788 research outputs found
Kepler-16b: safe in a resonance cell
The planet Kepler-16b is known to follow a circumbinary orbit around a system
of two main-sequence stars. We construct stability diagrams in the "pericentric
distance - eccentricity" plane, which show that Kepler-16b is in a hazardous
vicinity to the chaos domain - just between the instability "teeth" in the
space of orbital parameters. Kepler-16b survives, because it is close to the
stable half-integer 11/2 orbital resonance with the central binary, safe inside
a resonance cell bounded by the unstable 5/1 and 6/1 resonances. The
neighboring resonance cells are vacant, because they are "purged" by
Kepler-16b, due to overlap of first-order resonances with the planet. The newly
discovered planets Kepler-34b and Kepler-35b are also safe inside resonance
cells at the chaos border.Comment: 17 pages, including 5 figure
Raman tensor analysis of hexagonal polyoxymethylene and its application to study the molecular arrangement in highly crystalline electrospun nanofibers
The orientation dependence in space of Raman-active vibrations in the hexagonal structure of polyoxymethylene (POM) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. The variation of polarized intensity for the A1 and the E1 vibrational modes with respect to the POM molecular orientation is systematically studied, from both theoretical and experimental viewpoints, according to the symmetry assignments of each vibrational mode. A set of working equations including the Raman selection rules associated with the A1 and the E1 modes and the orientation distribution function are explicitly formulated and validated by means of a least-square fitting procedure on experimental data. In addition, an approach based on the introduction of orientation distribution functions is applied to quantitatively assess and compare on a statistical base the molecular orientation of two different types of electrospun POM nanofibers. Copyright (c) 2012 John Wiley & Sons, Ltd
Stability conditions and positivity of invariants of fibrations
We study three methods that prove the positivity of a natural numerical
invariant associated to parameter families of polarized varieties. All
these methods involve different stability conditions. In dimension 2 we prove
that there is a natural connection between them, related to a yet another
stability condition, the linear stability. Finally we make some speculations
and prove new results in higher dimension.Comment: Final version, to appear in the Springer volume dedicated to Klaus
Hulek on the occasion of his 60-th birthda
Lead Isotopes in Olivine-Phyric Shergottite Tissint: Implications for the Geochemical Evolution of the Shergottite Source Mantle
Geochemically-depleted shergottites are basaltic rocks derived from a martian mantle source reservoir. Geochemical evolution of the martian mantle has been investigated mainly based on the Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics of the shergottites [1]. Although potentially informative, U-Th- Pb isotope systematics have been limited because of difficulties in interpreting the analyses of depleted meteorite samples that are more susceptible to the effects of near-surface processes and terrestrial contamination. This study conducts a 5-step sequential acid leaching experiment of the first witnessed fall of the geochemically-depleted olivinephyric shergottite Tissint to minimize the effect of low temperature distrubence. Trace element analyses of the Tissint acid residue (mostly pyroxene) indicate that Pb isotope compositions of the residue do not contain either a martian surface or terrestrial component, but represent the Tissint magma source [2]. The residue has relatively unradiogenic initial Pb isotopic compositions (e.g., 206Pb/204Pb = 10.8136) that fall within the Pb isotope space of other geochemically-depleted shergottites. An initial -value (238U/204Pb = 1.5) of Tissint at the time of crystallization (472 Ma [3]) is similar to a time-integrated mu- value (1.72 at 472 Ma) of the Tissint source mantle calculated based on the two-stage mantle evolution model [1]. On the other hand, the other geochemically-depleted shergottites (e.g., QUE 94201 [4]) have initial -values of their parental magmas distinctly lower than those of their modeled source mantle. These results suggest that only Tissint potentially reflects the geochemical signature of the shergottite mantle source that originated from cumulates of the martian magma ocea
Lead Isotope Compositions of Acid Residues from Olivine-Phyric Shergottite Tissint: Implications for Heterogeneous Shergottite Source Reservoirs
Geochemical studies of shergottites suggest that their parental magmas reflect mixtures between at least two distinct geochemical source reservoirs, producing correlations between radiogenic isotope compositions and trace element abundances. These correlations have been interpreted as indicating the presence of a reduced, incompatible element- depleted reservoir and an oxidized, incompatible- element-enriched reservoir. The former is clearly a depleted mantle source, but there is ongoing debate regarding the origin of the enriched reservoir. Two contrasting models have been proposed regarding the location and mixing process of the two geochemical source reservoirs: (1) assimilation of oxidized crust by mantle derived, reduced magmas, or (2) mixing of two distinct mantle reservoirs during melting. The former requires the ancient Martian crust to be the enriched source (crustal assimilation), whereas the latter requires isolation of a long-lived enriched mantle domain that probably originated from residual melts formed during solidification of a magma ocean (heterogeneous mantle model). This study conducts Pb isotope and trace element concentration analyses of sequential acid-leaching fractions (leachates and the final residues) from the geochemically depleted olivine-phyric shergottite Tissint. The results suggest that the Tissint magma is not isotopically uniform and sampled at least two geochemical source reservoirs, implying that either crustal assimilation or magma mixing would have played a role in the Tissint petrogenesis
Tracking the Martian Mantle Signature in Olivine-Hosted Melt Inclusions of Basaltic Shergottites Yamato 980459 and Tissint
The Martian shergottite meteorites are basaltic to lherzolitic igneous rocks that represent a period of relatively young mantle melting and volcanism, approximately 600-150 Ma (e.g. [1,2]). Their isotopic and elemental composition has provided important constraints on the accretion, evolution, structure and bulk composition of Mars. Measurements of the radiogenic isotope and trace element concentrations of the shergottite meteorite suite have identified two end-members; (1) incompatible trace element enriched, with radiogenic Sr and negative epsilon Nd-143, and (2) incompatible traceelement depleted, with non-radiogenic Sr and positive epsilon 143-Nd(e.g. [3-5]). The depleted component represents the shergottite martian mantle. The identity of the enriched component is subject to debate, and has been proposed to be either assimilated ancient martian crust [3] or from enriched domains in the martian mantle that may represent a late-stage magma ocean crystallization residue [4,5]. Olivine-phyric shergottites typically have the highest Mg# of the shergottite group and represent near-primitive melts having experienced minimal fractional crystallization or crystal accumulation [6]. Olivine-hosted melt inclusions (MI) in these shergottites represent the most chemically primitive components available to understand the nature of their source(s), melting processes in the martian mantle, and origin of enriched components. We present trace element compositions of olivine hosted melt inclusions in two depleted olivinephyric shergottites, Yamato 980459 (Y98) and Tissint (Fig. 1), and the mesostasis glass of Y98, using Secondary Ionization Mass Spectrometry (SIMS). We discuss our data in the context of understanding the nature and origin of the depleted martian mantle and the emergence of the enriched component
Optical Conductivity and Electronic Structure of CeRu4Sb12 under High Pressure
Optical conductivity [s(w)] of Ce-filled skutterudite CeRu4Sb12 has been
measured at high pressure to 8 GPa and at low temperature, to probe the
pressure evolution of its electronic structures. At ambient pressure, a
mid-infrared peak at 0.1 eV was formed in s(w) at low temperature, and the
spectral weight below 0.1 eV was strongly suppressed, due to a hybridization of
the f electron and conduction electron states. With increasing external
pressure, the mid-infrared peak shifts to higher energy, and the spectral
weight below the peak was further depleted. The obtained spectral data are
analyzed in comparison with band calculation result and other reported physical
properties. It is shown that the electronic structure of CeRu4Sb12 becomes
similar to that of a narrow-gap semiconductor under external pressure.Comment: 8 pages, 9 figure
The International Urban Energy Balance Models Comparison Project: First Results from Phase 1
A large number of urban surface energy balance models now exist with different assumptions about the
important features of the surface and exchange processes that need to be incorporated. To date, no com-
parison of these models has been conducted; in contrast, models for natural surfaces have been compared
extensively as part of the Project for Intercomparison of Land-surface Parameterization Schemes. Here, the
methods and first results from an extensive international comparison of 33 models are presented. The aim of
the comparison overall is to understand the complexity required to model energy and water exchanges in
urban areas. The degree of complexity included in the models is outlined and impacts on model performance
are discussed. During the comparison there have been significant developments in the models with resulting
improvements in performance (root-mean-square error falling by up to two-thirds). Evaluation is based on a
dataset containing net all-wave radiation, sensible heat, and latent heat flux observations for an industrial area in
Vancouver, British Columbia, Canada. The aim of the comparison is twofold: to identify those modeling ap-
proaches that minimize the errors in the simulated fluxes of the urban energy balance and to determine the
degree of model complexity required for accurate simulations. There is evidence that some classes of models
perform better for individual fluxes but no model performs best or worst for all fluxes. In general, the simpler
models perform as well as the more complex models based on all statistical measures. Generally the schemes
have best overall capability to model net all-wave radiation and least capability to model latent heat flux
Root hydrotropism is controlled via a cortex-specific growth mechanism
Plants can acclimate by using tropisms to link the direction of growth to environmental conditions. Hydrotropism allows roots to forage for water, a process known to depend on abscisic acid (ABA) but whose molecular and cellular basis remains unclear. Here, we show that hydrotropism still occurs in roots after laser ablation removed the meristem and root cap. Additionally, targeted expression studies reveal that hydrotropism depends on the ABA signalling kinase, SnRK2.2, and the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing differential cell-length increases in the cortex, but not in other cell types. We conclude that root tropic responses to gravity and water are driven by distinct tissue-based mechanisms. In addition, unlike its role in root gravitropism, the elongation zone performs a dual function during a hydrotropic response, both sensing a water potential gradient and subsequently undergoing differential growth
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