4,661 research outputs found
Grain alignment by ferromagnetic impurities
The observed wavelength dependence of linear polarization, and its variation from region to region can be explained by the following assumptions. Interstellar grains resemble interplanetary grains, in that they are composed of collections of small particles coagulated together into elongated masses. A fraction of the small particles are ferromagnetic. Presumably these are either metallic Fe or magnetite, Fe3O4. If and only if a large grain contains one or more magnetic particles is the grain aligned in the galactic magnetic field. The magnetic particles stick only to silicate grains because of chemical similarities, or (equivalently) any pure carbon grains in the diffuse interstellar medium (ISM) are too spherical to produce polarization. Grains in dense regions, such as the outer parts of molecular clouds, are larger than those in the diffuse ISM because of coagulation of the grains rather than accretion of icy mantles. These regions are known to have larger than normal values of lambda (max), the wavelength of the maximum of linear polarization. The above assumptions are sufficient to allow the calculation of the wavelength dependence of the polarization
Transport and mixing in the radiation zones of rotating stars: I-Hydrodynamical processes
The purpose of this paper is to improve the modelization of the rotational
mixing which occurs in stellar radiation zones, through the combined action of
the thermally driven meridional circulation and of the turbulence generated by
the shear of differential rotation. The turbulence is assumed to be
anisotropic, due to the stratification, with stronger transport in the
horizontal directions than in the vertical. The main difference with the former
treatments by Zahn (1992) and Maeder & Zahn (1998) is that we expand here the
departures from spherical symmetry to higher order, and include explicitly the
differential rotation in latitude, to first order. This allows us to treat
simultaneously the bulk of a radiation zone and its tachocline(s). Moreover, we
take fully into account the non-stationarity of the problem, which will enable
us to tackle the rapid phases of evolution. The system of partial differential
equations, which govern the transport of angular momentum, heat and chemical
elements, is written in a form which makes it ready to implement in a stellar
evolution code. Here the effect of a magnetic field is deliberately ignored; it
will be included in forthcoming papers.Comment: 16 pages, no figures, accepted for publication in A&
Estimating Column Density in Molecular Clouds with FIR and Sub-mm Emission Maps
We have used a numerical simulation of a turbulent cloud to synthesize maps
of the thermal emission from dust at a variety of far-IR and sub-mm
wavelengths. The average column density and external radiation field in the
simulation is well matched to clouds such as Perseus and Ophiuchus. We use
pairs of single-wavelength emission maps to derive the dust color temperature
and column density, and we compare the derived column densities with the true
column density. We demonstrate that longer wavelength emission maps yield less
biased estimates of column density than maps made towards the peak of the dust
emission spectrum. We compare the scatter in the derived column density with
the observed scatter in Perseus and Ophiuchus. We find that while in Perseus
all of the observed scatter in the emission-derived versus the
extinction-derived column density can be attributed to the flawed assumption of
isothermal dust along each line of sight, in Ophiuchus there is additional
scatter above what can be explained by the isothermal assumption. Our results
imply that variations in dust emission properties within a molecular cloud are
not necessarily a major source of uncertainty in column density measurements.Comment: Accepted to ApJ Letter
Dependence of Gas Phase Abundances in the ISM on Column Density
Sightlines through high- and intermediate-velocity clouds allow measurements
of ionic gas phase abundances, A, at very low values of HI column density,
N(HI). Present observations cover over 4 orders of magnitude in N(HI).
Remarkably, for several ions we find that the A vs N(HI) relation is the same
at high and low column density and that the abundances have a relatively low
dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have
slightly higher values of A than disk gas at the same N(HI), suggesting that
part of the dispersion may be attributed to the environment. We note that the
dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to
large errors. Important implications of the low dispersions regarding the
physical nature of the ISM are: (a) because of clumping, over sufficiently long
pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the
H atoms along the sight line; (b) the destruction of grains does not mainly
take place in catastrophic events such as strong shocks, but is a continuous
function of the mean density; (c) the cycling of the ions becoming attached to
grains and being detached must be rapid, and the two rates must be roughly
equal under a wide variety of conditions; (d) in gas that has a low average
density the attachment should occur within denser concentrations
Unified derivation of phase-field models for alloy solidification from a grand-potential functional
In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness.Comment: 36 pages, 1 figur
Reconstruction of primordial density fields
The Monge-Ampere-Kantorovich (MAK) reconstruction is tested against
cosmological N-body simulations. Using only the present mass distribution
sampled with particles, and the assumption of homogeneity of the primordial
distribution, MAK recovers for each particle the non-linear displacement field
between its present position and its Lagrangian position on a primordial
uniform grid. To test the method, we examine a standard LCDM N-body simulation
with Gaussian initial conditions and 6 models with non-Gaussian initial
conditions: a chi-squared model, a model with primordial voids and four weakly
non-Gaussian models. Our extensive analyses of the Gaussian simulation show
that the level of accuracy of the reconstruction of the nonlinear displacement
field achieved by MAK is unprecedented, at scales as small as about 3 Mpc. In
particular, it captures in a nontrivial way the nonlinear contribution from
gravitational instability, well beyond the Zel'dovich approximation. This is
also confirmed by our analyses of the non-Gaussian samples. Applying the
spherical collapse model to the probability distribution function of the
divergence of the displacement field, we also show that from a
well-reconstructed displacement field, such as that given by MAK, it is
possible to accurately disentangle dynamical contributions induced by
gravitational clustering from possible initial non-Gaussianities, allowing one
to efficiently test the non-Gaussian nature of the primordial fluctuations. In
addition, a simple application of MAK using the Zel'dovich approximation allows
one to also recover accurately the present-day peculiar velocity field on
scales of about 8 Mpc.Comment: Version to appear in MNRAS, 24 pages, 21 figures appearing (uses 35
figure files), 1 tabl
The surface signature of the tidal dissipation of the core in a two-layer planet
Tidal dissipation, which is directly linked to internal structure, is one of
the key physical mechanisms that drive systems evolution and govern their
architecture. A robust evaluation of its amplitude is thus needed to predict
evolution time for spins and orbits and their final states. The purpose of this
paper is to refine recent model of the anelastic tidal dissipation in the
central dense region of giant planets, commonly assumed to retain a large
amount of heavy elements, which constitute an important source of dissipation.
The previous paper evaluated the impact of the presence of the static fluid
envelope on the tidal deformation of the core and on the associated anelastic
tidal dissipation, through the tidal quality factor Qc. We examine here its
impact on the corresponding effective anelastic tidal dissipation, through the
effective tidal quality factor Qp. We show that the strength of this mechanism
mainly depends on mass concentration. In the case of Jupiter- and Saturn-like
planets, it can increase their effective tidal dissipation by, around, a factor
2.4 and 2 respectively. In particular, the range of the rheologies compatible
with the observations is enlarged compared to the results issued from previous
formulations. We derive here an improved expression of the tidal effective
factor Qp in terms of the tidal dissipation factor of the core Qc, without
assuming the commonly used assumptions. When applied to giant planets, the
formulation obtained here allows a better match between the an elastic core's
tidal dissipation of a two-layer model and the observations.Comment: 5 pages, 2 figures, Accepted for publication in Astronomy &
Astrophysic
In situ ozone data for evaluation of the laser absorption spectrometer ozone remote sensor: 1979 southeastern Virginia urban plume study summer field program
Ozone data from the 1979 Southeastern Virginia Urban Study (SEV-UPS) field program are presented. The SEV-UPS was conducted for evaluation of an ozone remote sensor, the Laser Absorption Spectrometer. During the measurement program, remote-sensor evaluation was in two areas; (1) determination of the remote sensor's accuracy, repeatability, and operational characteristics, and (2) demonstration of the application of remotely sensed ozone data in air-quality studies. Data from six experiments designed to provide in situ ozone data for evaluation of the sensor in area 1, above, are presented. Experiments consisted of overflights of a test area with the remote sensor aircraft while in situ measurements with a second aircraft and selected surface stations provided correlative ozone data within the viewing area of the remote sensor
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