139 research outputs found
Rupture of a liposomal vesicle
We discuss pore dynamics in osmotically stressed vesicles. A set of equations
which govern the liposomal size, internal solute concentration, and pore
diameter is solved numerically. We find that dependent on the internal solute
concentration and vesicle size, liposomes can stay pore-free, nucleate a short
lived pore, or nucleate a long-lived pore. The phase diagram of pore stability
is constructed, and the different scaling regimes are deduced analytically.Comment: in press, Phys. Rev.
Ages of Elliptical Galaxies: Single versus Multi Population Interpretation
New calibrations of spectrophotometric indices of elliptical galaxies as
functions of spectrophotometric indices are presented, permitting estimates of
mean stellar population ages and metallicities. These calibrations are based on
evolutionary models including a two-phase interstellar medium, infall and a
galactic wind.Free parameters were fixed by requiring that models reproduce the
mean trend of data in the color-magnitude diagram as well as in the plane of
indices Hbeta-Mg2 and Mg2-. To improve the location of faint ellipticals(MB
> -20) in the Hbeta-Mg2 diagram, down-sizing was introduced. An application of
our calibrations to a sample of ellipticals and a comparison with results
derived from single stellar population models is given. Our models indicate
that mean population ages span an interval of 7-12 Gyr and are correlated with
metallicities, which range from approximately half up to three times solar.Comment: 10 pages and 6 figures. Accepted for publication in Monthly Notices
of the Royal Astronomical Society Main Journa
A Mean-Field Method for Generic Conductance-Based Integrate-and-Fire Neurons with Finite Timescales
The construction of transfer functions in theoretical neuroscience plays an
important role in determining the spiking rate behavior of neurons in networks.
These functions can be obtained through various fitting methods, but the
biological relevance of the parameters is not always clear. However, for
stationary inputs, such functions can be obtained without the adjustment of
free parameters by using mean-field methods. In this work, we expand current
Fokker-Planck approaches to account for the concurrent influence of colored and
multiplicative noise terms on generic conductance-based integrate-and-fire
neurons. We reduce the resulting stochastic system from the application of the
diffusion approximation to a one-dimensional Langevin equation. An effective
Fokker-Planck is then constructed using Fox Theory, which is solved numerically
to obtain the transfer function. The solution is capable of reproducing the
transfer function behavior of simulated neurons across a wide range of
parameters. The method can also be easily extended to account for different
sources of noise with various multiplicative terms, and it can be used in other
types of problems in principle.Comment: 11 pages, 6 figures, research articl
New UBVRI colour distributions in E-type galaxies I.The data
New colour distributions have been derived from wide field UBVRI frames for
36 northern bright elliptical galaxies and a few lenticulars. The classical
linear representations of colours against log r were derived, with some
improvements in the accuracy of the zero point colours and of the gradients.
The radial range of signicant measurements was enlarged both towards the
galaxian center and towards the outskirts of each object. Thus, the "central
colours", integrated within a radius of 3", and the "outermost colours"
averaged near the mu_V = 24 surface brightness, could also be obtained. Some
typical deviations of colour profiles from linearity are described.
Colour-colour relations of interest are presented. Very tight correlations are
found between the U-V colour and the Mg2 line-index, measured either at the
galaxian center or at the effective radius.Comment: 34 pages, 10 figures, to appear in A&A journa
Modeling microstructural effects in dilatational plasticity of polycrystalline materials
In a recent paper [1] we presented a new constitutive model for the viscoplastic response of polycrystalline aggregates accounting for local anisotropy induced by crystal plasticity and dilatational effects associated with the presence of intergranular cavities. In this contribution we provide a summary of our findings, as well as previously unpublished details of the numerical algorithm underlying this novel formulation. The formulation is based on homogenization and captures microstructural effects on the dilatational plastic behavior of polycrystalline materials. These effects are relevant to many engineering problems in which the presence of cavities embedded in a heterogeneous and anisotropic polycrystalline matrix must be accounted for, and for which standard polycrystalline models of incompressible plasticity, or dilatational plasticity formulations for voided materials with uniform properties of the matrix, have been proven to be insufficient. The present approach makes use of variational linearcomparison homogenization methods to develop constitutive models simultaneously accounting for texture of the matrix, porosity and average pore shape and orientation. The predictions of the models are compared with full-field numerical simulations based on fast Fourier transforms to study the influence of different microstructural features (e.g. overall porosity, single-crystal anisotropy, etc.) and triaxiality on the dilatational viscoplastic behavior of voided fcc polycrystals.Facultad de Ingenierí
Simulations of viscous shape relaxation in shuffled foam clusters
We simulate the shape relaxation of foam clusters and compare them with the
time exponential expected for Newtonian fluid. Using two-dimensional Potts
Model simulations, we artificially create holes in a foam cluster and shuffle
it by applying shear strain cycles. We reproduce the experimentally observed
time exponential relaxation of cavity shapes in the foam as a function of the
number of strain steps. The cavity rounding up results from local rearrangement
of bubbles, due to the conjunction of both a large applied strain and local
bubble wall fluctuations
Modeling microstructural effects in dilatational plasticity of polycrystalline materials
In a recent paper [1] we presented a new constitutive model for the viscoplastic response of polycrystalline aggregates accounting for local anisotropy induced by crystal plasticity and dilatational effects associated with the presence of intergranular cavities. In this contribution we provide a summary of our findings, as well as previously unpublished details of the numerical algorithm underlying this novel formulation. The formulation is based on homogenization and captures microstructural effects on the dilatational plastic behavior of polycrystalline materials. These effects are relevant to many engineering problems in which the presence of cavities embedded in a heterogeneous and anisotropic polycrystalline matrix must be accounted for, and for which standard polycrystalline models of incompressible plasticity, or dilatational plasticity formulations for voided materials with uniform properties of the matrix, have been proven to be insufficient. The present approach makes use of variational linearcomparison homogenization methods to develop constitutive models simultaneously accounting for texture of the matrix, porosity and average pore shape and orientation. The predictions of the models are compared with full-field numerical simulations based on fast Fourier transforms to study the influence of different microstructural features (e.g. overall porosity, single-crystal anisotropy, etc.) and triaxiality on the dilatational viscoplastic behavior of voided fcc polycrystals.Facultad de Ingenierí
The Metallicity of the Redshift 4.16 Quasar BR2248-1242
We estimate the metallicity in the broad emission-line region of the redshift
z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to
theoretical predictions. BR2248-1242 has unusually narrow emission lines with
large equivalent widths, thus providing a rare opportunity to measure several
line-ratio abundance diagnostics. The combined diagnostics indicate a
metallicity of ~2 times solar. This result suggests that an episode of vigorous
star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch.
The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for
H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence
for high metallicities and rapid star formation is consistent with the expected
early-epoch evolution of dense galactic nuclei.Comment: 8 pages, 3 figures. Prepared in AAStex. Submitted to the
Astrophysical Journal Revised version: added 1 referenc
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