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 9-12 201

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

Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures

Recent investigations have highlighted the failure of a sum of $R^{-6}$ 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

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

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