2,250,434 research outputs found
Non-LTE spectral analyses of the lately discovered DB-gap white dwarfs from the SDSS
For a long time, no hydrogen-deficient white dwarfs have been known that have
effective temperature between 30 kK and < 45 kK, i.e. exceeding those of DB
white dwarfs and having lower ones than DO white dwarfs. Therefore, this
temperature range was long known as the DB-gap. Only recently, the SDSS
provided spectra of several candidate DB-gap stars. First analyses based on
model spectra calculated under the assumption of local thermodynamic
equilibrium (LTE) confirmed that these stars had 30 kK < Teff < 45 kK
(Eisenstein et al. 2006). It has been shown for DO white dwarfs that the
relaxation of LTE is necessary to account for non local effects in the
atmosphere caused by the intense radiation field. Therefore, we calculated a
non-LTE model grid and re-analysed the aforementioned set of SDSS spectra. Our
results confirm the existence of DB-gap white dwarfs.Comment: 4 pages, 2 figures, to appear in: Proceedings of the 16th European
Workshop on White Dwarf
Probability in relativistic quantum mechanics and foliation of spacetime
The conserved probability densities (attributed to the conserved currents
derived from relativistic wave equations) should be non-negative and the
integral of them over an entire hypersurface should be equal to one. To satisfy
these requirements in a covariant manner, the foliation of spacetime must be
such that each integral curve of the current crosses each hypersurface of the
foliation once and only once. In some cases, it is necessary to use
hypersurfaces that are not spacelike everywhere. The generalization to the
many-particle case is also possible.Comment: 9 pages, 3 figures, revised, new references, to appear in Int. J.
Mod. Phys.
Discovery of secular variations in the atmospheric abundances of magnetic Ap stars
The stars of the middle main sequence have relatively quiescent outer layers,
and unusual chemical abundance patterns may develop in their atmospheres. The
presence of chemical peculiarities reveal the action of such subsurface
phenomena as gravitational settling and radiatively driven levitation of trace
elements, and their competition with mixing processes such as turbulent
diffusion.
We want to establish whether abundance peculiarities change as stars evolve
on the main sequence, and provide observational constraints to diffusion
theory.
We have performed spectral analysis of 15 magnetic Bp stars that are members
of open clusters (and thus have well-known ages), with masses between about 3
and 4 M_sun. For each star, we measured the abundances of He, O, Mg, Si, Ti,
Cr, Fe, Pr and Nd.
We have discovered the systematic time evolution of trace elements through
the main-sequence lifetime of magnetic chemically peculiar stars as their
atmospheres cool and evolve toward lower gravity. During the main sequence
lifetime, we observe clear and systematic variations in the atmospheric
abundances of He, Ti, Cr, Fe, Pr and Nd. For all these elements, except He, the
atmospheric abundances decrease with age. The abundances of Fe-peak elements
converge toward solar values, while the rare-earth elements converge toward
values at least 100 times more abundant than in the Sun. Helium is always
underabundant compared to the Sun, evolving from about 1% up to 10% of the
solar He abundance. We have attempted to interpret the observed abundance
variations in the context of radiatively driven diffusion theory, which appears
to provide a framework to understand some, but not all, of the observed
anomalous abundance levels and variations.Comment: 13 pages, 5 figures, 5 tables, accepted for publication in A&
Bivariate stochastic modeling of functional response with natural mortality
A correction due to Abbott (1925) is the standard method of dealing with control mortality in insect bioassay to estimate the mortality of an insect conditional on control mortality not having occurred. In this article a bivariate stochastic process for overall mortality is developed in which natural mortality and predation are jointly modeled to take account of the competing-risks associated with prey loss. The total mortality estimate from this model is essentially identical with that from more classical modeling. However, when predation loss is estimated in the absence of control mortality the results are somewhat different, with the estimate from the bivariate model being lower than that from using Abbottâs formula in conjunction with the classical model. It is argued that overdispersion in observed mortality data corresponds to correlated outcomes (death or survival) for the prey initially present, while Abbottâs correction relies implicitly on independence
Studies of superconductivity and structure for CaC6 to pressures above 15 GPa
The dependence of the superconducting transition temperature Tc of CaC6 has
been determined as a function of hydrostatic pressure in both helium-loaded gas
and diamond-anvil cells to 0.6 and 32 GPa, respectively. Following an initial
increase at the rate +0.39(1) K/GPa, Tc drops abruptly from 15 K to 4 K at 10
GPa. Synchrotron x-ray measurements to 15 GPa point to a structural transition
near 10 GPa from a rhombohedral to a higher symmetry phase
A dynamical systems model of unorganised segregation
We consider Schelling's bounded neighbourhood model (BNM) of unorganised
segregation of two populations from the perspective of modern dynamical systems
theory. We derive a Schelling dynamical system and carry out a complete
quantitative analysis of the system for the case of a linear tolerance schedule
in both populations. In doing so, we recover and generalise Schelling's
qualitative results. For the case of unlimited population movement, we derive
exact formulae for regions in parameter space where stable integrated
population mixes can occur. We show how neighbourhood tipping can be adequately
explained in terms of basins of attraction. For the case of limiting population
movement, we derive exact criteria for the occurrence of new population mixes
and identify the stable cases. We show how to apply our methodology to
nonlinear tolerance schedules, illustrating our approach with numerical
simulations. We associate each term in our Schelling dynamical system with a
social meaning. In particular we show that the dynamics of one population in
the presence of another can be summarised as follows
{rate of population change} = {intrinsic popularity of neighbourhood} -
{finite size of neighbourhood} - {presence of other population}
By approaching the dynamics from this perspective, we have a complementary
approach to that of the tolerance schedule.Comment: 17 pages (inc references), 9 figure
Nonclassical Nature of Dispersion Cancellation and Nonlocal Interferometry
Several recent papers have shown that some forms of dispersion cancellation
have classical analogs and that some aspects of nonlocal two-photon
interferometry are consistent with local realistic models. It is noted here
that the classical analogs only apply to local dispersion cancellation
experiments [A.M. Steinberg et al., Phys. Rev. Lett. 68, 2421 (1992)] and that
nonlocal dispersion cancellation [J.D. Franson, Phys. Rev. A 45, 3126 (1992)]
is inconsistent with any classical field theory and has no classical analog.
The local models that have been suggested for two-photon interferometry are
shown to be local but not realistic if the spatial extent of the
interferometers is taken into account. It is the inability of classical models
to describe all of the relevant aspects of these experiments that distinguishes
between quantum and classical physics, which is also the case in Bell's
inequality.Comment: 10 pages, 8 figures; minor revisions, to appear in Phys. Rev.
- âŠ