Evidence for dust grain growth in young circumstellar disks

Hundreds of circumstellar disks in the Orion nebula are being rapidly destroyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form planetary systems. Nevertheless, the first stage of planet formation-the growth of dust grains into larger particles-may have begun in these systems. Observational evidence for these large particles in Orion's disks is presented. A model of grain evolution in externally irradiated protoplanetary disks is developed and predicts rapid particle size evolution and sharp outer disk boundaries. We discuss implications for the formation rates of planetary systems.     

The onset of planet formation in brown dwarf disks

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub-micrometer-sized dust grains accompanied by dust settling toward the disk mid-plane. Here, we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.     

Substructure in the circumstellar disk around the young star AU Microscopii

Keck adaptive optics imaging with a physical resolution of 0.4 astronomical units (AU) resolves the inner (15 to 80 AU) disk of AU Microscopii (AU Mic, GJ 803, HD 197481), the nearest known scattered light disk to Earth. The inner disk is asymmetric and possesses a sharp change in structure at 35 AU. The disk also shows spatially localized enhancements and deficits at 25- to 40-AU separations. The overall morphology points to the influence of unseen larger bodies and resembles structures expected from recent planet formation. AU Mic is coeval with the archetypical debris disk system beta Pictoris, and the similarities between their two disks point to synchronous disk evolution. Multiple indications of substructure appear to be common in circumstellar disks at an age of approximately 12 million years.     

A young white dwarf companion to pulsar B1620-26: evidence for early planet formation

The pulsar B1620-26 has two companions, one of stellar mass and one of planetary mass. We detected the stellar companion with the use of Hubble Space Telescope observations. The color and magnitude of the stellar companion indicate that it is an undermassive white dwarf (0.34 +/- 0.04 solar mass) of age 480 x 10(6) +/- 140 x 10(6) years. This places a constraint on the recent history of this triple system and supports a scenario in which the current configuration arose through a dynamical exchange interaction in the cluster core. This implies that planets may be relatively common in low-metallicity globular clusters and that planet formation is more widespread and has happened earlier than previously believed.     

The formation and fragmentation of disks around primordial protostars

The very first stars to form in the universe heralded an end to the cosmic dark ages and introduced new physical processes that shaped early cosmic evolution. Until now, it was thought that these stars lived short, solitary lives, with only one extremely massive star, or possibly a very wide binary system, forming in each dark-matter minihalo. Here we describe numerical simulations that show that these stars were, to the contrary, often members of tight multiple systems. Our results show that the disks that formed around the first young stars were unstable to gravitational fragmentation, possibly producing small binary and higher-order systems that had separations as small as the distance between Earth and the Sun.     

The formation of substellar objects induced by the collision of protostellar disks

Simulations of a close encounter between two protostars, each surrounded by a relatively massive disk, resulted in the ejection of some of the disk material into a tidal tail. A portion of the tail condensed into an object with a mass in the range of 5 to 10 jovian masses. This mechanism may explain the existence of the single objects of substellar mass that have recently been discovered.     

Rotation of the planet mercury

The equations of motion for the rotation of Mercury are solved for the general case by an asymptotic expansion. The findings of Liu and O'Keefe, obtained by numerical integration of a special case, that it is possible for Mercury's rotation to be locked into a 2:3 resonance with its revolution, are confirmed in detail. The general solution has further applications.     

亚热带4种典型人工林幼树光合特征和生物量对土壤水肥因子的响应

以亚热带4种典型树种(杉木、马尾松、尾巨桉、闽楠)人工林2年生幼树为研究对象,比较不同土壤水分(40%、55%和70%饱和田间持水量)和土壤养分(未施肥和施肥)处理对叶净光合速率、叶绿素荧光特性、总生物量和各器官生物量分配的影响.结果显示,4种典型树种人工林幼树的叶净光合速率、叶绿素荧光特性、总生物量和各器官生物量分配存在显著的树种差异性.土壤水分处理能够显著影响幼树的叶净光合速率、总生物量和分配比例,且存在树种和水分交互性.随土壤水分增加,闽楠、杉木幼树的叶净光合速率呈增加趋势,但尾巨桉和马尾松幼树的叶净光合速率在不同水分处理间差异不显著.幼树总生物量、叶生物量的比例,随土壤水分的增加呈增加趋势,而根生物量所占比例呈下降趋势.回归分析结果表明,树种自身特性是决定植物光合速率、叶绿素荧光的关键因素,土壤水分是幼树总生物量的关键解释因子,而土壤养分因子并不能解释幼树的叶光合速率、叶绿素荧光特征和生物量.研究表明,树种、土壤水分是影响树木叶光合作用和生物量的关键因子,而土壤养分对幼树叶片的光合特性和生物量影响较小.因此,在亚热带人工林培育过程中应重视树种选择,并基于树种差异性进行合理的水肥管理.     

Dynamics of water molecules in aqueous solvation shells

We report on the direct measurement of the dynamics of water molecules in the solvation shell of an ion in aqueous solution. The hydrogen-bond dynamics of water molecules solvating a Cl-, Br-, or I- anion is slow compared with neat liquid water, indicating that the aqueous solvation shells of these ions are rigid. This rigidity can play an important role in the overall dynamics of chemical reactions in aqueous solution. The experiments were performed with femtosecond midinfrared nonlinear spectroscopy, because this technique allows the spectral response of the water molecules in the solvation shell to be distinguished clearly from that of the other water molecules in the solution.     

Managing soil quality for humanity and the planet

Rather than a human-centric, the basic strategy of achieving Sustainable Development Goals must be focused on restoring and sustaining planetary processes. The urgency of meeting the demands of the humanity must be reconciled with the necessity of enhancing the environment. Increasing and restoring soil organic matter content of the degraded and depleted soils is critical to strengthening planetary processes.     

Organic aerosol formation downwind from the Deepwater Horizon oil spill

A large fraction of atmospheric aerosols are derived from organic compounds with various volatilities. A National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft made airborne measurements of the gaseous and aerosol composition of air over the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico that occurred from April to August 2010. A narrow plume of hydrocarbons was observed downwind of DWH that is attributed to the evaporation of fresh oil on the sea surface. A much wider plume with high concentrations of organic aerosol (>25 micrograms per cubic meter) was attributed to the formation of secondary organic aerosol (SOA) from unmeasured, less volatile hydrocarbons that were emitted from a wider area around DWH. These observations provide direct and compelling evidence for the importance of formation of SOA from less volatile hydrocarbons.     

Detection of circumstellar material in a normal type Ia supernova

Type Ia supernovae are important cosmological distance indicators. Each of these bright supernovae supposedly results from the thermonuclear explosion of a white dwarf star that, after accreting material from a companion star, exceeds some mass limit, but the true nature of the progenitor star system remains controversial. Here we report the spectroscopic detection of circumstellar material in a normal type Ia supernova explosion. The expansion velocities, densities, and dimensions of the circumstellar envelope indicate that this material was ejected from the progenitor system. In particular, the relatively low expansion velocities suggest that the white dwarf was accreting material from a companion star that was in the red-giant phase at the time of the explosion.     

水匮乏指数及其在东北地区水安全评价中的应用

水安全问题对社会经济发展的影响已纳入综合安全的范畴,英国生态与水文中心针对水安全评价问题提出的水匮乏指数可以对地区水安全状况进行评价。通过对东北地区2009年水资源匮乏指数的计算,发现黑龙江省水匮乏指数最高,吉林省水匮乏指数最低,其水安全状况最差,需要从提高水资源开发利用程度、人口受教育程度、控制化肥用量、增加水土流失治理面积、提高社会经济发展水平等方面保障水资源安全体系建立。     

氢氧稳定同位素示踪旱区植物水分来源与利用策略

植物吸收和利用水分的模式决定了生态系统对环境水分的响应,示踪不同条件下植物水分来源,可以为植物水分利用策略研究提供科学依据。在土壤-植被-大气连续体系统水分传输研究中,传统方法越来越不能满足学者对水分传输机理的了解,而稳定氢氧同位素示踪技术因其高灵敏度和示踪性等特点已成为研究水分运动机制的重要手段。国内外相关学者已从多时间尺度、不同层次方向来研究,但对不同区域植物水分来源的定量区分、不同植物的水分利用策略及叶片水同位素分馏机制尚未达成共识。在简述氢氧同位素示踪原理的基础上,系统阐述了定量区分植物水分来源的方法,讨论了不同生境、不同季节、不同生长期、降雨前后的植物水分利用策略和植物叶片水同位素动力分馏过程及其影响因素,旨在为区域生态用水研究提供新的研究手段和理论依据。     

NaCl对杉木中水分子扩散影响的机理研究

【目的】NaCl的存在会改变木材中水分子的扩散行为,使水分子"易进入"或"难扩散",从而影响了木材的吸湿性和平衡含水率。研究其影响机理对含盐木质文物的保护具有重要意义。【方法】以杉木为研究对象,制作了不同含盐率的杉木试件,开展吸湿与干燥试验,探究盐分对杉木中水分扩散的宏观影响规律;建立了杉木的分子模型,采用分子动力学(MD)模拟方法分别研究了水分子进入含盐与不含盐两种模型的过程,以及水分子在两种模型中的扩散行为。通过平衡构型、相对浓度分布等参数分析水分子进入两种模型结果的差异;通过均方位移、吸附能、径向分布函数、氢键等参数分析两种模型中的水分子与纤维素分子、半纤维素分子、木质素分子等杉木主要组成成分之间的相互作用关系,定性解释了盐分影响杉木中水分扩散的微观机理。【结果】试验结果表明杉木试件的吸湿速率随着含盐率的升高而增大;在含水率低于80%范围内,干燥速率随含盐率的升高而减小。MD模拟结果表明,水分子更容易进入含有NaCl的杉木分子模型中,但NaCl会抑制水分子在杉木分子模型中的扩散。模型中各分子之间的相互作用分析结果显示,NaCl与水分子间有很强的吸附作用,但会降低水分子与杉木之间作...     

压载水中消毒副产物碘代三卤甲烷形成机制研究

由于海水中I~-、Br~-含量较高,通过电解法处理压载水过程中会产生一定含量的碘代三卤甲烷(ITHMs),该类物质具有极强的致癌和致突变特性,在电解处理工艺过程中极易生成,难以有效控制。本研究通过次氯酸钠投加模拟实验分析消毒副产物碘代三卤甲烷的形成机制与影响因素,结果显示:(1)Na Cl O浓度由5 mg/L增加至15 mg/L时,I-THMs生成量增加24.76μg/L;(2)I-THMs的生成量随着温度的升高而增加,当温度高于20℃时I-THMs的生成量增加迅速;(3)酸性条件下,I-THMs的生成量较小,碱性条件下,I-THMs的生成量明显增大;(4)对于溶液中不同溴碘浓度比,I-THMs的生成量和各组分比例差异较大。因此,建议在压载水电解处理工艺过程中应对反应条件进行最佳优化,以期保证其处理效果的同时有效控制消毒副产物DBPs的产生。     

节水栽培对小麦产量构成及品质的影响

节水栽培对小麦产量构成和品质影响的试验表明，节水栽培对有效穗数的影响是灌浆成熟期最小，分蘖期最大；水分胁迫对穗粒数影响是随着水分胁迫的加剧而增大，影响穗粒数最为敏感的时期是拔节孕穗期；对千粒重的影响最为敏感的时期是灌浆成熟期；水分胁迫不利于产量的形成，而有利于冬小麦的蛋白质含量的提高，轻度、中度和重度胁迫条件下，蛋白质含量分别比对照增加10．6％、12．9％和13．6％；同时水分胁迫使得湿面筋含量升高，粗淀粉的变化趋势相反。根据综合评判，小麦节水栽培以轻度水分胁迫(60％～65％)最优，重度水分胁迫(40％～45％)最差。