1,581 research outputs found
Spectral Evolution of an Earth-Like Planet
We have developed a characterization of the geological evolution of the
Earths atmosphere and surface in order to model the observable spectra of an
Earth-like planet through its geological history. These calculations are
designed to guide the interpretation of an observed spectrum of such a planet
by future instruments that will characterize exoplanets. Our models focus on
spectral features that either imply habitability or are required for
habitability. These features are generated by H2O, CO2, CH4, O2, O3, N2O, and
vegetation-like surface albedos. We chose six geological epochs to
characterize. These epochs exhibit a wide range in abundance for these
molecules, ranging from a CO2 rich early atmosphere, to a CO2/CH4-rich
atmosphere around 2 billion years ago to a present-day atmosphere. We analyzed
the spectra to quantify the strength of each important spectral feature in both
the visible and thermal infrared spectral regions, and the resolutions required
to unambiguously observe the features for each epoch. We find a wide range of
spectral resolutions required for observing the different features. For
example, H2O and O3 can be observed with relatively low resolution, while O2
and N2O require higher resolution. We also find that the inclusion of clouds in
our models significantly affects both the strengths and resolutions required to
observe all spectral features.Comment: 34 pages, 24 fig, pdf, ApJ, TB
Charged gravitational instantons in five-dimensional Einstein-Gauss-Bonnet-Maxwell theory
We study a solution of the Einstein-Gauus-Bonnet theory in 5 dimensions
coupled to a Maxwell field, whose euclidean continuation gives rise to an
instanton describing black hole pair production. We also discuss the dual
theory with a 3-form field coupled to gravity.Comment: 8 pages, plain Te
Volume elements and torsion
We reexamine here the issue of consistency of minimal action formulation with
the minimal coupling procedure (MCP) in spaces with torsion. In Riemann-Cartan
spaces, it is known that a proper use of the MCP requires that the trace of the
torsion tensor be a gradient, , and that the modified
volume element be
used in the action formulation of a physical model. We rederive this result
here under considerably weaker assumptions, reinforcing some recent results
about the inadequacy of propagating torsion theories of gravity to explain the
available observational data. The results presented here also open the door to
possible applications of the modified volume element in the geometric theory of
crystalline defects.Comment: Revtex, 8 pages, 1 figure. v2 includes a discussion on
-symmetr
Five-Dimensional Eguchi-Hanson Solitons in Einstein-Gauss-Bonnet Gravity
Eguchi-Hanson solitons are odd-dimensional generalizations of the
four-dimensional Eguchi-Hanson metric and are asymptotic to
AdS\mathbb{Z}_p$ when the cosmological constant is either positive or
negative. We find soliton solutions to Lovelock gravity in 5 dimensions that
are generalizations of these objects.Comment: 26 pages, 11 figure
Modeled CO2 Emissions from Coastal Wetland Transitions to Other Land Uses: Tidal Marshes, Mangrove Forests, and Seagrass Beds
The sediments of coastal wetlands contain large stores of carbon which are vulnerable to oxidation once disturbed, resulting in high levels of CO2 emissions that may be avoided if coastal ecosystems are conserved or restored. We used a simple model to estimate CO2 emissions from mangrove forests, seagrass beds, and tidal marshes based on known decomposition rates for organic matter in these ecosystems under either oxic or anoxic conditions combined with assumptions of the proportion of sediment carbon being deposited in either oxic or anoxic environments following a disturbance of the habitat. Our model found that over 40 years after disturbance the cumulative CO2 emitted from tidal marshes, mangrove forests, and seagrass beds were âŒ70â80% of the initial carbon stocks in the top meter of the sediment. Comparison of our estimates of CO2 emissions with empirical studies suggests that (1) assuming 50% of organic material moves to an oxic environment after disturbance gives rise to estimates that are similar to CO2 emissions reported for tidal marshes; (2) field measurements of CO2 emissions in disturbed mangrove forests were generally higher than our modeled emissions that assumed 50% of organic matter was deposited in oxic conditions, suggesting higher proportions of organic matter may be exposed to oxic conditions after disturbance in mangrove ecosystems; and (3) the generally low observed rates of CO2 emissions from disturbed seagrasses compared to our estimates, assuming removal of 50% of the organic matter to oxic environments, suggests that lower proportions may be exposed to oxic conditions in seagrass ecosystems. There are significant gaps in our knowledge of the fate of wetland sediment carbon in the marine environment after disturbance. Greater knowledge of the distribution, form, decomposition, and emission rates of wetland sediment carbon after disturbance would help to improve models
Gravitational dynamics for all tensorial spacetimes carrying predictive, interpretable and quantizable matter
Only a severely restricted class of tensor fields can provide classical
spacetime geometries, namely those that can carry matter field equations that
are predictive, interpretable and quantizable. These three conditions on matter
translate into three corresponding algebraic conditions on the underlying
tensorial geometry, namely to be hyperbolic, time-orientable and
energy-distinguishing. Lorentzian metrics, on which general relativity and the
standard model of particle physics are built, present just the simplest
tensorial spacetime geometry satisfying these conditions. The problem of
finding gravitational dynamics---for the general tensorial spacetime geometries
satisfying the above minimum requirements---is reformulated in this paper as a
system of linear partial differential equations, in the sense that their
solutions yield the actions governing the corresponding spacetime geometry.
Thus the search for modified gravitational dynamics is reduced to a clear
mathematical task.Comment: 47 pages, no figures, minor update
Algebraic Rainich theory and antisymmetrisation in higher dimensions
The classical Rainich(-Misner-Wheeler) theory gives necessary and sufficient
conditions on an energy-momentum tensor to be that of a Maxwell field (a
2-form) in four dimensions. Via Einstein's equations these conditions can be
expressed in terms of the Ricci tensor, thus providing conditions on a
spacetime geometry for it to be an Einstein-Maxwell spacetime. One of the
conditions is that is proportional to the metric, and it has previously
been shown in arbitrary dimension that any tensor satisfying this condition is
a superenergy tensor of a simple -form. Here we examine algebraic Rainich
conditions for general -forms in higher dimensions and their relations to
identities by antisymmetrisation. Using antisymmetrisation techniques we find
new identities for superenergy tensors of these general (non-simple) forms, and
we also prove in some cases the converse; that the identities are sufficient to
determine the form. As an example we obtain the complete generalisation of the
classical Rainich theory to five dimensions.Comment: 16 pages, LaTe
Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN
We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with blackâwhite contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated
Analysing Charges in even dimensions
Lanczos-Lovelock theories of gravity, in its first order version, are studied
on asymptotically locally anti de Sitter spaces. It is shown that
thermodynamics satisfies the standard behavior and an expression for entropy is
found for this formalism. Finally a short analysis of the algebra of conserved
charges is displayed
N-Alkyl-α-amino acids in Nature and their biocatalytic preparation
PWS would like to acknowledge the European Union for his current funding: âThis project has received funding from the European Unionâs Horizon 2020 research and innovation programme under Marie SkĆodowska-Curie grant agreement No 665919â.N-alkylated-α-amino acids are useful building blocks for the pharmaceutical and fine chemical industries. Enantioselective methods of N-alkylated-α-amino acid synthesis are therefore highly valuable and widely investigated. While there are a variety of chemical methods for their synthesis, they often employ stoichiometric quantities of hazardous reagents such as pyrophoric metal hydrides or genotoxic alkylating agents, whereas biocatalytic routes can provide a greener and cleaner alternative to existing methods. This review highlights the occurrence of the N-alkyl-α-amino acid motif and its role in nature, important applications towards human health and biocatalytic methods of preparation. Several enzyme classes that can be used to access chiral N-alkylated-α-amino acids and their substrate selectivities are detailed.PostprintPeer reviewe
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