4,087 research outputs found
Segregation in a fluidized binary granular mixture: Competition between buoyancy and geometric forces
Starting from the hydrodynamic equations of binary granular mixtures, we
derive an evolution equation for the relative velocity of the intruders, which
is shown to be coupled to the inertia of the smaller particles. The onset of
Brazil-nut segregation is explained as a competition between the buoyancy and
geometric forces: the Archimedean buoyancy force, a buoyancy force due to the
difference between the energies of two granular species, and two geometric
forces, one compressive and the other-one tensile in nature, due to the
size-difference. We show that inelastic dissipation strongly affects the phase
diagram of the Brazil nut phenomenon and our model is able to explain the
experimental results of Breu et al. (PRL, 2003, vol. 90, p. 01402).Comment: 5 pages, 2 figure
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism
An attractive diagnostic tool for investigating the magnetism of the solar
chromosphere is the observation and theoretical modeling of the Hanle and
Zeeman effects in spicules, as shown in this letter for the first time. Here we
report on spectropolarimetric observations of solar chromospheric spicules in
the He I 10830 \AA multiplet and on their theoretical modeling accounting for
radiative transfer effects. We find that the magnetic field in the observed
(quiet Sun) spicular material at a height of about 2000 km above the visible
solar surface has a strength of the order of 10 G and is inclined by
approximately with respect to the local vertical direction. Our
empirical finding based on full Stokes-vector spectropolarimetry should be
taken into account in future magnetohydrodynamical simulations of spicules.Comment: 12 pages and 2 figure
Putting hydrodynamic interactions to work: tagged particle separation
Separation of magnetically tagged cells is performed by attaching markers to
a subset of cells in suspension and applying fields to pull from them in a
variety of ways. The magnetic force is proportional to the field gradient, and
the hydrodynamic interactions play only a passive, adverse role. Here we
propose using a homogeneous rotating magnetic field only to make tagged
particles rotate, and then performing the actual separation by means of
hydrodynamic interactions, which thus play an active role. The method, which we
explore here theoretically and by means of numerical simulations, lends itself
naturally to sorting on large scales.Comment: Version accepted for publication - Europhysics Letter
Restrictions to the galaxy evolutionary models from the Hawaiian Deep Fields SSA13 and SSA22
Quantitative structural analysis of the galaxies present in the Hawaiian Deep Fields SSA13 and SSA22 is reported. The structural parameters of the galaxies have been obtained automatically by fitting a two-component model (Sérsic r1/n bulge and exponential disc) to the surface brightness of the galaxies. The galaxies were classified on the basis of the bulge-to-total luminosity ratio (B/T). The magnitude selection criteria and the reliability of our method have been checked by using Monte Carlo simulations. A complete sample of objects up to redshift 0.8 has been achieved. Spheroidal objects (E/S0) represent ≈33 per cent and spirals ≈41 per cent of the total number of galaxies, while mergers and unclassified objects represent ≈26 per cent. We have computed the comoving space density of the different kinds of object. In an Einstein-de Sitter universe, a decrease in the comoving density of E/S0 galaxies is observed as redshift increases (≈30 per cent less at z=0.8), while for spiral galaxies a relatively quiet evolution is reported. The framework of hierarchical clustering evolution models of galaxies seems to be the most appropriate to explain our result
- …