10,491 research outputs found
The connection between the Galactic halo and ancient Dwarf Satellites
We explore the hypothesis that the classical and ultra-faint dwarf spheroidal
satellites of the Milky Way have been the building blocks of the Galactic halo
by comparing their [O/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones
observed in Galactic halo stars. Oxygen abundances deviate substantially from
the observed abundances in the Galactic halo stars for [Fe/H] values larger
than -2 dex, while they overlap for lower metallicities. On the other hand, for
the [Ba/Fe] ratio the discrepancy is extended at all [Fe/H] values, suggesting
that the majority of stars in the halo are likely to have been formed in situ.
Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [O/Fe]
ratios. Moreover, we show the effects of an enriched infall of gas with the
same chemical abundances as the matter ejected and/or stripped from dwarf
satellites of the Milky Way on the chemical evolution of the Galactic halo. We
find that the resulting chemical abundances of the halo stars depend on the
assumed infall time scale, and the presence of a threshold in the gas for star
formation.Comment: To appear in Proceeding of Science: Frontier Research in Astrophysics
- II 23-28 May 2016 Mondello (Palermo), Ital
Hints for families of GRBs improving the Hubble diagram
As soon as their extragalactic origins were established, the hope to make
Gamma - Ray Bursts (GRBs) standardizeable candles to probe the very high - z
universe has opened the search for scaling relations between redshift
independent observable quantities and distance dependent ones. Although some
remarkable success has been achieved, the empirical correlations thus found are
still affected by a significant intrinsic scatter which downgrades the
precision in the inferred GRBs Hubble diagram. We investigate here whether this
scatter may come from fitting together objects belonging to intrinsically
different classes. To this end, we rely on a cladistics analysis to partition
GRBs in homogenous families according to their rest frame properties. Although
the poor statistics prevent us from drawing a definitive answer, we find that
both the intrinsic scatter and the coefficients of the \,-\,
and \,-\, correlations significantly change depending on which
subsample is fitted. It turns out that the fit to the full sample leads to a
scaling relation which approximately follows the diagonal of the region
delimited by the fits to each homogenous class. We therefore argue that a
preliminary identification of the class a GRB belongs to is necessary in order
to select the right scaling relation to be used in order to not bias the
distance determination and hence the Hubble diagram.Comment: 10 pages, 6 figures, 4 tables, accepted for publication on MNRA
Cosmological parameters from lenses distance ratio
Strong lensing provides popular techniques to investigate the mass
distribution of intermediate redshift galaxies, testing galaxy evolution and
formation scenarios. It especially probes the background cosmic expansion,
hence constraining cosmological parameters. The measurement of Einstein radii
and central velocity dispersions indeed allows to trace the ratio D_s/D_ls
between the distance D_s from the observer to the source and the distance D_ls
from the lens to the source. We present an improved method to explicitly
include the two - component structure in the galaxy lens modeling, in order to
analyze the role played by the redshift and the model dependence on a nuisance
parameter, F_E, which is usually marginalized in the cosmological applications.
We show how to deal with these problems and carry on a Fisher matrix analysis
to infer the accuracy on cosmological parameters achieved by this method.Comment: 8 pages, 2 figures, 2 tables, accepted for publication on Monthly
Notices of Royal Astronomical Societ
An updated analysis of two classes of f(R) theories of gravity
The observed accelerated cosmic expansion can be a signature of
fourth\,-\,order gravity theories, where the acceleration of the Universe is a
consequence of departures from Einstein General Relativity, rather than the
sign of the existence of a fluid with negative pressure. In the
fourth\,-\,order gravity theories, the gravity Lagrangian is described by an
analytic function of the scalar curvature subject to the demanding
conditions that no detectable deviations from standard GR is observed on the
Solar System scale. Here we consider two classes of theories able to
pass Solar System tests and investigate their viability on cosmological scales.
To this end, we fit the theories to a large dataset including the combined
Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter
data from passively evolving red galaxies, Baryon Acoustic Oscillations
extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS)
and the distance priors from the Wilkinson Microwave Anisotropy Probe seven
years (WMAP7) data. We find that both classes of fit very well this
large dataset with the present\,-\,day values of the matter density, Hubble
constant and deceleration parameter in agreement with previous estimates;
however, the strong degeneracy among the parameters prevents us from
strongly constraining their values. We also derive the growth factor , with the matter density
perturbation, and show that it can still be well approximated by . We finally constrain (on some representative
scales) and investigate its redshift dependence to see whether future data can
discriminate between these classes of theories and standard dark energy
models.Comment: 27 pages, 5 figures, 1 table, accepted for publication on JCAP. Note
that this paper updates and supersedes preprint arXiv:0907.468
K -> pi pi Phenomenology in the Presence of Electromagnetism
We describe the influence of electromagnetism on the phenomenology of K -> pi
pi decays. This is required because the present data were analyzed without
inclusion of electromagnetic radiative corrections, and hence contain several
ambiguities and uncertainties which we describe in detail. Our presentation
includes a full description of the infrared effects needed for a new
experimental analysis. It also describes the general treatment of final state
interaction phases, needed because Watson's theorem is no longer valid in the
presence of electromagnetism. The phase of the isospin-two amplitude A_2 may be
modified by 50% -> 100%. We provide a tentative analysis using present data in
order to illustrate the sensitivity to electromagnetic effects, and also
discuss how the standard treatment of epsilon'/epsilon is modified.Comment: 25 pages, 6 figure
Cosmography of f(R) - brane cosmology
Cosmography is a useful tool to constrain cosmological models, in particular
dark energy models. In the case of modified theories of gravity, where the
equations of motion are generally quite complicated, cosmography can contribute
to select realistic models without imposing arbitrary choices a priori. Indeed,
its reliability is based on the assumptions that the universe is homogeneous
and isotropic on large scale and luminosity distance can be "tracked" by the
derivative series of the scale factor a(t). We apply this approach to induced
gravity brane-world models where an f(R)-term is present in the brane effective
action. The virtue of the model is to self-accelerate the normal and healthy
DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We
show that the model, coming from a fundamental theory, is consistent with the
LCDM scenario at low redshift. We finally estimate the cosmographic parameters
fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors
from Baryon Acoustic Oscillations (BAO) and then provide constraints on the
present day values of f(R) and its second and third derivatives.Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Design of Wireless Sensor Nodes for Structural Health Monitoring applications
Enabling low-cost distributed monitoring, wireless sensor networks represents an interesting solution for the implementation of
structural health monitoring systems. This work deals with the design of wireless sensor networks for health monitoring of civil
structures, specifically focusing on node design in relation to the requirements of different structural monitoring application classes.
Design problems are analysed with specific reference to a large-scale experimental setup (the long-term structural monitoring of
the Basilica S. Maria di Collemaggio, L’Aquila, Italy). Main limitations emerged are highlighted, and adopted solution strategies
are outlined, both in the case of commercial sensing platform and of full custom solutions
- …