1,541 research outputs found
The Atomic and Electronic Structure of Liquid N- Methylformamide as Determined from Diffraction Experiments
The structure of liquid N-methylformamide (NMF) has been investigated using
synchrotron radiation at 77 and 95 keV. The use of high energy photons has
several advantages, in this case especially the large accessible momentum
transfer range, the low absorption and the direct comparability with neutron
diffraction. The range of momentum transfer covered is 0.6 \AA Q
24.0 \AA. Neutron diffraction data on the same sample in the same
momentum transfer range have been published previously. In that study two
differently isotope - substituted species were investigated. In order to
compare neutron and photon diffraction data properly Reverse Monte Carlo (RMC-)
simulations have been performed. Some modifications had to be added to the
standard RMC- code introducing different constraints for inter- and
intramolecular distances as these distances partly overlap in liquid NMF. RMC-
simulations having only the neutron data as input were carried out in order to
test the quality of the X-ray data. The photon structure factor calculated from
the RMC- configurations is found to agree well with the present experimental
data, while it deviates considerably from earlier X-ray work using low energy
photons (17 keV). Finally we discuss whether the different interaction
mechanisms of neutrons and photons can be used to directly access the
electronic structure in the liquid. Evidence is presented that the elastic self
scattering part of liquid NMF is changed with respect to the independent atom
approximation. This modification can be accounted for by a simple charged atoms
model.Comment: Accepted for publication in Molecular Physics, LaTex file, 12 pages,
figures not include
Modulation of human JAK-STAT pathway signaling by functionally conserved regulators
Both the core JAK-STAT pathway components and their in vivo roles have been widely conserved between vertebrates and invertebrate models such as Drosophila melanogaster. Misregulation of JAK-STAT pathway activity has also been identified as a key factor in the development of multiple human malignancies. Recently, whole genome RNA interference (RNAi) screens in cultured Drosophila cells have identified both positively and negatively acting JAK-STAT pathway regulators. Here, we describe the analysis of 73 human genes representing homologs of 56 Drosophila genes originally identified by genome-wide RNAi screening as regulators of JAK-STAT signaling. Using assays for human STAT1 and STAT3 protein levels and phosphorylation status, as well as assays measuring the expression of endogenous STAT1 and STAT3 transcriptional targets, we have tested siRNAs targeting these 73 human genes and have identified potential JAK-STAT pathway regulatory roles in 69 (95%) of these. The genes identified represent a wide range of human JAK-STAT pathway regulators and include genes not previously known to modulate this signaling cascade. These results underline the value of model system based approaches for the identification of pathway regulators and have led to the identification of loci whose misregulation may ultimately be implicated in JAK-STAT pathway-mediated human disease
Numerical Bifurcation Analysis of Conformal Formulations of the Einstein Constraints
The Einstein constraint equations have been the subject of study for more
than fifty years. The introduction of the conformal method in the 1970's as a
parameterization of initial data for the Einstein equations led to increased
interest in the development of a complete solution theory for the constraints,
with the theory for constant mean curvature (CMC) spatial slices and closed
manifolds completely developed by 1995. The first general non-CMC existence
result was establish by Holst et al. in 2008, with extensions to rough data by
Holst et al. in 2009, and to vacuum spacetimes by Maxwell in 2009. The non-CMC
theory remains mostly open; moreover, recent work of Maxwell on specific
symmetry models sheds light on fundamental non-uniqueness problems with the
conformal method as a parameterization in non-CMC settings. In parallel with
these mathematical developments, computational physicists have uncovered
surprising behavior in numerical solutions to the extended conformal thin
sandwich formulation of the Einstein constraints. In particular, numerical
evidence suggests the existence of multiple solutions with a quadratic fold,
and a recent analysis of a simplified model supports this conclusion. In this
article, we examine this apparent bifurcation phenomena in a methodical way,
using modern techniques in bifurcation theory and in numerical homotopy
methods. We first review the evidence for the presence of bifurcation in the
Hamiltonian constraint in the time-symmetric case. We give a brief introduction
to the mathematical framework for analyzing bifurcation phenomena, and then
develop the main ideas behind the construction of numerical homotopy, or
path-following, methods in the analysis of bifurcation phenomena. We then apply
the continuation software package AUTO to this problem, and verify the presence
of the fold with homotopy-based numerical methods.Comment: 13 pages, 4 figures. Final revision for publication, added material
on physical implication
Far-from-constant mean curvature solutions of Einstein's constraint equations with positive Yamabe metrics
In this article we develop some new existence results for the Einstein
constraint equations using the Lichnerowicz-York conformal rescaling method.
The mean extrinsic curvature is taken to be an arbitrary smooth function
without restrictions on the size of its spatial derivatives, so that it can be
arbitrarily far from constant. The rescaled background metric belongs to the
positive Yamabe class, and the freely specifiable part of the data given by the
traceless-transverse part of the rescaled extrinsic curvature and the matter
fields are taken to be sufficiently small, with the matter energy density not
identically zero. Using topological fixed-point arguments and global barrier
constructions, we then establish existence of solutions to the constraints. Two
recent advances in the analysis of the Einstein constraint equations make this
result possible: A new type of topological fixed-point argument without
smallness conditions on spatial derivatives of the mean extrinsic curvature,
and a new construction of global super-solutions for the Hamiltonian constraint
that is similarly free of such conditions on the mean extrinsic curvature. For
clarity, we present our results only for strong solutions on closed manifolds.
However, our results also hold for weak solutions and for other cases such as
compact manifolds with boundary; these generalizations will appear elsewhere.
The existence results presented here for the Einstein constraints are
apparently the first such results that do not require smallness conditions on
spatial derivatives of the mean extrinsic curvature.Comment: 4 pages, no figures, accepted for publication in Physical Review
Letters. (Abstract shortenned and other minor changes reflecting v4 version
of arXiv:0712.0798
Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. II. Optical and Near Infrared Star Formation History of the Starburst Cluster NGC 2070 in 30 Doradus
We present a study of the recent star formation of 30 Doradus in the Large
Magellanic Cloud (LMC) using the panchromatic imaging survey Hubble Tarantula
Treasury Project (HTTP). In this paper we focus on the stars within 20 pc of
the center of the massive ionizing cluster of 30 Doradus, NGC 2070. We
recovered the star formation history by comparing deep optical and NIR
color-magnitude diagrams (CMDs) with state-of-the-art synthetic CMDs generated
with the latest PARSEC models, which include all stellar phases from pre-main
sequence to post- main sequence. For the first time in this region we are able
to measure the star formation using intermediate and low mass stars
simultaneously. Our results suggest that NGC2070 experienced a prolonged
activity. In particular, we find that the star formation in the region: i)
exceeded the average LMC rate ~ 20 Myr ago; ii) accelerated dramatically ~ 7
Myr ago; and iii) reached a peak value 1-3 Myr ago. We did not find significant
deviations from a Kroupa initial mass function down to 0.5 Msun. The average
internal reddening E(B-V) is found to be between 0.3 and 0.4 mag.Comment: Submitted to Ap
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
The Newtonian Limit for Asymptotically Flat Solutions of the Vlasov-Einstein System
It is shown that there exist families of asymptotically flat solutions of the
Einstein equations coupled to the Vlasov equation describing a collisionless
gas which have a Newtonian limit. These are sufficiently general to confirm
that for this matter model as many families of this type exist as would be
expected on the basis of physical intuition. A central role in the proof is
played by energy estimates in unweighted Sobolev spaces for a wave equation
satisfied by the second fundamental form of a maximal foliation.Comment: 24 pages, plain TE
A theory of intense-field dynamic alignment and high harmonic generation from coherently rotating molecules and interpretation of intense-field ultrafast pump-probe experiments
A theory of ultra-fast pump-probe experiments proposed by us earlier [F.H.M.
Faisal et al., Phys. Rev. Lett. 98, 143001 (2007) and F.H.M. Faisal and A.
Abdurrouf, Phys. Rev. Lett. 100, 123005 (2008)] is developed here fully and
applied to investigate the phenomena of dynamic alignment and high harmonic
generation (HHG) from coherently rotating linear molecules. The theory provides
essentially analytical results for the signals that allow us to investigate the
simultaneous dependence of the HHG signals on the two externally available
control parameters, namely, the relative angle between the polarizations, and
the delay-time between the two pulses. It is applied to investigate the
characteristics of high harmonic emission from nitrogen and oxygen molecules
that have been observed experimentally in a number of laboratories. The results
obtained both in the time-domain and in the frequency-domain are compared with
the observed characteristics as well as directly with the data and are found to
agree remarkably well. In addition we have predicted the existence of a "magic"
polarization angle at which all modulations of the harmonic emission from
nitrogen molecule changes to a steady emission at the harmonic frequency. Among
other things we have also shown a correlation between the existence of the
"magic" or critical polarization angles and the symmetry of the active
molecular orbitals, that is deemed to be useful in connection with the "inverse
problem" of molecular imaging from the HHG data.Comment: 31 pages, 22 figures, and 140 equation
Unital Quantum Channels - Convex Structure and Revivals of Birkhoff's Theorem
The set of doubly-stochastic quantum channels and its subset of mixtures of
unitaries are investigated. We provide a detailed analysis of their structure
together with computable criteria for the separation of the two sets. When
applied to O(d)-covariant channels this leads to a complete characterization
and reveals a remarkable feature: instances of channels which are not in the
convex hull of unitaries can return to it when either taking finitely many
copies of them or supplementing with a completely depolarizing channel. In
these scenarios this implies that a channel whose noise initially resists any
environment-assisted attempt of correction can become perfectly correctable.Comment: 31 page
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