2,413 research outputs found
Constraining planet structure and composition from stellar chemistry: trends in different stellar populations
The chemical composition of stars that have orbiting planets provides
important clues about the frequency, architecture, and composition of exoplanet
systems. We explore the possibility that stars from different galactic
populations that have different intrinsic abundance ratios may produce planets
with a different overall composition. We compiled abundances for Fe, O, C, Mg,
and Si in a large sample of solar neighbourhood stars that belong to different
galactic populations. We then used a simple stoichiometric model to predict the
expected iron-to-silicate mass fraction and water mass fraction of the planet
building blocks, as well as the summed mass percentage of all heavy elements in
the disc. Assuming that overall the chemical composition of the planet building
blocks will be reflected in the composition of the formed planets, we show that
according to our model, discs around stars from different galactic populations,
as well as around stars from different regions in the Galaxy, are expected to
form rocky planets with significantly different iron-to-silicate mass
fractions. The available water mass fraction also changes significantly from
one galactic population to another. The results may be used to set constraints
for models of planet formation and chemical composition. Furthermore, the
results may have impact on our understanding of the frequency of planets in the
Galaxy, as well as on the existence of conditions for habitability.Comment: Accepted for publication in Astronomy & Astrophysic
Loop Equation in Two-dimensional Noncommutative Yang-Mills Theory
The classical analysis of Kazakov and Kostov of the Makeenko-Migdal loop
equation in two-dimensional gauge theory leads to usual partial differential
equations with respect to the areas of windows formed by the loop. We extend
this treatment to the case of U(N) Yang-Mills defined on the noncommutative
plane. We deal with all the subtleties which arise in their two-dimensional
geometric procedure, using where needed results from the perturbative
computations of the noncommutative Wilson loop available in the literature. The
open Wilson line contribution present in the non-commutative version of the
loop equation drops out in the resulting usual differential equations. These
equations for all N have the same form as in the commutative case for N to
infinity. However, the additional supplementary input from factorization
properties allowing to solve the equations in the commutative case is no longer
valid.Comment: 20 pages, 3 figures, references added, small clarifications adde
Constraining planet structure from stellar chemistry: the cases of CoRoT-7, Kepler-10, and Kepler-93
We explore the possibility that the stellar relative abundances of different
species can be used to constrain the bulk abundances of known transiting rocky
planets. We use high resolution spectra to derive stellar parameters and
chemical abundances for Fe, Si, Mg, O, and C in three stars hosting low mass,
rocky planets: CoRoT-7, Kepler-10, and Kepler-93. These planets follow the same
line along the mass-radius diagram, pointing toward a similar composition. The
derived abundance ratios are compared with the solar values. With a simple
stoichiometric model, we estimate the iron mass fraction in each planet,
assuming stellar composition. We show that in all cases, the iron mass fraction
inferred from the mass-radius relationship seems to be in good agreement with
the iron abundance derived from the host star's photospheric composition. The
results suggest that stellar abundances can be used to add constraints on the
composition of orbiting rocky planets.Comment: A&A Letters, in pres
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
Multiloop Calculations in the String-Inspired Formalism: The Single Spinor-Loop in QED
We use the worldline path-integral approach to the Bern-Kosower formalism for
developing a new algorithm for calculation of the sum of all diagrams with one
spinor loop and fixed numbers of external and internal photons. The method is
based on worldline supersymmetry, and on the construction of generalized
worldline Green functions. The two-loop QED -- function is calculated
as an example.Comment: uuencoded ps-file, 20 pages, 2 figures, final revised version to
appear in Phys. Rev.
N=1 SUSY Conformal Block Recursive Relations
We present explicit recursive relations for the four-point superconformal
block functions that are essentially particular contributions of the given
conformal class to the four-point correlation function. The approach is based
on the analytic properties of the superconformal blocks as functions of the
conformal dimensions and the central charge of the superconformal algebra. The
results are compared with the explicit analytic expressions obtained for
special parameter values corresponding to the truncated operator product
expansion. These recursive relations are an efficient tool for numerically
studying the four-point correlation function in Super Conformal Field Theory in
the framework of the bootstrap approach, similar to that in the case of the
purely conformal symmetry.Comment: 12 pages, typos corrected, reference adde
Morita Duality and Noncommutative Wilson Loops in Two Dimensions
We describe a combinatorial approach to the analysis of the shape and
orientation dependence of Wilson loop observables on two-dimensional
noncommutative tori. Morita equivalence is used to map the computation of loop
correlators onto the combinatorics of non-planar graphs. Several
nonperturbative examples of symmetry breaking under area-preserving
diffeomorphisms are thereby presented. Analytic expressions for correlators of
Wilson loops with infinite winding number are also derived and shown to agree
with results from ordinary Yang-Mills theory.Comment: 32 pages, 9 figures; v2: clarifying comments added; Final version to
be published in JHE
DNA Replication Origin Function Is Promoted by H3K4 Di-methylation in Saccharomyces cerevisiae
DNA replication is a highly regulated process that is initiated from replication origins, but the elements of chromatin structure that contribute to origin activity have not been fully elucidated. To identify histone post-translational modifications important for DNA replication, we initiated a genetic screen to identify interactions between genes encoding chromatin-modifying enzymes and those encoding proteins required for origin function in the budding yeast Saccharomyces cerevisiae. We found that enzymes required for histone H3K4 methylation, both the histone methyltransferase Set1 and the E3 ubiquitin ligase Bre1, are required for robust growth of several hypomorphic replication mutants, including cdc6-1. Consistent with a role for these enzymes in DNA replication, we found that both Set1 and Bre1 are required for efficient minichromosome maintenance. These phenotypes are recapitulated in yeast strains bearing mutations in the histone substrates (H3K4 and H2BK123). Set1 functions as part of the COMPASS complex to mono-, di-, and tri-methylate H3K4. By analyzing strains lacking specific COMPASS complex members or containing H2B mutations that differentially affect H3K4 methylation states, we determined that these replication defects were due to loss of H3K4 di-methylation. Furthermore, histone H3K4 di-methylation is enriched at chromosomal origins. These data suggest that H3K4 di-methylation is necessary and sufficient for normal origin function. We propose that histone H3K4 di-methylation functions in concert with other histone post-translational modifications to support robust genome duplication
Improvements in vision-related quality of life in blind patients implanted with the Argus II Epiretinal Prosthesis
Background: The purpose of this analysis is to report the change in quality of life (QoL) after
treatment with the Argus II Epiretinal Prosthesis in patients with end-stage retinitis
pigmentosa.
Methods: The Vision and Quality of Life Index (VisQoL) was used to assess changes in QoL
dimensions and overall utility score in a prospective 30-patient single-arm clinical study.
VisQoL is a multi-attribute instrument consisting of six dimensions (injury, life, roles,
assistance, activity and friendship) that may be affected by visual impairment. Within each
dimension, patients were divided into two groups based on how much their QoL was affected
by their blindness at baseline (moderate/severe or minimal). Outcomes were compared within
each dimension sub-group between baseline and the combined follow-up periods using the
Friedman test. In addition, data from the six dimensions were combined into a single utility
score, with baseline data compared to the combined follow-up periods.
Results: Overall, 80 per cent of the patients reported difficulty in one or more dimensions preimplant. Composite VisQoL utility scores at follow-up showed no statistically significant change
from baseline; however, in three of the six VisQoL dimensions (injury, life and roles), patients
with baseline deficits showed significant and lasting improvement after implantation with
Argus II. In two of the three remaining dimensions (assistance and activity), data trended
toward an improvement. In the final VisQoL dimension (friendship), none of the patients
reported baseline deficits, suggesting that patients had largely adjusted to this attribute.
Conclusion: Patients whose vision negatively affected them with respect to three VisQoL
dimensions (that is, getting injured, coping with the demands of their life and fulfilling their
life roles) reported significant improvement in QoL after implantation of the Argus II retinal
prosthesis. Furthermore, the benefit did not deteriorate at any point during the 36-month
follow-up, suggesting a long-term, durable improvement
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