Detection of Earth-like planets around nearby stars using a petal-shaped occulter

Direct observation of Earth-like planets is extremely challenging, because their parent stars are about 10(10) times brighter but lie just a fraction of an arcsecond away. In space, the twinkle of the atmosphere that would smear out the light is gone, but the problems of light scatter and diffraction in telescopes remain. The two proposed solutions--a coronagraph internal to a telescope and nulling interferometry from formation-flying telescopes--both require exceedingly clean wavefront control in the optics. An attractive variation to the coronagraph is to place an occulting shield outside the telescope, blocking the starlight before it even enters the optical path. Diffraction and scatter around or through the occulter, however, have limited effective suppression in practically sized missions. Here I report an occulter design that would achieve the required suppression and can be built with existing technology. The compact mission architecture of a coronagraph is traded for the inconvenience of two spacecraft, but the daunting optics challenges are replaced with a simple deployable sheet 30 to 50 m in diameter. When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs.     

The synthesis of organic and inorganic compounds in evolved stars

Recent isotopic analysis of meteorites and interplanetary dust has identified solid-state materials of pre-solar origin. We can now trace the origin of these inorganic grains to the circumstellar envelopes of evolved stars. Moreover, organic (aromatic and aliphatic) compounds have been detected in proto-planetary nebulae and planetary nebulae, which are the descendants of carbon stars. This implies that molecular synthesis is actively happening in the circumstellar environment on timescales as short as several hundred years. The detection of stellar grains in the Solar System suggests that they can survive their journey through the interstellar medium and that they are a major contributor of interstellar grains.     

Substantial reservoirs of molecular hydrogen in the debris disks around young stars

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.     

A close halo of large transparent grains around extreme red giant stars

An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure, entraining the gas and driving the wind. Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star. For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here we report spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung-Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains through photon scattering rather than absorption--a plausible mass loss mechanism for lower-amplitude pulsating stars.     

An abundance of small exoplanets around stars with a wide range of metallicities

The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.     

A disk of dust and molecular gas around a high-mass protostar

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (over eight times the Sun's mass, M(o)) stars remains poorly understood. Recent studies suggest that high-mass stars may form through accretion of material from a circumstellar disk, in essentially the same way as low-mass stars form, rather than through the merging of several low-mass stars. There is as yet, however, no conclusive evidence. Here we report the presence of a flattened disk-like structure around a massive 15M(o) protostar in the Cepheus A region, based on observations of continuum emission from the dust and line emission from the molecular gas. The disk has a radius of about 330 astronomical units (Au) and a mass of 1 to 8 M(o). It is oriented perpendicular to, and spatially coincident with, the central embedded powerful bipolar radio jet, just as is the case with low-mass stars, from which we conclude that high-mass stars can form through accretion.     

Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star

The slow but persistent collisions between asteroids in our Solar System generate a tenuous cloud of dust known as the zodiacal light (because of the light the dust reflects). In the young Solar System, such collisions were more common and the dust production rate should have been many times larger. Yet copious dust in the zodiacal region around stars much younger than the Sun has rarely been found. Dust is known to orbit around several hundred main-sequence stars, but this dust is cold and comes from a Kuiper-belt analogous region out beyond the orbit of Neptune. Despite many searches, only a few main-sequence stars reveal warm (> 120 K) dust analogous to zodiacal dust near the Earth. Signs of planet formation (in the form of collisions between bodies) in the regions of stars corresponding to the orbits of the terrestrial planets in our Solar System have therefore been elusive. Here we report an exceptionally large amount of warm, small, silicate dust particles around the solar-type star BD+20,307 (HIP 8920, SAO 75016). The composition and quantity of dust could be explained by recent frequent or huge collisions between asteroids or other 'planetesimals' whose orbits are being perturbed by a nearby planet.     

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.     

元素核合成理论

在星际演化过程中 ,通过热核聚变和中子俘获等核反应 ,合成了氢、氦直到原子序数为 92的铀的所有原子核。本文简单介绍元素核合成的 8种核反应过程。     

一种碳酸盐岩流体替换新方法

针对碳酸盐岩的基质矿物类型非单一且孔隙结构复杂多变的特点,提出一种适用于碳酸盐岩的流体替换方法。以常规Gassmann方程为基础,采用流体因子分析的基质矿物体积模量反演方法和碳酸盐岩岩石物理模型的孔隙结构反演方法,得到岩石基质矿物的体积模量,并考察复杂孔隙结构的影响,实现碳酸盐岩的流体替换。岩石物理测试分析数据和实际测井资料的计算结果均证明了新方法的正确性和有效性。     

Low-temperature crystallization of silicate dust in circumstellar disks.

Silicate dust in the interstellar medium is observed to be amorphous, yet silicate dust in comets and interplanetary dust particles is sometimes partially crystalline. The dust in disks that are thought to be forming planets around some young stars also appears to be partially crystalline. These observations suggest that as the dust goes from the precursor clouds to a planetary system, it must undergo some processing, but the nature and extent of this processing remain unknown. Here we report observations of highly crystalline silicate dust in the disks surrounding binary red-giant stars. The dust was created in amorphous form in the outer atmospheres of the red giants, and therefore must be processed in the disks to become crystalline. The temperatures in these disks are too low for the grains to anneal; therefore, some low-temperature process must be responsible. As the physical properties of the disks around young stars and red giants are similar, our results suggest that low-temperature crystallization of silicate grains also can occur in protoplanetary systems.     

A giant stream of metal-rich stars in the halo of the galaxy M31.

Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.     

巍山某锑矿厂周围水土中锑含量的检测分析

目的：检测巍山某锑矿厂周围水和土壤中金属锑的含量,为环保部门开展环境治理工作提供参考。方法：对样品预处理（将土样用捣碎机捣碎,再用分析天平称取5．0000g用浓流酸和浓硝酸在数控消化仪中进行消化）后用AA6200型火焰原子吸收光谱仪测定锑含量。再用标准曲线法对结果进行分析比较。结果：测定表明该矿区周围池塘水有锑污染,生活用水、土壤锑的含量微少。结论：按国家GB3838—2002标准,该厂应对排出的废水进行处理,以免给环境带来影响。     

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'.     

Evidence that mechanisms of fin development evolved in the midline of early vertebrates

The origin of paired appendages was a major evolutionary innovation for vertebrates, marking the first step towards fin- (and later limb-) driven locomotion. The earliest vertebrate fossils lack paired fins but have well-developed median fins, suggesting that the mechanisms of fin development were assembled first in the midline. Here we show that shark median fin development involves the same genetic programs that operate in paired appendages. Using molecular markers for different cell types, we show that median fins arise predominantly from somitic (paraxial) mesoderm, whereas paired appendages develop from lateral plate mesoderm. Expression of Hoxd and Tbx18 genes, which specify paired limb positions, also delineates the positions of median fins. Proximodistal development of median fins occurs beneath an apical ectodermal ridge, the structure that controls outgrowth of paired appendages. Each median fin bud then acquires an anteroposteriorly-nested pattern of Hoxd expression similar to that which establishes skeletal polarity in limbs. Thus, despite their different embryonic origins, paired and median fins utilize a common suite of developmental mechanisms. We extended our analysis to lampreys, which diverged from the lineage leading to gnathostomes before the origin of paired appendages, and show that their median fins also develop from somites and express orthologous Hox and Tbx genes. Together these results suggest that the molecular mechanisms for fin development originated in somitic mesoderm of early vertebrates, and that the origin of paired appendages was associated with re-deployment of these mechanisms to lateral plate mesoderm.     

Revealing the dust transport processes of the 2021 mega dust storm event in northern China

The dust cycle is an important component of the Earth system,and dust storms are a severe environmental issue affecting many countries around the world [1].East...     

含铁尘泥在转炉中的利用

为实现含铁尘泥有效成分的资源化利用——用于转炉炼钢,考察铁碳球的使用对炼钢指标的影响以及经济效益。理论和试验分析表明,含铁尘泥制备成铁碳复合球团后完全可应用于转炉冶炼,并能促进化渣脱磷;也可作为炼钢终点的降温剂。铁碳复合球团的使用可以替代部分废钢,在适量使用的情况下,对工序的最终成本几乎无影响。     

兰州市大气降尘的化学特性

对定期收集的兰州市降尘样品,用X射线荧光法分析了其中24种元素的相对浓度,并用富集因子法对数据进行了处理,分析,结果表明兰州市现代降尘中主要有SiO2,Al2O3,CaO,铁的氧化物,MgO,Na2O,K2O,TiO2等,同时含有微量P,Mn,Zn,Ba,Pb等元素,从元素浓度的年内变化情况看,Fe,Co,Cu,Cr,As等元素在采暖期（11月－次年3月）浓度均高于非采暖期,Al,Si,Ca,Pb等元素浓度年内差异不大,而Na,Mg,K等元素浓度在采暖期明显降低,在降尘中富集程度最高的元素Zn,Pb,As主要来自人为污染,Si,Al,Mg,K,Na,V,Co,Ni则基本源于本地和邻近地区的土壤表土,Fe,Ca,Ti,P,Cu,Ba,Cr,Mn人为污染亦占有一定比例。     

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.     

A substantial population of low-mass stars in luminous elliptical galaxies

The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na?(I) doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3M(⊙) (where M(⊙) is the mass of the Sun) and are weak or absent in all other types of stars. We unambiguously detect both signatures, consistent with previous studies that were based on data of lower signal-to-noise ratio. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1M(⊙) to 1M(⊙).