Mid-infrared polarization studies of SgrA: a three-dimensional study of the central parsec

Thermal emission from magnetically aligned dust grains produces the observed mid-infrared polarization in the northern arm and east–west bar of SgrA West; recent arcsecond-resolution imaging polarimetry at 12.5 μm of these ionized filaments is presented, which confirms and extends previous studies. A lower limit ∼2 mG is found for the magnetic field in the northern arm and the IRS16 complex appears to be displaced from the northern arm by ∼ 0.15 pc along the line of sight. It is shown that the physical conditions in the ionized filaments of the central parsec lead to a very uniform grain alignment that is directed along the local magnetic field. The position angle of polarized emission will then be at right angles to the projection of the field direction on the plane of the sky and its amplitude a measure of the component of field along the line of sight; this makes possible a partial reconstruction of the field in three dimensions. We present the first application of the use of polarimetry in this way. This partial reconstruction is compared with the H92α observations of Roberts et al. and the implications are that the northern arm and east–west bar do not define either an orbital path or a spiral arm but rather represent a tidally stretched structure in free fall about SgrA^★ with significant deviations from a single plane, and most likely represent the inner ionized rim of a more extended neutral cloud.     

Self-gravitating warped discs around supermassive black holes

We consider warped equilibrium configurations for stellar and gaseous discs in the Keplerian force field of a supermassive black hole, assuming that the self-gravity of the disc provides the only acting torques. Modelling the disc as a collection of concentric circular rings and computing the torques in the non-linear regime, we show that stable, strongly warped precessing equilibria are possible. These solutions exist for a wide range of disc-to-black-hole mass ratios   M _d/ M _bh  , can span large warp angles of up to  ±∼120°  , have inner and outer boundaries, and extend over a radial range of a factor of typically two to four. These equilibrium configurations obey a scaling relation such that in good approximation     where     is the (retrograde) precession frequency and Ω is a characteristic orbital frequency in the disc. Stability was determined using linear perturbation theory and, in a few cases, confirmed by numerical integration of the equations of motion. Most of the precessing equilibria are found to be stable, but some are unstable. The main result of this study is that highly warped discs near black holes can persist for long times without any persistent forcing other than by their self-gravity. The possible relevance of this to galactic nuclei is briefly discussed.     

Kinematics of hypervelocity stars in the triaxial halo of the Milky Way

Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic Centre have unique kinematic properties compared to other halo stars. Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disc and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection. We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances ( r ≳ 50 kpc). We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10°–180°. Future astrometric and deep wide-field surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo.     

Hypervelocity collisions of binary stars at the Galactic Centre

Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N -body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It is possible for a coalescing binary to have sufficient velocity to escape the galaxy. Furthermore, some of the massive S-stars near Sgr A* might be the merger remnants of binary systems, however this production method can not account for most of the S-stars.     

Polarization of X-ray emission from the Sgr B2 cloud

The Sgr B2 giant molecular cloud is claimed to be an 'X-ray reflection nebula'– the reprocessing site of a powerful flare of the Sgr A* source, which occurred a few hundred years ago. The shape of the X-ray spectrum and the strength of the iron fluorescent line support this hypothesis. We argue that the cleanest test of the origin of X-rays from Sgr B2 would be a detection of polarized emission from this source.     

校园数据中心层次安全策略应用研究

校园数据中心是数字化校园的核心基础，包含了各种网络应用和服务的数据，其安全性关系到各项网络应用系统的安全和稳定运行。参考OSI模型原理和根据现实运行情况，提出了校园数据中心层次性安全防范策略，分级分层部署，保证了校园网络系统应用安全，具有一定的使用价值。     

中国火山岩型铀矿的主要地质特征

火山岩型铀矿主要分布于环太平洋的中、新生代多金属成矿带,产于以酸性岩或碱性岩为主的陆相火山岩系中,据对火山岩的^87Sr／^88 Sr含量初始比值测定、火山岩的稀土元素分布模式和火山岩中的熔融包体测温研究,推测火山岩浆是硅铝壳高温大部分熔融的产物,并不同程度受到地幔物质的混染。火山喷发受区域大断裂带控制,形成喷发岩带。火山喷发岩带可进一步划分为几条喷发亚带,喷发亚带则由一系列的火山活动中心组成。火山岩型铀矿均产于断陷红盆旁侧,或在其附近存在基性脉岩群,显示铀矿化与深部构造岩浆活动存在成因联系,适中的剥蚀程度是重要的找矿前提。     

关于曹妃甸某排水管道CIPP壁厚设计的歧义

本文根据曹妃甸某排水管道的招标文件,概要介绍了曹妃甸厂区待修复排水管道的运行状况和修复实践,并分别就不同CIPP工法（软管翻转法、软管拉入加热固化法、软管拉入紫外线固化法）遵照ASTMF1216—05规定,计算出内衬层厚度,并与招标文件对比,指出其不足。希望引起排水业界对CIPP工艺适用范围的关注,在应用中加强技术评价工作,以进一步促进管道非开挖修复事业在我国的健康发展。