Two Earth-sized planets orbiting Kepler-20

Since the discovery of the first extrasolar giant planets around Sun-like stars, evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically passes in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered has a radius 1.42 times that of the Earth's radius (R(⊕)), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03R(⊕)) and the other smaller than the Earth (0.87R(⊕)), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets. The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.     

A closely packed system of low-mass, low-density planets transiting Kepler-11

When an extrasolar planet passes in front of (transits) its star, its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal much more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star, which we call Kepler-11, that reveal six transiting planets, five with orbital periods between 10 and 47?days and a sixth planet with a longer period. The five inner planets are among the smallest for which mass and size have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation.     

利用黑体辐射理论研究行星和矮行星的表面温度

行星和矮行星不断地吸收来自太阳的热辐射,同时也向外辐射能量.假定行星和矮行星处于热平衡状态,利用黑体模型估计了它们的表面温度.结果显示,行星和矮行星表面温度与其自身大小无关,主要受它们与太阳距离远近的影响,距离太阳越远,温度越低.理论计算得到的表面温度大体上与实际表面平均温度相吻合,但由于行星大气或内部热源等因素,实际温度稍微高于理论计算值.文中还研究了星体表面能够达到的最高温度以及辐射冷却后的温度,对于周围没有大气的星体,结果与实际符合较好.     

关于波导管内不能传播TEM波的简单证明和唯象解释

本文从·E=0及×E=iωμH两式出发,无须解出波导管内电磁场的各个分量或横向分量与纵向分量的关系,即可证明波导管内不能传播TEM型电磁波,并从物理概念出发给出一个更简捷的唯象说明。     

SAPO-34和MeAPSO-34分子筛的合成及催化性能

采用水热合成法制备了具有CHA骨架结构的SAPO-34和MeAPSO-34(Me=Ni、Zn、Fe、Cu和Ni-Zn)分子筛。以甲醇裂解制取低碳烯烃(MTO)为模型反应,采用固定床反应装置,对所制备的分子筛进行了催化性能评价。结果表明,所制备的MeAPSO-34分子筛具有比SAPO-34分子筛更高的乙烯选择性和乙烯与丙烯总选择性,且Ni-Zn双金属改性比单金属改性更有利于乙烯与丙烯总选择性的提高,但金属离子的添加会导致丙烯选择性的降低。与SAPO-34相比,单金属Ni的加入可使乙烯的选择性增加7.7%(由SAPO-34的40.7%增加至NiAPSO-34的48.4%),乙烯与丙烯总选择性增加2.5%;而双金属Ni-Zn的加入可使乙烯的选择性增加6.5%,乙烯与丙烯总选择性增加3.2%。     

Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry

The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes. Of these, the (182)Hf-to-(182)W decay is ideally suited for dating core formation in planetary bodies. In an earlier study, the W isotope composition of the Earth's mantle was used to infer that core formation was late (> or = 60 million years after the beginning of the Solar System) and that accretion was a protracted process. The correct interpretation of Hf-W data depends, however, on accurate knowledge of the initial abundance of (182)Hf in the Solar System and the W isotope composition of chondritic meteorites. Here we report Hf-W data for carbonaceous and H chondrite meteorites that lead to timescales of accretion and core formation significantly different from those calculated previously. The revised ages for Vesta, Mars and Earth indicate rapid accretion, and show that the timescale for core formation decreases with decreasing size of the planet. We conclude that core formation in the terrestrial planets and the formation of the Moon must have occurred during the first approximately 30 million years of the life of the Solar System.     

35SiMn调质齿轮弯曲疲劳强度的试验研究

在齿轮设计中,国产材料齿轮的弯曲疲劳极限应力仍然缺乏可靠的试验数据,大多是沿用ISO6336提供的数据。而国内、外的齿轮材料在许多方面都存在一定的差异,以致影响齿轮设计的可靠性。笔者就35SiMn调质齿轮弯曲疲劳强度作了试验研究。试验在进口的STRON1603型电磁谐振疲劳试验机上完成。短寿命区采用四级恒定应力水平的成组试验法,长寿命区采用应力升降法。在试验数据的基础上,根据统计学原理,拟合出完整的C-R-S-N曲线,并获得各种可靠度下的弯曲疲劳曲线,从而为国产材料齿轮设计提供了必要的基础数据。     

Nb_(30)Hf_(35)Ni_(35–x)Co_x氢分离合金组织及其渗氢性能

通过制备Nb30Hf35Ni35-x Cox(x=0,5,···,35)合金,利用X射线衍射仪、扫描电子显微镜等设备研究合金相组成和铸态组织随Co含量的变化规律.通过在不同温度、压力差条件下进行渗氢实验,测定各成分合金的渗氢性能,并分析合金成分和渗氢性能之间的关系.研究结果表明,随着Co含量的增加,合金中杂质化合物相的含量逐渐降低,初生bcc-Nb相的体积分数逐渐减小,共晶相中的bcc-Nb相的体积分数逐渐增大,合金的共晶形态由棒状共晶形态逐渐演变为层片状共晶形态,且合金的渗氢性能逐渐提高.     

Deep and stable interferometric nulling of broadband light with implications for observing planets around nearby stars

The number of indirectly detected planetary systems around nearby stars has grown tremendously since their initial discovery five years ago. But the direct observation of light reflected from these systems remains a formidable task, because of the high contrast ratios between them and their parent stars, and because of the tiny angular separations. Theoretically, these difficulties can be overcome by using a dual-aperture stellar interferometer in which the starlight is cancelled, or 'nulled' by broadband destructive interference, leaving the planet's light visible. Although the basic requirement of equal and oppositely directed electric fields is easy to state, an experimental demonstration of deep broadband nulling has been lacking, owing to difficulties engendered by the needs for extreme symmetry and stability, and low dispersion in the optical system. Here we report the deep (10(-4)) and stable nulling of broadband (18% bandwidth) thermal light. These results validate the physical principles underlying future planet-searching interferometers, and our laboratory instrument will serve as a prototype for the nulling instrument to be implemented on the Keck interferometer in 2001.     

V.34调制解调器技术及DSP仿真

采用话带调制解调器进行数据信息传输 ,是目前广为人们采用的一种数据传输方式。首先介绍了 V.34建议的基本特点 ;然后简单讨论了在 V.34建议中为了提高信息传输速率而采用的新技术 ;最后 ,采用 DSP对 V.34 Modem进行了仿真 ,并给出了仿真结果     

ZG35CrMnSiRe热处理的试验研究

ZG35CrMnSiRe铸钢经不同的热处理,对其强度、硬度、塑性和冲击韧性等机械性能进行测定,实验结果表明其综合机械性能良好。     

妊娠合并癫痫34例临床分析

目的：探讨妊娠合并癫痫的临床表现及处理。方法：回顾性分析妊娠合并癫痫34例的临床表现、处理及结局。结果：妊娠合并癫痫34例中8例孕期未行抗癫痫治疗也未出现癫痫发作,26例行抗癫痫治疗,孕期发作增加5例,发作无增加21例;并发轻度子痫前期3例,胎膜早破1例,早产3例,胎儿宫内生长受限2例;阴道分娩5例,剖宫产29例;新生儿轻度窒息3例,低体重儿2例,新生儿畸形1例。结论：患有癫痫的妇女建议孕前咨询,优化抗癫痫治疗药物,孕前加服叶酸,孕期提倡单药治疗,不建议换药。     

对称群S35与S36的OD-刻画

利用有限群的阶和它的度数型对对称群S35与S36进行了刻画,得到:对称群S35和S36都是3-重OD-刻画的.     

35CrMo钢调质处理工艺的改进

对35CrMo钢常规调质处理时,采用油淬火和水(盐水)淬火进行了实验及生产性实验比较。结果表明,水(盐水)淬火其综合力学性能完全达标,σb略有降低,塑性韧性指标有较大幅度提高。     

喷射沉积ZA35合金组织与耐蚀性

采用喷射沉积技术制备ZA35合金沉积坯,并对沉积坯进行固态热挤压制备ZA35合金。对此种方法制备的合金进行抗拉强度与耐蚀性检测,分析了喷射沉积对ZA35力学性能与耐蚀性的影响。试验表明:喷射沉积与热挤压制备的ZA35合金抗拉强度优于金属型铸造ZA35合金,采用喷射沉积与挤压制备的合金组织均匀,晶粒细小,断口观察发现随拉伸温度升高喷射沉积合金的断裂表现为穿晶断裂,韧性提高。在KOH介质中,喷射沉积ZA35合金与铸态ZA35合金相比,自腐蚀电位变正,腐蚀电流减小,耐蚀性增强。     

35CrMo钢调质处理工艺的改进

对35CrMo钢常规调质处理时,采用油淬火和水(盐水)淬火进行了实验及生产性实验比较.结果表明,水(盐水)淬火其综合力学性能完全达标,σb略有降低,塑性韧性指标有较大幅度提高.     

椎管内脊膜瘤34例的MRI诊断分析及临床意义的评价

目的:探讨脊膜瘤的磁共振(MRI)表现特征及临床意义。方法:收集并分析行MRI检查且经手术及病理证实的脊膜瘤34例,观察肿瘤的部位、瘤体信号表现、增强后的信号特征及邻近脊膜改变。结果:位于胸段26例,颈段4例,腰段4例。肿瘤信号不均5例,囊变1例,邻近硬脊膜增厚3例,宽基底征象20例。增强后肿瘤均有强化信号。结论:脊膜瘤好发于女性,多位于胸段硬膜下间隙,大部分呈实体性,囊变出血少见,肿瘤信号不均匀,主要原因是瘤内钙化程度不一。注入钆喷酸葡胺(GD-DTPA)后肿瘤均有其强化特征。脊髓呈慢性受压缩细征象。脊膜瘤可引起相邻脊膜增厚,增强后呈"鼠尾征"。