1,242 research outputs found
Local dynamics and gravitational collapse of a self-gravitating magnetized Fermi gas
We use the Bianchi-I spacetime to study the local dynamics of a magnetized
self-gravitating Fermi gas. The set of Einstein-Maxwell field equations for
this gas becomes a dynamical system in a 4-dimensional phase space. We consider
a qualitative study and examine numeric solutions for the degenerate zero
temperature case. All dynamic quantities exhibit similar qualitative behavior
in the 3-dimensional sections of the phase space, with all trajectories
reaching a stable attractor whenever the initial expansion scalar H_{0} is
negative. If H_{0} is positive, and depending on initial conditions, the
trajectories end up in a curvature singularity that could be isotropic(singular
"point") or anisotropic (singular "line"). In particular, for a sufficiently
large initial value of the magnetic field it is always possible to obtain an
anisotropic type of singularity in which the "line" points in the same
direction of the field.Comment: 6 pages, 3 figures (accepted in General Relativity and Gravitation
Pseudospectral versus finite-differences schemes in the numerical integration of stochastic models of surface growth
We present a comparison between finite differences schemes and a
pseudospectral method applied to the numerical integration of stochastic
partial differential equations that model surface growth. We have studied, in
1+1 dimensions, the Kardar, Parisi and Zhang model (KPZ) and the Lai, Das Sarma
and Villain model (LDV). The pseudospectral method appears to be the most
stable for a given time step for both models. This means that the time up to
which we can follow the temporal evolution of a given system is larger for the
pseudospectral method. Moreover, for the KPZ model, a pseudospectral scheme
gives results closer to the predictions of the continuum model than those
obtained through finite difference methods. On the other hand, some numerical
instabilities appearing with finite difference methods for the LDV model are
absent when a pseudospectral integration is performed. These numerical
instabilities give rise to an approximate multiscaling observed in the
numerical simulations. With the pseudospectral approach no multiscaling is seen
in agreement with the continuum model.Comment: 13 single column pages, RevTeX, 6 eps fig
Kardar-Parisi-Zhang asymptotics for the two-dimensional noisy Kuramoto-Sivashinsky equation
We study numerically the Kuramoto-Sivashinsky (KS) equation forced by
external white noise in two space dimensions, that is a generic model for e.g.
surface kinetic roughening in the presence of morphological instabilities.
Large scale simulations using a pseudospectral numerical scheme allow us to
retrieve Kardar-Parisi-Zhang (KPZ) scaling as the asymptotic state of the
system, as in the 1D case. However, this is only the case for sufficiently
large values of the coupling and/or system size, so that previous conclusions
on non-KPZ asymptotics are demonstrated as finite size effects. Crossover
effects are comparatively stronger for the 2D case than for the 1D system.Comment: 5 pages, 3 figures; supplemental material available at journal web
page and/or on reques
Vertical Diffusivities of Active and Passive Tracers
The climate models that include a carbon-cycle need the vertical diffusivity of a passive tracer. Since an expression for the latter is not available, it has been common practice to identify it with that of salt. The identification is questionable since T, S are active, not passive tracers. We present the first derivation of the diffusivity of a passive tracer in terms of Ri (Richardson number) and Rq (density ratio, ratio of salinity over temperature z-gradients). The following results have emerged: (a) The passive tracer diffusivity is an algebraic function of Ri, Rq. (b) In doubly stable regimes (DS, partial derivative of T with respect to z > 0, partial derivative of S with respect to z 0. (c) In DC regimes (diffusive convection, partial derivative of T with respect to z 1), the passive scalar diffusivity is larger than that of salt. At Ri = O(1), it can be more than twice as large. (d) In SF regimes (salt fingers, partial derivative of T with respect to z > 0, partial derivative of S with respect to z > 0, Rq < 1), the passive scalar diffusivity is smaller than that of salt. At Ri = O(1), it can be less than half of it. (e) The passive tracer diffusivity predicted at the location of NATRE (North Atlantic Tracer Release Experiment) is discussed. (f) Perhaps the most relevant conclusion is that the common identification of the tracer diffusivity with that of salt is valid only in DS regimes. In the Southern Ocean, where there is the largest CO2 absorption, the dominant regime is diffusive convection discussed in (c) above
Theoretical study of turbulent channel flow: Bulk properties, pressure fluctuations, and propagation of electromagnetic waves
In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr--Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O--S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend stongly on the structure of the turbulence spectrun at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel
A Comparison of Measured Crab and Vela Glitch Healing Parameters with Predictions of Neutron Star Models
There are currently two well-accepted models that explain how pulsars exhibit
glitches, sudden changes in their regular rotational spin-down. According to
the starquake model, the glitch healing parameter, Q, which is measurable in
some cases from pulsar timing, should be equal to the ratio of the moment of
inertia of the superfluid core of a neutron star (NS) to its total moment of
inertia. Measured values of the healing parameter from pulsar glitches can
therefore be used in combination with realistic NS structure models as one test
of the feasibility of the starquake model as a glitch mechanism. We have
constructed NS models using seven representative equations of state of
superdense matter to test whether starquakes can account for glitches observed
in the Crab and Vela pulsars, for which the most extensive and accurate glitch
data are available. We also present a compilation of all measured values of Q
for Crab and Vela glitches to date which have been separately published in the
literature. We have computed the fractional core moment of inertia for stellar
models covering a range of NS masses and find that for stable NSs in the
realistic mass range 1.4 +/- 0.2 solar masses, the fraction is greater than
0.55 in all cases. This range is not consistent with the observational
restriction Q < 0.2 for Vela if starquakes are the cause of its glitches. This
confirms results of previous studies of the Vela pulsar which have suggested
that starquakes are not a feasible mechanism for Vela glitches. The much larger
values of Q observed for Crab glitches (Q > 0.7) are consistent with the
starquake model predictions and support previous conclusions that starquakes
can be the cause of Crab glitches.Comment: 8 pages, including 3 figures and 1 table. Accepted for publication in
Ap
Uncertainties of Synthetic Integrated Colors as Age Indicators
We investigate the uncertainties in the synthetic integrated colors of simple
stellar populations. Three types of uncertainties are from the stellar models,
the population synthesis techniques, and from the spectral libraries. Despite
some skepticism, synthetic colors appear to be reliable age indicators when
used for select age ranges. Rest-frame optical colors are good age indicators
at ages 2 -- 7Gyr. At ages sufficiently large to produce hot HB stars, the
UV-to-optical colors provide an alternative means for measuring ages. This UV
technique may break the age-metallicity degeneracy because it separates old
populations from young ones even in the lack of metallicity information. One
can use such techniques on extragalactic globular clusters and perhaps even for
high redshift galaxies that are passively evolving to study galaxy evolution
history.Comment: 38 pages, 21 figures, LaTex, 2003, ApJ, 582 (Jan 1), in pres
Effect of convective outer layers modeling on non-adiabatic seismic observables of delta Scuti stars
The identification of pulsation modes in delta Scuti stars is mandatory to
constrain the theoretical stellar models. The non-adiabatic observables used in
the photometric identification methods depend, however,on convection modeling
in the external layers. Our aim is to determine how the treatment of convection
in the atmospheric and sub-atmospheric layers affects the mode identification,
and what information about the thermal structure of the external layers can be
obtained from amplitude ratios and phase lags in Str\"omgren photometric bands.
We derive non-adiabatic parameters for delta Scuti stars by using, for the
first time, stellar models with the same treatment of convection in the
interior and in the atmosphere. We compute classical non-gray mixing length
models, and as well non-gray ``Full Spectrum of Turbulence'' models.
Furthermore, we compute the photometric amplitudes and phases of pulsation by
using the colors and the limb-darkening coefficents as derived from the same
atmosphere models used in the stellar modeling. We show that the non-adiabatic
phase-lag is mainly sensitive to the thermal gradients in the external layers,
(and hence to the treatment of convection), and that this sensitivity is also
clearly reflected in the multi-color photometric phase differences.Comment: 14 pag. 19 figs. accepted for publication in Astronomy and
Astrophysic
Compostos clorados: aspectos gerais e sua utilização como agente sanitizante na agricultura, micropropagação e pecuária.
Compostos clorados; Modo de ação dos compostos clorados; Fatores que Influenciam no processo de desinfecção; Utilizando compostos clorados; Tipos de compostos clorados e sua utilização; Compostos clorados de origemHipoclorito de sódio; Aspectos negativos mais comuns da utilização de compostos clorados como agentes sanitizantes e como contorná-los; Utilização de compostos clorados como agentes sanitizantes na agricultura; Utilização de compostos clorados como agentes sanitizantes na pecuária; Utilização de compostos clorados como agentes sanitizantes na micropropagação.bitstream/CPATSA/37828/1/SDC207.pd
- …