19,245 research outputs found
Spin Connection and Renormalization of Teleparallel Action
In general relativity, inertia and gravitation are both included in the
Levi-Civita connection. As a consequence, the gravitational action, as well as
the corresponding energy-momentum density, are in general contaminated by
spurious contributions coming from inertial effects. In teleparallel gravity,
on the other hand, because the spin connection represents inertial effects
only, it is possible to separate inertia from gravitation. Relying on this
property, it is shown that to each tetrad there is naturally associated a spin
connection that locally removes the inertial effects from the action. The use
of the appropriate spin connection can be viewed as a renormalization process
in the sense that the computation of energy and momentum naturally yields the
physically relevant values. A self-consistent method for solving field
equations and determining the appropriate spin connection is presented.Comment: v2: 13 pages, minor corrections, new section about solving the field
equations include
InGaN epilayer characterization by microfocused x-ray reciprocal space mapping
We report the use of microfocused three-dimensional x-ray reciprocal space mapping to study InGaN epilayers with average InN content 20%-22%. Analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused x-ray beam, allowed us to gain valuable information about the nanostructure of InN-rich InGaN epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain-free. The initial stages of InGaN-on-GaN epitaxial growth, therefore, conform to the Volmer-Weber growth mechanism with “seeds” nucleated on strain fields generated by the a-type edge dislocations
How realistic are solar model atmospheres?
Recently, new solar model atmospheres have been developed to replace
classical 1D LTE hydrostatic models and used to for example derive the solar
chemical composition. We aim to test various models against key observational
constraints. In particular, a 3D model used to derive the solar abundances, a
3D MHD model (with an imposed 10 mT vertical magnetic field), 1D models from
the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of
Holweger & M\"uller. We confront the models with observational diagnostics of
the temperature profile: continuum centre-to-limb variations (CLV), absolute
continuum fluxes, and the wings of hydrogen lines. We also test the 3D models
for the intensity distribution of the granulation and spectral line shapes. The
predictions from the 3D model are in excellent agreement with the continuum CLV
observations, performing even better than the Holweger & M\"uller model
(constructed largely to fulfil such observations). The predictions of the 1D
theoretical models are worse, given their steeper temperature gradients. For
the continuum fluxes, predictions for most models agree well with the
observations. No model fits all hydrogen lines perfectly, but again the 3D
model comes ahead. The 3D model also reproduces the observed continuum
intensity fluctuations and spectral line shapes very well. The excellent
agreement of the 3D model with the observables reinforces the view that its
temperature structure is realistic. It outperforms the MHD simulation in all
diagnostics, implying that recent claims for revised abundances based on MHD
modelling are premature. Several weaknesses in the 1D models are exposed. The
differences between the PHOENIX LTE and NLTE models are small. We conclude that
the 3D hydrodynamical model is superior to any of the tested 1D models, which
gives further confidence in the solar abundance analyses based on it.Comment: 17 pages, 15 figures. Accepted for publication in A&
Resistivity of non-Fermi liquid U2Pt2In under pressure
Non-Fermi liquid behaviour in single-crystalline U2Pt2In has been studied by
means of resistivity experiments (I||c) under hydrostatic pressure (P<1.5 GPa).
At ambient pressure the resistivity rho(T) follows a power law rho~T^alpha with
alpha~0.5. Upon applying pressure alpha increases. For P>1 GPa a minimum
develops in rho(T). A study of the field dependence of the minimum confirms its
magnetic origin. The ratio c/a is proposed as the effective control parameter,
rather than the unit cell volume.Comment: 5 pages (incl. 2 figures), submitted to SCES'99, Nagan
An X-ray Study of Local Infrared Bright Galaxies
We are carrying out detailed study of the X-ray and infrared (IR) properties
of a sample of local (d < 70 Mpc) luminous infrared galaxies (LIRGs) using
XMM-Newton and Spitzer (imaging and spectroscopy). The main goal is to study
the extreme processes of star formation and/or active galactic nuclei (AGN)
taking place in this cosmologically important class of galaxies. In this
proceedings we present the preliminary results obtained from the analysis of
the XMM-Newton X-ray images and the X-ray spectral modeling.Comment: 5 pages, to appear in Highlights of Spanish Astrophysics VI,
Proceedings of the IX Scientific Meeting of the Spanish Astronomical Society
held on September 13-17, 2010, in Madrid, Spai
Genetic Stratigraphy of Key Demographic Events in Arabia
The issue of admixture in human populations is normally addressed by genome-wide (GW) studies, and several approaches have been developed to date admixture events [1,2,3,4,5]. Admixed populations bear chromosomes with segments of DNA from all contributing source groups, the size of which decreases over successive generations until recombination renders them undetectably short. Several algorithms attempt to date admixture events by inferring the size of the nuclear ancestry segments, and these can work well when dating recent episodes in human history, such as the sub-Saharan African input into the New World [6], but they fail to detect several known episodes that took place at earlier times, such as the African input into Iberia [1] and genetic exchanges across the Red Sea [7]. Simulations with the suite of methods available at the ADMIXTOOLS package indicated that these methods could detect admixture events as early as 500 generation ago, but real data did not allow the tracing of such old events [8]. A recent improved algorithm, called GLOBETROTTER, has been used to tackle the detection of the co-occurrence of several mixture events by decomposing each chromosome into a series of haplotypic chunks and then analysing each chunk independently [3], but the problem of detecting ancient events remains. Its application to the systematic screening of worldwide admixture events was able to reveal around 100 events, but all occurring over only the past 4,000 years [3
Single-layer economic model predictive control for periodic operation
In this paper we consider periodic optimal operation of constrained periodic linear systems. We propose an economic model predictive controller based on a single layer that unites dynamic real time optimization and control. The proposed controller guarantees closed-loop convergence to the optimal periodic trajectory that minimizes the average operation cost for a given economic criterion. A priori calculation of the optimal trajectory is not required and if the economic cost function is changed, recursive feasibility and convergence to the new periodic optimal trajectory is guaranteed. The results are demonstrated with two simulation examples, a four tank system, and a simplified model of a section of Barcelona's water distribution network.Peer ReviewedPostprint (author’s final draft
Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping
We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of “seeds” on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth
Outflow of hot and cold molecular gas from the obscured secondary nucleus of NGC3256: closing in on feedback physics
The nuclei of merging galaxies are often deeply buried in dense layers of gas
and dust. In these regions, gas outflows driven by starburst and AGN activity
are believed to play a crucial role in the evolution of these galaxies.
However, to fully understand this process it is essential to resolve the
morphology and kinematics of such outflows. Using near-IR integral-field
spectroscopy obtained with VLT/SINFONI, we detect a kpc-scale structure of
high-velocity molecular hydrogen (H2) gas associated with the deeply buried
secondary nucleus of the IR-luminous merger NGC3256. We show that this
structure is likely the hot component of a molecular outflow, which is detected
also in the cold molecular gas by Sakamoto et al. This outflow, with a
molecular gas mass of M(H2)~2x10^7 Msun, is among the first to be spatially
resolved in both the hot H2 gas with VLT/SINFONI and the cold CO-emitting gas
with ALMA. The hot and cold components share a similar morphology and
kinematics, with a hot-to-cold molecular gas mass ratio of ~6x10^-5. The high
(~100 pc) resolution at which we map the geometry and velocity structure of the
hot outflow reveals a biconical morphology with opening angle ~40 deg and gas
spread across a FWZI~1200 km/s. Because this collimated outflow is oriented
close to the plane of the sky, the molecular gas may reach maximum intrinsic
outflow velocities of ~1800 km/s, with an average mass outflow rate of at least
~20 Msun/yr. By modeling the line-ratios of various near-IR H2 transitions, we
show that the H2 gas in the outflow is heated through shocks or X-rays to a
temperature of ~1900K. The energy needed to drive the outflow is likely
provided by a hidden Compton-thick AGN or by the nuclear starburst. We show
that the global kinematics of the molecular outflow in NGC3256 mimic those of
CO-outflows that have been observed at low spatial resolution in starburst- and
active galaxies.Comment: Accepted in Astronomy and Astrophysics (accepted 29 Aug 2014 v.3,
initial submission v.1 14 March 2014), 13 pages, 8 figure
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