9,739 research outputs found
A possible theoretical explanation of metallicity gradients in elliptical galaxies
Models of chemical evolution of elliptical galaxies taking into account
different escape velocities at different galactocentric radii are presented. As
a consequence of this, the chemical evolution develops differently in different
galactic regions; in particular, we find that the galactic wind, powered by
supernovae (of type II and I) starts, under suitable conditions, in the outer
regions and successively develops in the central ones. The rate of star
formation (SFR) is assumed to stop after the onset of the galactic wind in each
region. The main result found in the present work is that this mechanism is
able to reproduce metallicity gradients, namely the gradients in the
index, in good agreement with observational data. We also find that in order to
honor the constant [Mg/Fe] ratio with galactocentric distance, as inferred from
metallicity indices, a variable initial mass function as a function of
galactocentric distance is required. This is only a suggestion since trends on
abundances inferred just from metallicity indices are still uncertain.Comment: 18 pages, LaTeX file with 4 figures using mn.sty, submitted to MNRA
Logical Specification and Analysis of Fault Tolerant Systems through Partial Model Checking
This paper presents a framework for a logical characterisation of fault tolerance and its formal analysis based on partial model checking techniques. The framework requires a fault tolerant system to be modelled using a formal calculus, here the CCS process algebra. To this aim we propose a uniform modelling scheme in which to specify a formal model of the system, its failing behaviour and possibly its fault-recovering procedures. Once a formal model is provided into our scheme, fault tolerance - with respect to a given property - can be formalized as an equational ”-calculus formula. This formula expresses in a logic formalism, all the fault scenarios satisfying that fault tolerance property. Such a characterisation understands the analysis of fault tolerance as a form of analysis of open systems and thank to partial model checking strategies, it can be made independent on any particular fault assumption. Moreover this logical characterisation makes possible the fault-tolerance verification problem be expressed as a general ”-calculus validation problem, for solving which many theorem proof techniques and tools are available. We present several analysis methods showing the flexibility of our approach
The phase diagrams of iron-based superconductors: theory and experiments
Phase diagrams play a primary role in the understanding of materials
properties. For iron-based superconductors (Fe-SC), the correct definition of
their phase diagrams is crucial because of the close interplay between their
crystallo-chemical and magnetic properties, on one side, and the possible
coexistence of magnetism and superconductivity, on the other. The two most
difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin
of the structural transformation taking place during cooling and its
relationship with magnetism; 2) the correct description of the region where a
crossover between the magnetic and superconducting electronic ground states
takes place. Hence a proper and accurate definition of the structural, magnetic
and electronic phase boundaries provides an extremely powerful tool for
material scientists. For this reason, an exact definition of the thermodynamic
phase fields characterizing the different structural and physical properties
involved is needed, although it is not easy to obtain in many cases. Moreover,
physical properties can often be strongly dependent on the occurrence of
micro-structural and other local-scale features (lattice micro-strain, chemical
fluctuations, domain walls, grain boundaries, defects), which, as a rule, are
not described in a structural phase diagram. In this review, we critically
summarize the results for the most studied 11-, 122- and 1111-type compound
systems, providing a correlation between experimental evidence and theory
Linear feedback control of transient energy growth and control performance limitations in subcritical plane Poiseuille flow
Suppression of the transient energy growth in subcritical plane Poiseuille
flow via feedback control is addressed. It is assumed that the time derivative
of any of the velocity components can be imposed at the walls as control input,
and that full-state information is available. We show that it is impossible to
design a linear state-feedback controller that leads to a closed-loop flow
system without transient energy growth.
In a subsequent step, full-state feedback controllers -- directly targeting
the transient growth mechanism -- are designed, using a procedure based on a
Linear Matrix Inequalities approach. The performance of such controllers is
analyzed first in the linear case, where comparison to previously proposed
linear-quadratic optimal controllers is made; further, transition thresholds
are evaluated via Direct Numerical Simulations of the controlled
three-dimensional Poiseuille flow against different initial conditions of
physical interest, employing different velocity components as wall actuation.
The present controllers are effective in increasing the transition thresholds
in closed loop, with varying degree of performance depending on the initial
condition and the actuation component employed
Electromagnetic and strong isospin-breaking corrections to the muon from Lattice QCD+QED
We present a lattice calculation of the leading-order electromagnetic and
strong isospin-breaking corrections to the hadronic vacuum polarization (HVP)
contribution to the anomalous magnetic moment of the muon. We employ the gauge
configurations generated by the European Twisted Mass Collaboration (ETMC) with
dynamical quarks at three values of the lattice spacing ( fm) with pion masses between and
MeV. The results are obtained adopting the RM123 approach in the
quenched-QED approximation, which neglects the charges of the sea quarks. Quark
disconnected diagrams are not included. After the extrapolations to the
physical pion mass and to the continuum and infinite-volume limits the
contributions of the light, strange and charm quarks are respectively equal to
, and . At leading order in and we obtain , which is currently the most accurate determination of the
isospin-breaking corrections to .Comment: 23 pages, 7 figures, 5 tables. Version to appear in PRD. A bug in the
update of the strange and charm contributions is removed and an extended
discussion on the identification of the ground-state is included. arXiv admin
note: text overlap with arXiv:1808.00887, arXiv:1707.0301
Electromagnetic corrections to leptonic decay rates of charged pseudoscalar mesons: finite-volume effects
In Carrasco et al. we have recently proposed a method to calculate
electromagnetic corrections to leptonic decay widths of pseudoscalar mesons.
The method is based on the observation that the infrared divergent
contributions (that appear at intermediate stages of the calculation and that
cancel in physical quantities thanks to the Bloch-Nordsieck mechanism) are
universal, i.e. depend on the charge and the mass of the meson but not on its
internal structure. In this talk we perform a detailed analysis of the
finite-volume effects associated with our method. In particular we show that
also the leading finite-volume effects are universal and perform an
analytical calculation of the finite-volume leptonic decay rate for a
point-like meson
Finite-Volume QED Corrections to Decay Amplitudes in Lattice QCD
We demonstrate that the leading and next-to-leading finite-volume effects in
the evaluation of leptonic decay widths of pseudoscalar mesons at
are universal, i.e. they are independent of the structure of the meson. This is
analogous to a similar result for the spectrum but with some fundamental
differences, most notably the presence of infrared divergences in decay
amplitudes. The leading non-universal, structure-dependent terms are of
(compared to the leading non-universal corrections in the
spectrum). We calculate the universal finite-volume effects, which requires an
extension of previously developed techniques to include a dependence on an
external three-momentum (in our case, the momentum of the final state lepton).
The result can be included in the strategy proposed in
Ref.\,\cite{Carrasco:2015xwa} for using lattice simulations to compute the
decay widths at , with the remaining finite-volume effects starting
at order . The methods developed in this paper can be generalised to
other decay processes, most notably to semileptonic decays, and hence open the
possibility of a new era in precision flavour physics
- âŠ