996 research outputs found
Lithium in the Symbiotic Mira V407 Cyg
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
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
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
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=1200ânm and up to 16-fold enhancement at λ=1050ânm for 1.25âÎŒm 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
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
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
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
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 Councilâs Centres of Excellence Scheme
Point Sources from a Spitzer IRAC Survey of the Galactic Center
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
- âŠ