832 research outputs found
TESS: A Relativistic Hydrodynamics Code on a Moving Voronoi Mesh
We have generalized a method for the numerical solution of hyperbolic systems
of equations using a dynamic Voronoi tessellation of the computational domain.
The Voronoi tessellation is used to generate moving computational meshes for
the solution of multi-dimensional systems of conservation laws in finite-volume
form. The mesh generating points are free to move with arbitrary velocity, with
the choice of zero velocity resulting in an Eulerian formulation. Moving the
points at the local fluid velocity makes the formulation effectively
Lagrangian. We have written the TESS code to solve the equations of
compressible hydrodynamics and magnetohydrodynamics for both relativistic and
non-relativistic fluids on a dynamic Voronoi mesh. When run in Lagrangian mode,
TESS is significantly less diffusive than fixed mesh codes and thus preserves
contact discontinuities to high precision while also accurately capturing
strong shock waves. TESS is written for Cartesian, spherical and cylindrical
coordinates and is modular so that auxilliary physics solvers are readily
integrated into the TESS framework and so that the TESS framework can be
readily adapted to solve general systems of equations. We present results from
a series of test problems to demonstrate the performance of TESS and to
highlight some of the advantages of the dynamic tessellation method for solving
challenging problems in astrophysical fluid dynamics.Comment: ApJS, 197, 1
Viable tax constitutions
Taxation is only sustainable if the general public complies with it. This observation is uncontroversial with tax practitioners but has been ignored by the public finance tradition, which has interpreted tax constitutions as binding contracts by which the power to tax is irretrievably conferred by individuals to government, which can then levy any tax it chooses. However, in the absence of an outside party enforcing contracts between members of a group, no arrangement within groups can be considered to be a binding contract, and therefore the power of tax must be sanctioned by individuals on an ongoing basis. In this paper we offer, for the first time, a theoretical analysis of this fundamental compliance problem associated with taxation, obtaining predictions that in some cases point to a re-interptretation of the theoretical constructions of the public finance tradition while in others call them into question
A Pilot Search for Population III Supernova Candidates in the Spitzer/IRAC Dark Field
We have undertaken a systematic search for candidate supernovae from
high-redshift Population III stars in a field that has been observed with
repeated imaging on a cadence of 2-3 weeks over a 2.2 year baseline, the
Spitzer/IRAC Dark Field. The individual epochs reach a typical 5-sigma depth of
1 uJy in IRAC Channel 1 (3.6 um). Requiring a minimum of four epochs coverage,
the total effective area searched is 214 sq arcminutes. The unprecedented depth
and multi-epochal nature of these data make it ideal for a first foray to
detect transient objects which may be candidate luminous Pair Instability
Supernovae from the primordial-metallicity first stars. The search was
conducted over a broad range of timescales, allowing for different durations of
the putative candidates' light curve plateau phases. All candidates were vetted
by inspection of the Spitzer imaging data, as well as deep HST/ACS F814W
imaging available over the full field. While many resolved-source objects were
found with Spitzer variability, no transient objects could plausibly be
identified as high-redshift supernovae candidates. The resulting 95% confidence
level upper limit is 23/sq deg/yr, for sources with plateau timescales under
400/(1+z) days and brightnesses above ~1 uJy.Comment: ApJL, in pres
Versatile transporter apparatus for experiments with optically trapped Bose-Einstein condensates
We describe a versatile and simple scheme for producing magnetically and
optically-trapped Rb-87 Bose-Einstein condensates, based on a moving-coil
transporter apparatus. The apparatus features a TOP trap that incorporates the
movable quadrupole coils used for magneto-optical trapping and long-distance
magnetic transport of atomic clouds. As a stand-alone device, this trap allows
for the stable production of condensates containing up to one million atoms. In
combination with an optical dipole trap, the TOP trap acts as a funnel for
efficient loading, after which the quadrupole coils can be retracted, thereby
maximizing optical access. The robustness of this scheme is illustrated by
realizing the superfluid-to-Mott insulator transition in a three-dimensional
optical lattice
The Familial Clustering of Age at Menarche in Extended Twin Families
The timing of puberty is complex, possibly involving many genetic factors that may interact with environmental influences. Familial resemblance for age at menarche was studied in a sample of 4,995 female twins, 1,296 sisters, 2,946 mothers and 635 female spouses of male twins. They had indicated their age at menarche as part of a larger longitudinal survey. We assessed assortative mating for age at menarche, gene–environment interaction effects and estimated the heritability of individual differences in pubertal timing. There was significant evidence of gene–environment interaction, accounting for 1.5% of the variance. There was no indication of consistent mate assortment on age at menarche. Individual differences in age at menarche are highly heritable, with additive genetic factors explaining at least 70% of the true variation. An additional 1.5% of the variation can be explained by a genotype–environment interaction effect where environmental factors are more important in individuals genetically predisposed for late menarche
Quantum and classical criticality in a dimerized quantum antiferromagnet
A quantum critical point (QCP) is a singularity in the phase diagram arising
due to quantum mechanical fluctuations. The exotic properties of some of the
most enigmatic physical systems, including unconventional metals and
superconductors, quantum magnets, and ultracold atomic condensates, have been
related to the importance of the critical quantum and thermal fluctuations near
such a point. However, direct and continuous control of these fluctuations has
been difficult to realize, and complete thermodynamic and spectroscopic
information is required to disentangle the effects of quantum and classical
physics around a QCP. Here we achieve this control in a high-pressure,
high-resolution neutron scattering experiment on the quantum dimer material
TlCuCl3. By measuring the magnetic excitation spectrum across the entire
quantum critical phase diagram, we illustrate the similarities between quantum
and thermal melting of magnetic order. We prove the critical nature of the
unconventional longitudinal ("Higgs") mode of the ordered phase by damping it
thermally. We demonstrate the development of two types of criticality, quantum
and classical, and use their static and dynamic scaling properties to conclude
that quantum and thermal fluctuations can behave largely independently near a
QCP.Comment: 6 pages, 4 figures. Original version, published version available
from Nature Physics websit
Chinese cross‐border mergers and acquisitions in the developing world: Is Africa unique?
China has emerged as an important partner for Africa. Not surprisingly, Chinese business and investment relations with Africa have been growing. This article contends that Africa offers a different proposition to Chinese business interests in non-African developing economies. In this optic, it takes a “comparative” institution-based view treating factors that determine Chinese multinationals' cross-border merger and acquisition (CBMA) decisions as comparatively different for Africa to the rest of the developing world. From a panel data estimation of the number of Chinese CBMAs from 2007 to 2016, we find among market size, natural resources, strategic assets, labor productivity, and institutional governance, only natural resources and market size have a distinctive effect, with Chinese investors being more attracted to African natural resources than the African market. The drive for natural resources provides impetus for Chinese MNEs to choose CBMAs over greenfield investment, and through majority ownership to exercise control. Our inference is that Africa is “significantly” different from other developing regions, in terms of CBMAs, as Chinese multinationals have a strong motive to control access to natural resources
- …