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rss_1.0 Recursos de colección

Caltech Authors (80.503 recursos)
Repository of works by Caltech published authors.

Status = Published

Mostrando recursos 1 - 20 de 35.446

1. Reconstructing Farallon plate subduction beneath North America back to the Late Cretaceous - Liu, Lijun; Spasojevic, Sonja; Gurnis, Michael
Using an inverse mantle convection model that assimilates seismic structure and plate motions, we reconstruct Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow- dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten....

2. A d^10 Ni–(H_2) Adduct as an Intermediate in H-H Oxidative Addition across a Ni-B Bond - Harman, W. Hill; Lin, Tzu-Pin; Peters, Jonas C.
Bifunctional E-H activation offers a promising approach for the design of two-electron-reduction catalysts with late first-row metals, such as Ni. To this end, we have been pursuing H_2 activation reactions at late-metal boratranes and herein describe a diphosphine–borane-supported Ni—(H_2) complex, [(^(Ph)DPB^(iPr))Ni(H_2)], which has been characterized in solution. ^1H NMR spectroscopy confirms the presence of an intact H_2 ligand. A range of data, including electronic-structure calculations, suggests a d^(10) configuration for [(^(Ph)DPB^(iPr))Ni(H_2)] as most appropriate. Such a configuration is highly unusual among transition-metal H_2 adducts. The nonclassical H_2 adduct is an intermediate in the complete activation of H_2 across the Ni-B...

3. Cytochrome P450-catalyzed insertion of carbenoids into N–H bonds - Wang, Z. Jane; Peck, Nicole E.; Renata, Hans; Arnold, Frances H.
Expanding nature’s catalytic repertoire to include reactions important in synthetic chemistry will open new opportunities for ‘green’ chemistry and biosynthesis. We demonstrate the first enzyme-catalyzed insertion of carbenoids into N–H bonds. This type of bond disconnection, which has no counterpart in nature, can be mediated by variants of the cytochrome P450 from Bacillus megaterium. The N–H insertion reaction takes place in water, provides the desired products in 26–83% yield, forms the single addition product exclusively, and does not require slow addition of the diazo component.

4. The future of the Higgs boson - Lykken, Joseph; Spiropulu, Maria
Experimentalists and theorists are still celebrating the Nobel-worthy discovery of the Higgs boson that was announced in July 2012 at CERN’s Large Hadron Collider. Now they are working on the profound implications of that discovery.

5. Convex optimization for the planted k-disjoint-clique problem - Ames, Brendan P. W.; Vavasis, Stephen A.
We consider the k -disjoint-clique problem. The input is an undirected graph G in which the nodes represent data items, and edges indicate a similarity between the corresponding items. The problem is to find within the graph k disjoint cliques that cover the maximum number of nodes of G. This problem may be understood as a general way to pose the classical ‘clustering’ problem. In clustering, one is given data items and a distance function, and one wishes to partition the data into disjoint clusters of data items, such that the items in each cluster are close to each other....

6. Nanometallic Glasses: Size Reduction Brings Ductility, Surface State Drives Its Extent - Chen, D. Z.; Jang, D.; Guan, K. M.; An, Q.; Goddard, W. A., III; Greer, J. R.
We report tensile experiments on Ni_(80)P_20 metallic glass samples fabricated via a templated electroplating process and via focused ion beam milling, which differed only in their surface energy states: Ga-ion-irradiated and as-electroplated. Molecular dynamics simulations on similar Ni_(80)Al_20 systems corroborate the experimental results, which suggest that the transition from brittle to ductile behavior is driven by sample size, while the extent of ductility is driven by surface state.

7. Biogenic and biomass burning organic aerosol in a boreal forest at Hyytiälä, Finland, during HUMPPA-COPEC 2010 - Corrigan, A. L.; Russell, L. M.; Takahama, S.; Äijälä, M.; Ehn, M.; Junninen, H.; Rinne, J.; Petäjä, T.; Kulmala, M.; Vogel, A. L.; Hoffmann, T.; Ebben, C. J.; Geiger, F. M.; Chhabra, P.; Seinfeld, J. H.; Worsnop, D. R.; Song, W.; Auld, J.; Williams, J.
Submicron aerosol particles were collected during July and August 2010 in Hyytiälä, Finland, to determine the composition and sources of aerosol at that boreal forest site. Submicron particles were collected on Teflon filters and analyzed by Fourier transform infrared (FTIR) spectroscopy for organic functional groups (OFGs). Positive matrix factorization (PMF) was applied to aerosol mass spectrometry (AMS) measurements and FTIR spectra to identify summertime sources of submicron aerosol mass at the sampling site. The two largest sources of organic mass (OM) in particles identified at Hyytiälä were (1) biogenic aerosol from surrounding local forest and (2) biomass burning aerosol, transported...

8. Evidence for anomalous cosmic-ray hydrogen - Christian, E. R.; Cummings, A. C.; Stone, E. C.
We present evidence for the emergence of a measurable anomalous cosmic-ray hydrogen component in 1987 which may account for 20%-40% of the total hydrogen flux at 60 MeV. Comparing this flux with that of anomalous cosmic-ray helium, we estimate that the H I/He I ratio in the very local interstellar medium is ~4, consistent with determinations from solar ultraviolet backscatter measurements.

9. The isotopic composition of cosmic-ray B, C, N, and O: Evidence for an overabundance of ^(18)O - Gibner, P. S.; Mewaldt, R. A.; Schindler, S. M.; Stone, E. C.; Webber, W. R.
New observations of galactic cosmic rays at higher energies (~400-780 MeV nucleon^(-1)) than obtained in previous direct mass measurements have resulted in determinations of the isotopic composition of B, C, N, and O. The derived cosmic-ray source abundances provide evidence for an enhancement of ^(18)O/^(16)O by a factor of 5.6 ± 1.9 over the solar system value and give a ^(13)C/^(12)C ratio consistent with the solar system abundance ratio, but lower than some recent measurements of the local interstellar medium.

10. Cosmic-ray abundances of elements with atomic number 26 ≤ Z ≤ 40 measured on HEAO 3 - Binns, W. R.; Fickle, R. K.; Garrard, T. L.; Israel, M. H.; Klarmann, J.; Stone, E. C.; Waddington, C. J.
Individual elements in the cosmic radiation of even atomic number (Z) in the interval 26 ≤ Z ≤ 40 have been resolved and their relative abundances measured. The results are inconsistent with a cosmic-ray source whose composition in this charge interval is dominated by r-process nucleosynthests. The ratios of cosmic-ray source abundances to solar system abundances in this interval follow the same general correlation with first ionization potential as for the lighter elements although there are deviations in detail.

11. The isotopic composition of solar flare accelerated magnesium - Mewaldt, R. A.; Spalding, J. D.; Stone, E. C.; Vogt, R. E.
We report observations of energetic solar particles from the 1978 September 23 solar flare in which the three stable isotopes of magnesium are clearly resolved with a mass resolution of 0.23 amu. We find ^(25)Mg/^(24)Mg = 0.14 (+0.05, -0.02) and ^(26)Mg/^(24)Mg = 0.15 (+0.04, -0.03) in the energy interval 12-36 MeV per nucleon. These observations are consistent with terrestrial magnesium isotopic abundances.

12. Neuroscience: Transforming Visual Percepts into Memories - Rutishauser, Ueli
A new study shows that local field potential oscillations in the human entorhinal cortex and hippocampus are correlated with visual awareness.

13. Voyager 1 Encounter with the Saturnian System - Stone, E. C.; Miner, E. D.
An overview of the Voyager 1 encounter with Saturn is presented, including a brief discussion of the flight, trajectory, science plan formulation, and highlights of the results described in the subsequent reports.

14. Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513–46546: Broadband Coherent Emission at the Substellar Boundary? - Hallinan, G.; Antonova, A.; Doyle, J. G.; Bourke, S.; Brisken, W. F.; Golden, A.
The Very Large Array was used to observe the ultracool rapidly rotating M9 dwarf TVLM 513-46546 simultaneously at 4.88 and 8.44 GHz. The radio emission was determined to be persistent, variable, and periodic at both frequencies with a period of ~2 hr. This periodicity is in excellent agreement with the estimated period of rotation of the dwarf based on its v sin i of ~60 km s^(-1). This rotational modulation places strong constraints on the source size of the radio-emitting region and hence the brightness temperature of the associated emission. We find the resulting high brightness temperature, together with the...

15. Ti Isotope Abundances in Whole Meteorites - Kastenmayer, P.; Papanastassiou, D. A.; Wasserburg, G. J.
Previous work has identified the presence of nonlinear isotope effects for Ti in CM and CV chondrites (NPW, 1984, 1985; KPW, 1985; Niemeyer and Lugmair, 1984). We report new measurements on CO and CV meteorites, two chondrites (Parnallee, LL3; Parsa, E4) and a shergottite. Since our earlier reports (NPW, 1985; KPW, 1985) on the absolute Ti composition, we have concentrated on the measurement of Ti abundances normalized to ^(46)Ti/^(48)Ti, in order to establish first the extent of nonlinear effects.

16. An experimental study of Mg self-diffusion in spinel - Sheng, Y. J.; Hutcheon, I. D.; Wasserburg, G. J.
Mg isotope heterogeneity, both between coexisting spinel and silicates and among spinels, is a prominent feature of Plagioclase-Olivine Inclusions (POIs) (1). The preservation of isotopic heterogeneity and relict spinel (Sp) in inclusions with igneous textures indicates that the thermal event which partially melted the precursor material either had too low a temperature or was too brief to allow Mg isotope homogenization between Sp and the melt. Since the temperature history required to homogenize Mg isotopes depends on the diffusion rate of Mg in Sp, we designed experiments using an isotope tracer method to determine this critical rate (2).

17. Somewhere Over the Rainbow - Wasserburg, G. J.
The study of meteorites has produced a wealth of information about these objects. Petrologic and chemical observations of meteorites have shown the presence of planetary differentiates and of undifferentiated aggregates resulting from nebular condensation and the interaction of these aggregates due to metamorphism. Equilibrium chemistry in a solar gas and in planetary environments has been an excellent guide to our studies. Studies by several groups have yielded remarkable discoveries about the early stages of planetary evolution and of precursor components from the solar nebula and from the interstellar medium (ISM).

18. The Voyager 2 Encounter with the Uranian System - Stone, E. C.; Miner, E. D.
An overview of the Voyager 2 encounter with Uranus is presented, including a brief discussion of the trajectory and the planned observations as well as the highlights of the results described in the 11 companion papers.

19. Latitude variation of recurrent Mev-energy proton flux enhancements in the heliocentric radial range 11 to 20 AU and possible correlation with solar coronal hole dynamics - Christon, S. P.; Stone, E. C.
Recurrent low energy ( ≳0.5 MeV) proton flux enhancements, reliable indicators of corotating plasma interaction regions in interplanetary space, have been observed on the Voyager 1 and 2 and Pioneer 11 spacecraft in the heliographic latitude range 2°S to 23°N and the heliocentric radial range 11 to 20 AU. After a period of rather high correlation between fluxes at different latitudes in early 1983, distinct differences develop. The evolution of the fluxes appears to be related to the temporal and latitudinal dynamics of solar coronal holes, suggesting that information about the latitudinal structure of solar wind stream sources propagates to...

20. Neodymium and strontium isotopic characteristics of New Zealand granitoids and related rocks - Pickett, David A.; Wasserburg, G. J.
Initial ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd ratios of Phanerozoic granitoids and related intrusions of the New Zealand block display a mixing-type array indicative of the involvement in their sources of old continental crustal material, most likely of Proterozoic age.ɛ Sr(T) values range from −4 to +273 (^(87)Sr/^(86)Sr=0.7041–0.7233), while ɛ Nd(T) ranges from +2.7 to −11.0. Preexisting metasedimentary rocks have generally higher ɛ_Sr and lower ɛ_Nd (ranging to present-day values of +646 and −15.0, respectively), and, particularly for the Mesozoic intrusives, are isotopically appropriate mixing end-members. The widespread, early Paleozoic Greenland Group graywackes, which are derived from Proterozoic sources, are modeled as the...

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