8,595 research outputs found
On the Sensitivity of 3-D Thermal Convection Codes to Numerical Discretization: A Model Intercomparison
Fully 3-D numerical simulations of thermal convection in a spherical shell have become a standard for studying the dynamics of pattern formation and its stability under perturbations to various parameter values. The question arises as to how does the discretization of the governing equations affect the outcome and thus any physical interpretation. This work demonstrates the impact of numerical discretization on the observed patterns, the value at which symmetry is broken, and how stability and stationary behavior is dependent upon it. Motivated by numerical simulations of convection in the Earth\u27s mantle, we consider isoviscous Rayleigh-Bénard convection at infinite Prandtl number, where the aspect ratio between the inner and outer shell is 0.55. We show that the subtleties involved in development mantle convection models are considerably more delicate than has been previously appreciated, due to the rich dynamical behavior of the system. Two codes with different numerical discretization schemes: an established, community-developed, and benchmarked finite element code (CitcomS) and a novel spectral method that combines Chebyshev polynomials with radial basis functions (RBF) are compared. A full numerical study is investigated for the following three cases. The first case is based on the cubic (or octahedral) initial condition (spherical harmonics of degree ℓ =4). How variations in the behavior of the cubic pattern to perturbations in the initial condition and Rayleigh number between the two numerical discrezations is studied. The second case investigates the stability of the dodecahedral (or icosahedral) initial condition (spherical harmonics of degree ℓ = 6). Although both methods converge first to the same pattern, this structure is ultimately unstable and systematically degenerates to cubic or tetrahedral symmetries, depending on the code used. Lastly, a new steady state pattern is presented as a combination of order 3 and 4 spherical harmonics leading to a five cell or a hexahedral pattern and stable up to 70 times the critical Rayleigh number. This pattern can provide the basis for a new accuracy benchmark for 3-D spherical mantle convection codes
Stationary axisymmetric exteriors for perturbations of isolated bodies in general relativity, to second order
Perturbed stationary axisymmetric isolated bodies, e.g. stars, represented by
a matter-filled interior and an asymptotically flat vacuum exterior joined at a
surface where the Darmois matching conditions are satisfied, are considered.
The initial state is assumed to be static. The perturbations of the matching
conditions are derived and used as boundary conditions for the perturbed Ernst
equations in the exterior region. The perturbations are calculated to second
order. The boundary conditions are overdetermined: necessary and sufficient
conditions for their compatibility are derived. The special case of
perturbations of spherical bodies is given in detail.Comment: RevTeX; 32 pp. Accepted by Phys. Rev. D. Added references and extra
comments in introductio
The Character of Z-pole Data Constraints on Standard Model Parameters
Despite the impressive precision of the Z-pole measurements made at LEP and
SLC, the allowed region for the principle Standard Model parameters responsible
for radiative corrections (the mass of the Higgs, the mass of the top and
alpha(Mz)) is still large enough to encompass significant non-linearities. The
nature of the experimental constraints therefore depends in an interesting way
on the "accidental" relationships among the various measurements. In
particular, the fact that the Z-pole measurements favor values of the Higgs
mass excluded by direct searches leads us to examine the effects of external
Higgsstrahlung, a process ignored by the usual precision electroweak
calculations.Comment: 9 pages, 6 figures, REVTeX format; added reference in section IV;
added paragraph on widths and a few cosmetic changes to correspond to
published versio
Fostering Application Opportunites for the NASA Soil Moisture Active Passive (SMAP) Mission
The NASA Soil Moisture Active Passive (SMAP) Mission will provide global observations of soil moisture and freeze/thaw state from space. We outline how priority applications contributed to the SMAP mission measurement requirements and how the SMAP mission plans to foster applications and applied science
Mesoscopic Phase Coherence in a Quantum Spin Fluid
Mesoscopic quantum phase coherence is important because it improves the
prospects for handling quantum degrees of freedom in technology. Here we show
that the development of such coherence can be monitored using magnetic neutron
scattering from a one-dimensional spin chain Y2BaNiO5, a quantum spin fluid
where no classical, static magnetic order is present. In the cleanest samples,
the quantum coherence length is 20 nm, almost an order of magnitude larger than
the classical antiferromagnetic correlation length of 3 nm. We also demonstrate
that the coherence length can be modified by static and thermally activated
defects in a quantitatively predictable manner
Stability of sub-surface oxygen at Rh(111)
Using density-functional theory (DFT) we investigate the incorporation of
oxygen directly below the Rh(111) surface. We show that oxygen incorporation
will only commence after nearly completion of a dense O adlayer (\theta_tot =
1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested
octahedral sub-surface site occupancy, inducing a site-switch of the on-surface
species from fcc to hcp sites, is indeed found to be a rather low energy
structure. Our results indicate that at even higher coverages oxygen
incorporation is followed by oxygen agglomeration in two-dimensional
sub-surface islands directly below the first metal layer. Inside these islands,
the metastable hcp/octahedral (on-surface/sub-surface) site combination will
undergo a barrierless displacement, introducing a stacking fault of the first
metal layer with respect to the underlying substrate and leading to a stable
fcc/tetrahedral site occupation. We suggest that these elementary steps,
namely, oxygen incorporation, aggregation into sub-surface islands and
destabilization of the metal surface may be more general and precede the
formation of a surface oxide at close-packed late transition metal surfaces.Comment: 9 pages including 9 figure files. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Chandra detection of extended X-ray emission from the recurrent nova RS Ophiuchi
Radio, infrared, and optical observations of the 2006 eruption of the
symbiotic recurrent nova RS Ophiuchi (RS Oph) showed that the explosion
produced non-spherical ejecta. Some of this ejected material was in the form of
bipolar jets to the east and west of the central source. Here we describe Xray
observations taken with the Chandra X-ray Observatory one and a half years
after the beginning of the outburst that reveal narrow, extended structure with
a position angle of approximately 300 degrees (east of north). Although the
orientation of the extended feature in the X-ray image is consistent with the
readout direction of the CCD detector, extensive testing suggests that the
feature is not an artifact. Assuming it is not an instrumental effect, the
extended X-ray structure shows hot plasma stretching more than 1,900 AU from
the central binary (taking a distance of 1.6 kpc). The X-ray emission is
elongated in the northwest direction - in line with the extended infrared
emission and some minor features in the published radio image. It is less
consistent with the orientation of the radio jets and the main bipolar optical
structure. Most of the photons in the extended X-ray structure have energies of
less than 0.8 keV. If the extended X-ray feature was produced when the nova
explosion occurred, then its 1".2 length as of 2007 August implies that it
expanded at an average rate of more than 2 mas/d, which corresponds to a flow
speed of greater than 6,000 km/s (d/1.6 kpc) in the plane of the sky. This
expansion rate is similar to the earliest measured expansion rates for the
radio jets.Comment: accepted in Ap
The TWA 3 Young Triple System: Orbits, Disks, Evolution
We have characterized the spectroscopic orbit of the TWA 3A binary and
provide preliminary families of probable solutions for the TWA 3A visual orbit
as well as for the wide TWA 3A--B orbit. TWA 3 is a hierarchical triple located
at 34 pc in the 10 Myr old TW Hya association. The wide component
separation is 1."55; the close pair was first identified as a possible binary
almost 20 years ago. We initially identified the 35-day period orbital solution
using high-resolution infrared spectroscopy which angularly resolved the A and
B components. We then refined the preliminary orbit by combining the infrared
data with a re-analysis of our high-resolution optical spectroscopy. The
orbital period from the combined spectroscopic solution is 35 days, the
eccentricity is 0.63, and the mass ratio is 0.84; although this
high mass ratio would suggest that optical spectroscopy alone should be
sufficient to identify the orbital solution, the presence of the tertiary B
component likely introduced confusion in the blended optical spectra. Using
millimeter imaging from the literature, we also estimate the inclinations of
the stellar orbital planes with respect to the TWA 3A circumbinary disk
inclination and find that all three planes are likely misaligned by at least
30 degrees. The TWA 3A spectroscopic binary components have spectral
types of M4.0 and M4.5; TWA 3B is an M3. We speculate that the system formed as
a triple, is bound, and that its properties were shaped by dynamical
interactions between the inclined orbits and disk.Comment: Accepted to Ap
Integral Equations for Heat Kernel in Compound Media
By making use of the potentials of the heat conduction equation the integral
equations are derived which determine the heat kernel for the Laplace operator
in the case of compound media. In each of the media the parameter
acquires a certain constant value. At the interface of the media the
conditions are imposed which demand the continuity of the `temperature' and the
`heat flows'. The integration in the equations is spread out only over the
interface of the media. As a result the dimension of the initial problem is
reduced by 1. The perturbation series for the integral equations derived are
nothing else as the multiple scattering expansions for the relevant heat
kernels. Thus a rigorous derivation of these expansions is given. In the one
dimensional case the integral equations at hand are solved explicitly (Abel
equations) and the exact expressions for the regarding heat kernels are
obtained for diverse matching conditions. Derivation of the asymptotic
expansion of the integrated heat kernel for a compound media is considered by
making use of the perturbation series for the integral equations obtained. The
method proposed is also applicable to the configurations when the same medium
is divided, by a smooth compact surface, into internal and external regions, or
when only the region inside (or outside) this surface is considered with
appropriate boundary conditions.Comment: 26 pages, no figures, no tables, REVTeX4; two items are added into
the Reference List; a new section is added, a version that will be published
in J. Math. Phy
The effect of monomer evaporation on a simple model of submonolayer growth
We present a model for thin film growth by particle deposition that takes
into account the possible evaporation of the particles deposited on the
surface. Our model focuses on the formation of two-dimensional structures. We
find that the presence of evaporation can dramatically affect the growth
kinetics of the film, and can give rise to regimes characterized by different
``growth'' exponents and island size distributions. Our results are obtained by
extensive computer simulations as well as through a simple scaling approach and
the analysis of rate equations describing the system. We carefully discuss the
relationship of our model with previous studies by Venables and Stoyanov of the
same physical situation, and we show that our analysis is more general.Comment: 41 pages including figures, Revtex, to be published in Physical
Review
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