6,772 research outputs found
Saddles and softness in simple model liquids
We report a numerical study of saddles properties of the potential energy
landscape for soft spheres with different softness, i.e. different power n of
the interparticle repulsive potential. We find that saddle-based quantities
rescale into master curves once energies and temperatures are scaled by
mode-coupling temperature T_MCT, confirming and generalizing previous findings
obtained for Lennard-Jones like models.Comment: 2 pages, 2 figure
Profiling Web Archive Coverage for Top-Level Domain and Content Language
The Memento aggregator currently polls every known public web archive when
serving a request for an archived web page, even though some web archives focus
on only specific domains and ignore the others. Similar to query routing in
distributed search, we investigate the impact on aggregated Memento TimeMaps
(lists of when and where a web page was archived) by only sending queries to
archives likely to hold the archived page. We profile twelve public web
archives using data from a variety of sources (the web, archives' access logs,
and full-text queries to archives) and discover that only sending queries to
the top three web archives (i.e., a 75% reduction in the number of queries) for
any request produces the full TimeMaps on 84% of the cases.Comment: Appeared in TPDL 201
Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo
In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through ß-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein
Spectral properties of a two-orbital Anderson impurity model across a non-Fermi liquid fixed point
We study by NRG the spectral properties of a two-orbital Anderson impurity
model in the presence of an exchange splitting which follows either regular or
inverted Hund's rules. The phase diagram contains a non-Fermi liquid fixed
point separating a screened phase, where conventional Kondo effect occurs, from
an unscreened one, where the exchange-splitting takes care of quenching the
impurity degrees of freedom. On the Kondo screened side close to this fixed
point the impurity density of states shows a narrow Kondo-peak on top of a
broader resonance. This narrow peak transforms in the unscreened phase into a
narrow pseudo-gap inside the broad resonance. Right at the fixed point only the
latter survives. The fixed point is therefore identified by a jump of the
density of states at the chemical potential. We also show that particle-hole
perturbations which simply shift the orbital energies do not wash out the fixed
point, unlike those perturbations which hybridize the two orbitals.
Consequently the density-of-state jump at the chemical potential remains finite
even away from particle-hole symmetry, and the pseudo-gap stays pinned at the
chemical potential, although it is partially filled in. We also discuss the
relevance of these results for lattice models which map onto this Anderson
impurity model in the limit of large lattice-coordination. Upon approaching the
Mott metal-insulator transition, these lattice models necessarily enter a
region with a local criticality which reflects the impurity non-Fermi liquid
fixed point. However, unlike the impurity, the lattice can get rid of the
single-impurity fixed-point instability by spontaneously developing
bulk-coherent symmetry-broken phases, which we identify for different lattice
models.Comment: 43 pages, 11 figures. Minor corrections in the Appendi
Impact of URI Canonicalization on Memento Count
Quantifying the captures of a URI over time is useful for researchers to
identify the extent to which a Web page has been archived. Memento TimeMaps
provide a format to list mementos (URI-Ms) for captures along with brief
metadata, like Memento-Datetime, for each URI-M. However, when some URI-Ms are
dereferenced, they simply provide a redirect to a different URI-M (instead of a
unique representation at the datetime), often also present in the TimeMap. This
infers that confidently obtaining an accurate count quantifying the number of
non-forwarding captures for a URI-R is not possible using a TimeMap alone and
that the magnitude of a TimeMap is not equivalent to the number of
representations it identifies. In this work we discuss this particular
phenomena in depth. We also perform a breakdown of the dynamics of counting
mementos for a particular URI-R (google.com) and quantify the prevalence of the
various canonicalization patterns that exacerbate attempts at counting using
only a TimeMap. For google.com we found that 84.9% of the URI-Ms result in an
HTTP redirect when dereferenced. We expand on and apply this metric to TimeMaps
for seven other URI-Rs of large Web sites and thirteen academic institutions.
Using a ratio metric DI for the number of URI-Ms without redirects to those
requiring a redirect when dereferenced, five of the eight large web sites' and
two of the thirteen academic institutions' TimeMaps had a ratio of ratio less
than one, indicating that more than half of the URI-Ms in these TimeMaps result
in redirects when dereferenced.Comment: 43 pages, 8 figure
Numerical modeling of the electron beam welding and its experimental validation
Electron Beam Welding (EBW) is a highly efficient and precise welding method increasingly used within the manufacturing chain and of growing importance in different industrial environments such as the aeronautical and aerospace sectors. This is because, compared to other welding processes, EBW induces lower distortions and residual stresses due to the lower and more focused heat input along the welding line.
This work describes the formulation adopted for the numerical simulation of the EBW process as well as the experimental work carried out to calibrate and validate it.
The numerical simulation of EBW involves the interaction of thermal, mechanical and metallurgical phenomena. For this reason, in this work the numerical framework couples the heat transfer process to the stress analysis to maximize accuracy. An in-house multi-physics FE software is used to deal with the numerical simulation. The definition of an ad hoc moving heat source is proposed to simulate the EB power surface distribution and the corresponding absorption within the work-piece thickness. Both heat conduction and heat radiation models are considered to dissipate the heat through the boundaries of the component. The material behavior is characterized by an apropos thermo-elasto-viscoplastic constitutive model. Titanium-alloy Ti6A14V is the target material of this work.
From the experimental side, the EB welding machine, the vacuum chamber characteristics and the corresponding operative setting are detailed. Finally, the available facilities to record the temperature evolution at different thermo-couple locations as well as to measure both distortions and residual stresses are described. Numerical results are compared with the experimental evidence.Peer ReviewedPostprint (author's final draft
The Planetary Nebulae Population in the Nuclear Regions of M31: the SAURON view
Following a first study of the central regions of M32 that illustrated the
power of integral-field spectroscopy (IFS) in detecting and measuring the [O
III]{\lambda}5007 emission of PNe against a strong stellar background, we turn
to the very nuclear PN population of M31, within 80 pc of its centre. We show
that PNe can also be found in the presence of emission from diffuse gas and
further illustrate the excellent sensitivity of IFS in detecting extragalactic
PNe through a comparison with narrowband images obtained with the Hubble Space
Telescope. We find the nuclear PNe population of M31 is only marginally
consistent with the generally adopted form of the PNe luminosity function
(PNLF). In particular, this is due to a lack of PNe with absolute magnitude
M5007 brighter than -3, which would only result from a rather unfortunate draw
from such a model PNLF. We suggest that the observed lack of bright PNe in the
nuclear regions of M31 is due to a horizontal-branch population that is more
tilted toward less massive and hotter He-burning stars, so that its progeny
consists mostly of UV-bright stars that fail to climb back up the asymptotic
giant branch (AGB) and only of few, if any, bright PNe powered by central
post-AGB stars. These results are also consistent with recent reports on a
dearth of bright post-AGB stars towards the nucleus of M31, and lend further
support to the idea that the metallicity of a stellar population has an impact
on the way the horizontal branch is populated and to the loose anticorrelation
between the strength of the UV-upturn and the specific number of PNe that is
observed in early-type galaxies. Finally, our investigation also serves to
stress the importance of considering the same spatial scales when comparing the
PNe population of galaxies with the properties of their stellar populations.Comment: 11 pages, 10 figures, accepted for publication on Monthly Notices of
the Royal Astronomical Societ
Dynamical and photometric imprints of feedback processes on the early evolution of E/S0 galaxies
We show that the observed Velocity Dispersion Function of E/S0 galaxies
matches strikingly well the distribution function of virial velocities of
massive halos virializing at z > 1.5, as predicted by the standard hierarchical
clustering scenario in a \LambdaCDM cosmology, for a constant ratio sigma/V_vir
= 0.55 \pm 0.05, close to the value expected at virialization if it typically
occurred at z > 3. This strongly suggests that dissipative processes and later
merging events had little impact on the matter density profile. Adopting the
above sigma/V_vir ratio, the observed relationships between photometric and
dynamical properties which define the fundamental plane of elliptical galaxies,
such as the luminosity-sigma (Faber-Jackson) and the luminosity-effective
radius relations, as well as the M_BH-sigma relation, are nicely reproduced.
Their shapes turn out to be determined by the mutual feedback of star-formation
(and supernova explosions)and nuclear activity, along the lines discussed by
Granato et al. (2004). To our knowledge, this is the first semi-analytic model
for which simultaneous fits of the fundamental plane relations and of the
epoch-dependent luminosity function of spheroidal galaxies have been presented.Comment: 14 pages, 6 figures, submitted to Ap
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