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排序方式: 共有183条查询结果,搜索用时 15 毫秒
31.
R Sanchis-Ojeda DC Fabrycky JN Winn T Barclay BD Clarke EB Ford JJ Fortney JC Geary MJ Holman AW Howard JM Jenkins D Koch JJ Lissauer GW Marcy F Mullally D Ragozzine SE Seader M Still SE Thompson 《Nature》2012,487(7408):449-453
The Sun's equator and the planets' orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion , magnetic interactions or torques from neighbouring stars. Indeed, isolated 'hot Jupiters' are often misaligned and even orbiting retrograde. Here we report an analysis of transits of planets over starspots on the Sun-like star Kepler-30 (ref. 8), and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star-disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters. 相似文献
32.
Humans regularly produce new utterances that are understood by other members of the same language community. Linguistic theories account for this ability through the use of syntactic rules (or generative grammars) that describe the acceptable structure of utterances. The recursive, hierarchical embedding of language units (for example, words or phrases within shorter sentences) that is part of the ability to construct new utterances minimally requires a 'context-free' grammar that is more complex than the 'finite-state' grammars thought sufficient to specify the structure of all non-human communication signals. Recent hypotheses make the central claim that the capacity for syntactic recursion forms the computational core of a uniquely human language faculty. Here we show that European starlings (Sturnus vulgaris) accurately recognize acoustic patterns defined by a recursive, self-embedding, context-free grammar. They are also able to classify new patterns defined by the grammar and reliably exclude agrammatical patterns. Thus, the capacity to classify sequences from recursive, centre-embedded grammars is not uniquely human. This finding opens a new range of complex syntactic processing mechanisms to physiological investigation. 相似文献
33.
Ultrasensitive solution-cast quantum dot photodetectors 总被引:4,自引:0,他引:4
Konstantatos G Howard I Fischer A Hoogland S Clifford J Klem E Levina L Sargent EH 《Nature》2006,442(7099):180-183
Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10(3) A W(-1). The best devices exhibited a normalized detectivity D* of 1.8 x 10(13) jones (1 jones = 1 cm Hz(1/2) W(-1)) at 1.3 microm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 10(12) jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 10(11) jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices. 相似文献
34.
Qiang Li Kittiya Wongkhan XianCai Luo Andrei S. Batsanov Judith A. K. Howard Yu Lan YunDong Wu Todd B. Marder AiWen Lei 《科学通报(英文版)》2010,55(25):2794-2798
While stable in CH2Cl2, hexane or THF, in the presence of MeOH, self-promoted dimerization of the triarylphosphine-alkene 1, a ligand for Pd-catalyzed reactions, produced an unusual racemic bis(phosphine) 2 in high yield. The reaction of 2 with Pd(dba)2, followed by oxidative addition of p-IC6H4NO2, yielded a trans-chelated Pd(II) aryl iodide complex. 相似文献
35.
Jakobsson M Scholz SW Scheet P Gibbs JR VanLiere JM Fung HC Szpiech ZA Degnan JH Wang K Guerreiro R Bras JM Schymick JC Hernandez DG Traynor BJ Simon-Sanchez J Matarin M Britton A van de Leemput J Rafferty I Bucan M Cann HM Hardy JA Rosenberg NA Singleton AB 《Nature》2008,451(7181):998-1003
Genome-wide patterns of variation across individuals provide a powerful source of data for uncovering the history of migration, range expansion, and adaptation of the human species. However, high-resolution surveys of variation in genotype, haplotype and copy number have generally focused on a small number of population groups. Here we report the analysis of high-quality genotypes at 525,910 single-nucleotide polymorphisms (SNPs) and 396 copy-number-variable loci in a worldwide sample of 29 populations. Analysis of SNP genotypes yields strongly supported fine-scale inferences about population structure. Increasing linkage disequilibrium is observed with increasing geographic distance from Africa, as expected under a serial founder effect for the out-of-Africa spread of human populations. New approaches for haplotype analysis produce inferences about population structure that complement results based on unphased SNPs. Despite a difference from SNPs in the frequency spectrum of the copy-number variants (CNVs) detected--including a comparatively large number of CNVs in previously unexamined populations from Oceania and the Americas--the global distribution of CNVs largely accords with population structure analyses for SNP data sets of similar size. Our results produce new inferences about inter-population variation, support the utility of CNVs in human population-genetic research, and serve as a genomic resource for human-genetic studies in diverse worldwide populations. 相似文献
36.
Quintana FJ Basso AS Iglesias AH Korn T Farez MF Bettelli E Caccamo M Oukka M Weiner HL 《Nature》2008,453(7191):65-71
37.
Schlesinger Y Straussman R Keshet I Farkash S Hecht M Zimmerman J Eden E Yakhini Z Ben-Shushan E Reubinoff BE Bergman Y Simon I Cedar H 《Nature genetics》2007,39(2):232-236
Many genes associated with CpG islands undergo de novo methylation in cancer. Studies have suggested that the pattern of this modification may be partially determined by an instructive mechanism that recognizes specifically marked regions of the genome. Using chromatin immunoprecipitation analysis, here we show that genes methylated in cancer cells are specifically packaged with nucleosomes containing histone H3 trimethylated on Lys27. This chromatin mark is established on these unmethylated CpG island genes early in development and then maintained in differentiated cell types by the presence of an EZH2-containing Polycomb complex. In cancer cells, as opposed to normal cells, the presence of this complex brings about the recruitment of DNA methyl transferases, leading to de novo methylation. These results suggest that tumor-specific targeting of de novo methylation is pre-programmed by an established epigenetic system that normally has a role in marking embryonic genes for repression. 相似文献
38.
Molecular evolution. How old is a polymorphism? 总被引:1,自引:0,他引:1
J C Howard 《Nature》1988,332(6165):588-590
39.
A novel heterodimeric cysteine protease is required for interleukin-1 beta processing in monocytes. 总被引:81,自引:0,他引:81
N A Thornberry H G Bull J R Calaycay K T Chapman A D Howard M J Kostura D K Miller S M Molineaux J R Weidner J Aunins 《Nature》1992,356(6372):768-774
Interleukin-1 beta (IL-1 beta)-converting enzyme cleaves the IL-1 beta precursor to mature IL-1 beta, an important mediator of inflammation. The identification of the enzyme as a unique cysteine protease and the design of potent peptide aldehyde inhibitors are described. Purification and cloning of the complementary DNA indicates that IL-1 beta-converting enzyme is composed of two nonidentical subunits that are derived from a single proenzyme, possibly by autoproteolysis. Selective inhibition of the enzyme in human blood monocytes blocks production of mature IL-1 beta, indicating that it is a potential therapeutic target. 相似文献
40.
Howard Sankey 《Studies in history and philosophy of science》2009,40(2):210-212
In this response, doubts are expressed relating to the treatment by Hoyningen-Huene and Oberheim of the relation between incommensurability and content comparison. A realist response is presented to their treatment of ontological replacement. Further questions are raised about the coherence of the neo-Kantian idea of the world-in-itself as well as the phenomenal worlds hypothesis. The notion of common sense is clarified. Meta-incommensurability is dismissed as a rhetorical device which obstructs productive discussion. 相似文献