩席在走向上进行了连接,对其数量进行了识别.在此基础上,提出了复杂岩席成因的斜冲褶皱模式,即在北西向应力场持续作用下,岩席向南东斜冲,并在其水平位移分量作用下,沿走向发生褶皱和斜冲错断:横向断裂是多排岩席褶皱翼部扭性断层的发展和连通.     

Unconsolidated sediment distribution patterns in the KwaZulu-Natal Bight,South Africa: the role of wave ravinement in separating relict versus active sediment populations

This paper examines the distribution of unconsolidated sediment in the KwaZulu-Natal Bight located along the east coast of South Africa. Results show that there is a general shelf-wide sediment distribution of coarser grain sizes between depths of 60 and 100?m, punctuated by a broad swathe of mud offshore of the Thukela River. Seasonal changes in sediment distribution patterns are small, being restricted to seaward fining on the inner shelf off the fluvial sources. Sediment distribution reflects a partitioning between sediment populations that are current- influenced and relict (palimpsest) populations associated with submerged shorelines. Wave ravinement during the deglacial transgression, the reworking of submerged shorelines during sea-level stillstands and, to a lesser extent, the Agulhas Current system, are the dominant controls on sediment distribution.     

Numerical simulations of protostellar encounters — II. Coplanar disc–disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs.   We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc–disc encounters lead efficiently to the formation of multiple star systems and small- N clusters, including substellar objects.     

胶东金矿金成色及其指示意义

影响金成色的因素较多,包括矿床成因类型、形成深度、变质程度、成矿时代、成矿温度和矿化阶段等,其中金矿床形成深度是最关键的因素。矿床形成深度大,成矿的物化参数变化慢,形成的金成色高;形成深度小,成矿的物化参数变化快,形成的金成色低。以胶东金矿为例,统计并定性分析了这种关系,证明胶东金矿区存在类似的规律,据此可以推广,以矿床金的成色来判断矿床的形成深度和矿体的延深情况。胶东金矿金成色较低,以５５０～７５０居多,反映了金矿成矿以中浅成为主。南张家金矿金成色最低,暗示其形成深度最浅（小于１．５ｋｍ）。     

东营凹陷下第三系深部碎屑岩储层次生孔隙垂向分布及成因分析

东营凹陷下第三系埋深大于 30 0 0 m的深部碎屑岩储层普遍发育 2～ 4个次生孔隙带 ,Φ=10 %～ 30 % ,K=0 .1× 10 - 3μm2～ 30× 10 - 3μm2 ,为中孔低渗低孔低渗油气储层。综合有机质热演化过程、成岩作用和构造断裂等分析 ,认为东营凹陷埋深大于 30 0 0m碎屑岩地层中 ,第一、二个次生孔隙带埋深在 30 0 0 m～ 390 0 m之间 ,其形成主要与有机质成熟过程释放有机酸对长石等颗粒的溶解有密切关系 ,在深大断裂附近储层同时受到大气淡水的影响 ,而深陷带包裹于暗色泥岩中的浊积砂岩的次生孔隙发育情况还与泥质岩异常压力带有关 ;第三个次生孔隙带埋深一般为 390 0 m～ 430 0 m,主要成因于粘土矿物转化造成的还原环境 ,同时受到硫酸盐热化学氧化还原反应的影响。这三个次生孔隙发育带对储集油气有效。第四个孔隙发育带埋深在 470 0 m以下 ,主要是构造成因的微裂缝 ,较难成为有效的油气储层。