996 research outputs found

    Lithium in the Symbiotic Mira V407 Cyg

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    We report an identification of the lithium resonance doublet LiI 6708A in the spectrum of V407 Cyg, a symbiotic Mira with a pulsation period of about 745 days. The resolution of the spectra used was R~18500 and the measured equivalent width of the line is ~0.34A. It is suggested that the lithium enrichment is due to hot bottom burning in the intermediate mass AGB variable, although other possible origins cannot be totally ruled out. In contrast to lithium-rich AGB stars in the Magellanic clouds, ZrO 5551A, 6474A absorption bands were not found in the spectrum of V407Cyg. These are the bands used to classify the S-type stars at low-resolution. Although we identified weak ZrO 5718A, 6412A these are not visible in the low-resolution spectra, and we therefore classify the Mira in V407 Cyg as an M type. This, together with other published work, suggests lithium enrichment can precede the third dredge up of s-process enriched material in galactic AGB stars.Comment: 4 pages, 2 figures, to be published in MNRA

    A conceptual model of light coupling by pillar diffraction gratings

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    Diffractivestructures such as pillar gratings are a promising way of coupling light into or out of thin semiconductor devices, for applications in thin film solar cells and light-emitting diodes. In this paper we show that the diffuse transmittance behavior of pillar gratings can be understood using the concept of grating mode interference and that the optimum heights of the grating and an estimate of the optimum period can be predicted with the effective index method. Furthermore, the method also gives good results for structures outside the range for which it was derived, including circular pillars and quasiperiodic structures. We also show that pillar gratings offer substantially improved performance over groove gratings for thin film silicon solar cells.One of the authors K.R.C. acknowledges the support of an Australian Research Council fellowship. The Centre of Excellence for Advanced Silicon Photovoltaics and Photonics is supported by the Australian Research Council

    The Age and Structure of the Galactic Bulge from Mira Variables

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    We report periods and JHKL observations for 648 oxygen-rich Mira variables found in two outer bulge fields at b=-7 degrees and l=+/-8 degrees and combine these with data on 8057 inner bulge Miras from the OGLE, Macho and 2MASS surveys, which are concentrated closer to the Galactic centre. Distance moduli are estimated for all these stars. Evidence is given showing that the bulge structure is a function of age. The longer period Miras (log P > 2.6, age about 5 Gyr and younger) show clear evidence of a bar structure inclined to the line of sight in both the inner and outer regions. The distribution of the shorter period (metal-rich globular cluster age) Miras, appears spheroidal in the outer bulge. In the inner region these old stars are also distributed differently from the younger ones and possibly suggest a more complex structure. These data suggest a distance to the Galactic centre, R0, of 8.9 kpc with an estimated uncertainty of 0.4 kpc. The possible effect of helium enrichment on our conclusions is discussed.Comment: Accepted for MNRAS, 12 pages, 12 figure

    Surface plasmon enhanced silicon solar cells

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    Thin-film solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-film crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the suitability of localized surface plasmons on silvernanoparticles for enhancing the absorbance of silicon solar cells. We find that surface plasmons can increase the spectral response of thin-film cells over almost the entire solar spectrum. At wavelengths close to the band gap of Si we observe a significant enhancement of the absorption for both thin-film and wafer-based structures. We report a sevenfold enhancement for wafer-based cells at λ=1200 nm and up to 16-fold enhancement at λ=1050 nm for 1.25 Όm thin silicon-on-insulator (SOI) cells, and compare the results with a theoretical dipole-waveguide model. We also report a close to 12-fold enhancement in the electroluminescence from ultrathin SOI light-emitting diodes and investigate the effect of varying the particle size on that enhancement.S. Pillai would like to acknowledge the UNSW Faculty of Engineering Research Scholarship. K.R. Catchpole acknowledges the support of an Australian Research Council fellowship

    The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions

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    The excitation of surface plasmons on metallic nanoparticles has the potential to significantly improve the performance of solar cells, in particular thin-film structures. In this article, we investigate the effect of the dielectric spacer layer thickness on the photocurrent enhancement of 2 ÎŒm thick, thin-film poly-Si on glass solar cells, due to random arrays of self-assembled Ag nanoparticlesdeposited on the front or the rear of the cells. We report a strong asymmetry in the external quantum efficiency (EQE) of the cell for front and rear located particles for different spacer thicknesses, which is attributed to differences in the scattering behavior of the nanoparticles. We find that for random arrays, with spectrally broad scattering resonances, the strength of the driving field and the coupling efficiency are more important for light trapping than the resonance wavelength. For particles located on the front of the cells it is desirable to have a thin dielectric spacer layer to enhance the scattering from the Ag nanoparticles. Additionally, light trapping provided by the random sized particles on the front can overcome suppression of light transmitted in the visible wavelength regions for thin layers of Si, to result in overall EQE enhancements. However, for particles deposited on the rear it is more beneficial to have the particles as close to the Si substrate as possible to increase both the scattering and the coupling efficiency.K.R.C. acknowledges the support of an Australian Research Council fellowship and the EU FP7 PRIMA project

    Impact of Human Presence and Visual Access on Barking Behavior in Shelter Dogs

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    Shelters can be stressful for dogs due to lack of predictability and control, social isolation, and busy environments. Providing dogs with more social opportunities and environmental predictability may improve their welfare. Barking may indicate stress and contribute to noise levels that are harmful to dogs and people. We investigated the impact of human presence and line of sight on barking. We manipulated line of sight by partially removing a crate barrier to allow the dogs visual access to other dogs and a better view of the room. We collected data on barking on 17 focal dogs as well as overall barking in the room during pre-treatment (no visual access), treatment (visual access), and post-treatment (no visual access) and noted if a person other than the observer was in the room. We found that in-room barking was significantly higher when a person was in the room (Wilcoxon Signed Ranks Test, Z= -4.048, p \u3c .001). Based on these results, shelters should consider limiting the human activity in the room to reduce noise levels. Since barking did not significantly increase with the addition of visual access, shelters may also consider providing the dogs visual access as a way to allow beneficial social interaction

    The visibility of the Galactic bulge in optical surveys. Application to the Gaia mission

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    The bulge is a region of the Galaxy which is of tremendous interest for understanding Galaxy formation. However, measuring photometry and kinematics in it raises several inherent issues, like high extinction in the visible and severe crowding. Here we attempt to estimate the problem of the visibility of the bulge at optical wavelengths, where large CCD mosaics allow to easily cover wide regions from the ground, and where future astrometric missions are planned. Assuming the Besancon Galaxy model and high resolution extinction maps, we estimate the stellar density as a function of longitude, latitude and apparent magnitude and we deduce the possibility of reaching and measuring bulge stars. The method is applied to three Gaia instruments, the BBP and MBP photometers, and the RVS spectrograph. We conclude that, while in the BBP most of the bulge will be accessible, in the MBP there will be a small but significant number of regions where bulge stars will be detected and accurately measured in crowded fields. Assuming that the RVS spectra may be extracted in moderately crowded fields, the bulge will be accessible in most regions apart from the strongly absorbed inner plane regions, because of high extinction, and in low extinction windows like the Baades's window where the crowding is too severe.Comment: 11 pages, 9 figures, accepted for publication in A&A, latex using A&A macro

    Enhanced emission from Si-based light-emitting diodes using surface plasmons

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    Excitation of surface plasmons on metallic nanoparticles has potential for increasing the absorption and emission from thin Si devices. We report an eight-fold enhancement in electroluminescence from silicon-on-insulator light-emitting diodes at 900nm via excitation of surface plasmon resonance in silvernanoparticles, along with a redshift in the electroluminescence by 70nm by overcoating the nanoparticles with ZnS. The enhancement is due to coupling between the electromagnetic excitations of the silvernanoparticles and the waveguide modes.The Centre of Excellence for Advanced Silicon Photovoltaics and Photonics is supported under the Australian Research Council’s Centres of Excellence Scheme

    Point Sources from a Spitzer IRAC Survey of the Galactic Center

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    We have obtained Spitzer/IRAC observations of the central 2.0 x 1.4 degrees (~280 x 200 pc) of the Galaxy at 3.6-8.0 microns. A point source catalog of 1,065,565 objects is presented. The catalog includes magnitudes for the point sources at 3.6, 4.5, 5.8, and 8.0 microns, as well as JHK photometry from 2MASS. The point source catalog is confusion limited with average limits of 12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0], respectively. We find that the confusion limits are spatially variable because of stellar surface density, background surface brightness level, and extinction variations across the survey region. The overall distribution of point source density with Galactic latitude and longitude is essentially constant, but structure does appear when sources of different magnitude ranges are selected. Bright stars show a steep decreasing gradient with Galactic latitude, and a slow decreasing gradient with Galactic longitude, with a peak at the position of the Galactic center. From IRAC color-magnitude and color-color diagrams, we conclude that most of the point sources in our catalog have IRAC magnitudes and colors characteristic of red giant and AGB stars.Comment: 44 pages, 13 figures, ApJS in pres
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