932 research outputs found
Can the James Webb Space Telescope detect isolated population III stars?
Isolated population III stars are postulated to exist at approximately
z=10-30 and may attain masses up to a few hundred solar masses. The James Webb
Space telescope (JWST) is the next large space based infrared telescope and is
scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will
probably be able to detect some of the first galaxies forming in the early
Universe. A natural question is whether it will also be able to see any
isolated population III stars. Here, we calculate the apparent broadband
AB-magnitudes for 300 solar masses population III stars in JWST filters at
z=10-20. Our calculations are based on realistic stellar atmospheres and take
into account the potential flux contribution from the surrounding HII region.
The gravitational magnification boost achieved when pointing JWST through a
foreground galaxy cluster is also considered. Using this machinery, we derive
the conditions required for JWST to be able to detect population III stars in
isolation. We find that a detection of individual population III stars with
JWST is unlikely at these redshifts. However, the main problem is not
necessarily that these stars are too faint, once gravitational lensing is taken
into account, but that their surface number densities are too low.Comment: 6 pages, 3 figures, proceedings of CRF2010, DESY Hamburg, Nov 9-12
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Van der Waals Density Functional for General Geometries
A scheme within density functional theory is proposed that provides a
practical way to generalize to unrestricted geometries the method applied with
some success to layered geometries [H. Rydberg, et al., Phys. Rev. Lett. 91,
126402 (2003)]. It includes van der Waals forces in a seamless fashion. By
expansion to second order in a carefully chosen quantity contained in the long
range part of the correlation functional, the nonlocal correlations are
expressed in terms of a density-density interaction formula. It contains a
relatively simple parametrized kernel, with parameters determined by the local
density and its gradient. The proposed functional is applied to rare gas and
benzene dimers, where it is shown to give a realistic description.Comment: 4 pages, 4 figure
Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results
The dispersive interaction between nanotubes is investigated through ab
initio theory calculations and in an analytical approximation. A van der Waals
density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to
determine and compare the binding of a pair of nanotubes as well as in a
nanotube crystal. To analyze the interaction and determine the importance of
morphology, we furthermore compare results of our ab initio calculations with a
simple analytical result that we obtain for a pair of well-separated nanotubes.
In contrast to traditional density functional theory calculations, the vdW-DF
study predicts an intertube vdW bonding with a strength that is consistent with
recent observations for the interlayer binding in graphitics. It also produce a
nanotube wall-to-wall separation which is in very good agreement with
experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal
binding energy can be approximated by a sum of nanotube-pair interactions when
these are calculated in vdW-DF. This observation suggests a framework for an
efficient implementation of quantum-physical modeling of the CNT bundling in
more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure
Metal-free galaxy candidates discovered in CLASH
Abstract. The first metals in the universe are expected to form in population III starsprimordial stars consisting entirely of hydrogen and helium. However, these stars have so far remained elusive. Simulations indicate that galaxies consisting exclusively, or almost exclusively, of population III stars may form at z > 6, and such galaxies may provide one of the best probes of the properties of the population III star formation mode. By fitting Yggdrasil model spectra to multiband photometry data, we have identified four population III galaxy candidates in the Cluster Lensing and Supernova survey with Hubble (CLASH). We rule out alternative mundane galaxies and low redshift interlopers through similar fits to catalogs of spectra from more mundane objects. If confirmed through spectroscopy, this would constitute the first detection of the "missing link" between the early pristine universe and the metal-enriched universe
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
Numerical Studies of a Confocal Resonator Pick-Up with FEMLAB
Diagnostic devices aimed at measuring beam profiles in high intensity accelerators are often perturbed by microwave fields generated by the beam itself upstream of the detection device, which propagate inside the vacuum pipe. These parasitic waveguide modes can significantly reduce the signal-to-noise ratio and thus the sensitivity of the beam monitor. This warrants investigation of detection devices that are sensitive to the direct electromagnetic fields of the beam, but largely ignore the parasitic waveguide modes. A new pick-up based on a confocal resonator configuration situated transversely to the direction of propagation of the beam is currently under development at Uppsala University, Sweden. Since a confocal resonator can have a high quality factor for the diffraction losses, then reciprocity suggests that it only couples weakly to external fields while keeping anyway a significant coupling to the direct fields of the beam. Numerical simulations were performed with FEMLAB to better characterize the electromagnetic properties of a confocal resonator pick-up to be operated in the multi-GHz range, especially in terms of eigen-frequencies and coupling to external electromagnetic fields. Our results were then compared to analytical predictions and a good agreement was found, despite a few limitations in the computation of the resonant modes. Having recently built a first confocal resonator prototype, we also performed experimental cross-checks of our numerical studies with a microwave network analyzer. Our results are presented in detail in this report and we discuss further applications of the confocal resonator microwave pick-up
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