17,580 research outputs found
Complexity and anisotropy in host morphology make populations safer against epidemic outbreaks
One of the challenges in epidemiology is to account for the complex
morphological structure of hosts such as plant roots, crop fields, farms,
cells, animal habitats and social networks, when the transmission of infection
occurs between contiguous hosts. Morphological complexity brings an inherent
heterogeneity in populations and affects the dynamics of pathogen spread in
such systems. We have analysed the influence of realistically complex host
morphology on the threshold for invasion and epidemic outbreak in an SIR
(susceptible-infected-recovered) epidemiological model. We show that disorder
expressed in the host morphology and anisotropy reduces the probability of
epidemic outbreak and thus makes the system more resistant to epidemic
outbreaks. We obtain general analytical estimates for minimally safe bounds for
an invasion threshold and then illustrate their validity by considering an
example of host data for branching hosts (salamander retinal ganglion cells).
Several spatial arrangements of hosts with different degrees of heterogeneity
have been considered in order to analyse separately the role of shape
complexity and anisotropy in the host population. The estimates for invasion
threshold are linked to morphological characteristics of the hosts that can be
used for determining the threshold for invasion in practical applications.Comment: 21 pages, 8 figure
A dynamical study of Galactic globular clusters under different relaxation conditions
We perform a systematic combined photometric and kinematic analysis of a
sample of globular clusters under different relaxation conditions, based on
their core relaxation time (as listed in available catalogs), by means of two
well-known families of spherical stellar dynamical models. Systems
characterized by shorter relaxation time scales are expected to be better
described by isotropic King models, while less relaxed systems might be
interpreted by means of non-truncated, radially-biased anisotropic f^(\nu)
models, originally designed to represent stellar systems produced by a violent
relaxation formation process and applied here for the first time to the study
of globular clusters. The comparison between dynamical models and observations
is performed by fitting simultaneously surface brightness and velocity
dispersion profiles. For each globular cluster, the best-fit model in each
family is identified, along with a full error analysis on the relevant
parameters. Detailed structural properties and mass-to-light ratios are also
explicitly derived. We find that King models usually offer a good
representation of the observed photometric profiles, but often lead to less
satisfactory fits to the kinematic profiles, independently of the relaxation
condition of the systems. For some less relaxed clusters, f^(\nu) models
provide a good description of both observed profiles. Some derived structural
characteristics, such as the total mass or the half-mass radius, turn out to be
significantly model-dependent. The analysis confirms that, to answer some
important dynamical questions that bear on the formation and evolution of
globular clusters, it would be highly desirable to acquire larger numbers of
accurate kinematic data-points, well distributed over the cluster field.Comment: 18 pages, 7 figures. Accepted for publication in Astronomy &
Astrophysic
Sterile neutrino production via active-sterile oscillations: the quantum Zeno effect
We study several aspects of the kinetic approach to sterile neutrino
production via active-sterile mixing. We obtain the neutrino propagator in the
medium including self-energy corrections up to , from which
we extract the dispersion relations and damping rates of the propagating modes.
The dispersion relations are the usual ones in terms of the index of refraction
in the medium, and the damping rates are where
is the active neutrino scattering rate and
is the mixing angle in the medium. We provide a generalization of
the transition probability in the \emph{medium from expectation values in the
density matrix}: and
study the conditions for its quantum Zeno suppression directly in real time. We
find the general conditions for quantum Zeno suppression, which for sterile neutrinos with \emph{may
only be} fulfilled near an MSW resonance. We discuss the implications for
sterile neutrino production and argue that in the early Universe the wide
separation of relaxation scales far away from MSW resonances suggests the
breakdown of the current kinetic approach.Comment: version to appear in JHE
A Revised Effective Temperature Scale for the Kepler Input Catalog
We present a catalog of revised effective temperatures for stars observed in
long-cadence mode in the Kepler Input Catalog (KIC). We use SDSS griz filters
tied to the fundamental temperature scale. Polynomials for griz
color-temperature relations are presented, along with correction terms for
surface gravity effects, metallicity, and statistical corrections for binary
companions or blending. We compare our temperature scale to the published
infrared flux method (IRFM) scale for VJKs in both open clusters and the Kepler
fields. We find good agreement overall, with some deviations between (J -
Ks)-based temperatures from the IRFM and both SDSS filter and other diagnostic
IRFM color-temperature relationships above 6000 K. For field dwarfs we find a
mean shift towards hotter temperatures relative to the KIC, of order 215 K, in
the regime where the IRFM scale is well-defined (4000 K to 6500 K). This change
is of comparable magnitude in both color systems and in spectroscopy for stars
with Teff below 6000 K. Systematic differences between temperature estimators
appear for hotter stars, and we define corrections to put the SDSS temperatures
on the IRFM scale for them. When the theoretical dependence on gravity is
accounted for we find a similar temperature scale offset between the
fundamental and KIC scales for giants. We demonstrate that statistical
corrections to color-based temperatures from binaries are significant. Typical
errors, mostly from uncertainties in extinction, are of order 100 K.
Implications for other applications of the KIC are discussed.Comment: Corrected for sign flip errors in the gravity corrections. Erratum to
this paper is attached in Appendix. Full version of revised Table 7 can be
found at http://home.ewha.ac.kr/~deokkeun/kic/sdssteff_v2.dat.g
Urban identity through quantifiable spatial attributes: coherence and dispersion of local identity through the automated comparative analysis of building block plans
This analysis investigates whether and to what degree quantifiable spatial attrib-utes, as expressed in plan representations, can capture elements related to the ex-perience of spatial identity. By combining different methods of shape and spatial analysis it attempts to quantify spatial attributes, predominantly derived from plans, in order to illustrate patterns of interrelations between spaces through an ob-jective automated process. The study focuses on the scale of the urban block as the basic modular unit for the formation of urban configurations and the issue of spa-tial identity is perceived through consistency and differentiation within and amongst urban neighbourhoods
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Effective Hamiltonian for transition-metal compounds. Application to Na_xCoO_2
We describe a simple scheme to construct a low-energy effective Hamiltonian
H_eff for highly correlated systems containing non-metals like O, P or As (O in
what follows) and a transition-metal (M) as the active part in the electronic
structure, eliminating the O degrees of freedom from a starting Hamiltonian
that contains all M d orbitals and all non-metal p orbitals. We calculate all
interaction terms between d electrons originating from Coulomb repulsion, as a
function of three parameters (F_0, F_2 and F_4) and write them in a basis of
orbitals appropriate for cubic, tetragonal, tetrahedral or hexagonal symmetry
around M. The approach is based on solving exactly (numerically if necessary) a
MO_n cluster containing the transition-metal atom and its n nearest O atoms
(for example a CoO_6 cluster in the case of the cobaltates, or a CuO_n cluster
in the case of the cuprates, in which n depends on the number of apical O
atoms), and mapping them into many-body states of the same symmetry containing
d holes only. We illustrate the procedure for the case of Na_xCoO_2. The
resulting H_eff, including a trigonal distortion D, has been studied recently
and its electronic structure agrees well with angle-resolved photoemission
spectra [A. Bourgeois, A. A. Aligia, and M. J. Rozenberg, Phys. Rev. Lett. 102,
066402 (2009)]. Although H_eff contains only 3d t_2g holes, the highly
correlated states that they represent contain an important amount not only of O
2p holes but also of 3d e_g holes. When more holes are added, a significant
redistribution of charge takes place. As a consequence of these facts, the
resulting values of the effective interactions between t_2g states are smaller
than previously assumed, rendering more important the effect of D in obtaining
only one sheet around the center of the Brillouin zone for the Fermi surface
(without additional pockets).Comment: 11 pages, 1 figure, accepted for publication in Phys.Rev.
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