830 research outputs found
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
The Detectability of Pair-Production Supernovae at z < 6
Nonrotating, zero metallicity stars with initial masses 140 < M < 260 solar
masses are expected to end their lives as pair-production supernovae (PPSNe),
in which an electron-positron pair-production instability triggers explosive
nuclear burning. Interest in such stars has been rekindled by recent
theoretical studies that suggest primordial molecular clouds preferentially
form stars with these masses. Since metal enrichment is a local process, the
resulting PPSNe could occur over a broad range of redshifts, in pockets of
metal-free gas. Using the implicit hydrodynamics code KEPLER, we have
calculated a set of PPSN light curves that addresses the theoretical
uncertainties and allows us to assess observational strategies for finding
these objects at intermediate redshifts. The peak luminosities of typical PPSNe
are only slightly greater than those of Type Ia, but they remain bright much
longer (~ 1 year) and have hydrogen lines. Ongoing supernova searches may soon
be able to limit the contribution of these very massive stars to < 1% of the
total star formation rate density out to z=2 which already provides useful
constraints for theoretical models. The planned Joint Dark Energy Mission
satellite will be able to extend these limits out to z=6.Comment: 12 pages, 6 figures, ApJ in press; slightly revised version, a few
typos correcte
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
Evaporation of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young clusters (CYCs) near
the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5
Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they
appear to be as massive as the smallest Galactic globular clusters (~10^4
Msun). A survey of cluster lifetimes for various initial mass functions,
cluster masses, and galactocentric radii is presented. Short relaxation times
due to the compactness of CYCs, and the strong tidal fields near the GC make
clusters evaporate fairly quickly. Depending on cluster parameters, mass
segregation may occur on a time scale shorter than the lifetimes of most
massive stars, which accelerates the cluster's dynamical evolution even more.
When the difference between the upper and lower mass boundaries of the initial
mass function is large enough, strongly selective ejection of lighter stars
makes massive stars dominate even in the outer regions of the cluster, so the
dynamical evolution of those clusters is weakly dependent on the lower mass
boundary. The mass bins for Fokker-Planck simulations were carefully chosen to
properly account for a relatively small number of the most massive stars. We
find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple
calculation based on the total masses in observed CYCs and the lifetimes
obtained here indicates that the massive CYCs comprise only a fraction of the
star formation rate (SFR) in the inner bulge estimated from Lyman continuum
photons and far-IR observations.Comment: 20 pages in two-column format, accepted for publication in Ap
Design Principles for Plasmonic Nanoparticle Devices
For all applications of plasmonics to technology it is required to tailor the
resonance to the optical system in question. This chapter gives an
understanding of the design considerations for nanoparticles needed to tune the
resonance. First the basic concepts of plasmonics are reviewed with a focus on
the physics of nanoparticles. An introduction to the finite element method is
given with emphasis on the suitability of the method to nanoplasmonic device
simulation. The effects of nanoparticle shape on the spectral position and
lineshape of the plasmonic resonance are discussed including retardation and
surface curvature effects. The most technologically important plasmonic
materials are assessed for device applicability and the importance of
substrates in light scattering is explained. Finally the application of
plasmonic nanoparticles to photovoltaic devices is discussed.Comment: 29 pages, 15 figures, part of an edited book: "Linear and Non-Linear
Nanoplasmonics
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
SN 2009E: a faint clone of SN 1987A
In this paper we investigate the properties of SN 2009E, which exploded in a
relatively nearby spiral galaxy (NGC 4141) and that is probably the faintest
1987A-like supernova discovered so far. Spectroscopic observations which
started about 2 months after the supernova explosion, highlight significant
differences between SN 2009E and the prototypical SN 1987A. Modelling the data
of SN 2009E allows us to constrain the explosion parameters and the properties
of the progenitor star, and compare the inferred estimates with those available
for the similar SNe 1987A and 1998A. The light curve of SN 2009E is less
luminous than that of SN 1987A and the other members of this class, and the
maximum light curve peak is reached at a slightly later epoch than in SN 1987A.
Late-time photometric observations suggest that SN 2009E ejected about 0.04
solar masses of 56Ni, which is the smallest 56Ni mass in our sample of
1987A-like events. Modelling the observations with a radiation hydrodynamics
code, we infer for SN 2009E a kinetic plus thermal energy of about 0.6 foe, an
initial radius of ~7 x 10^12 cm and an ejected mass of ~19 solar masses. The
photospheric spectra show a number of narrow (v~1800 km/s) metal lines, with
unusually strong Ba II lines. The nebular spectrum displays narrow emission
lines of H, Na I, [Ca II] and [O I], with the [O I] feature being relatively
strong compared to the [Ca II] doublet. The overall spectroscopic evolution is
reminiscent of that of the faint 56Ni-poor type II-plateau supernovae. This
suggests that SN 2009E belongs to the low-luminosity, low 56Ni mass, low-energy
tail in the distribution of the 1987A-like objects in the same manner as SN
1997D and similar events represent the faint tail in the distribution of
physical properties for normal type II-plateau supernovae.Comment: 19 pages, 9 figures (+7 in appendix); accepted for publication in A&A
on 3 November 201
Fishery Discards: Factors Affecting Their Variability within a Demersal Trawl Fishery
Discards represent one of the most important issues within current commercial fishing. It occurs for a range of reasons and is influenced by an even more complex array of factors. We address this issue by examining the data collected within the Danish discard observer program and describe the factors that influence discarding within the Danish Kattegat demersal fleet over the period 1997 to 2008. Generalised additive models were used to assess how discards of the 3 main target species, Norway lobster, cod and plaice, and their subcomponents (under and over minimum landings size) are influenced by important factors and their potential relevance to management. Our results show that discards are influenced by a range of different factors that are different for each species and portion of discards. We argue that knowledge about the factors influential to discarding and their use in relation to potential mitigation measures are essential for future fisheries management strategies
An investigation of polymorphisms in innate and adaptive immune response genes in canine leishmaniosis
The outcome of infection with Leishmania infantum in dogs is variable, which is thought to be due to the nature of the immune response mounted by the host. As a consequence, the clinical signs and severity of canine leishmaniosis vary between individual dogs. Host immunogenetic factors might play an important role in determining the outcome of infection. The aim of this study was to examine polymorphisms in innate and adaptive immune response genes, to determine whether any of these were associated with susceptibility or resistance to L. infantum infection. Genomic DNA was obtained from two groups: pet dogs in endemic regions of Europe and a group of Beagles exposed to sand fly infection as part of a vaccine study. Genotyping was performed using a SNP (single nucleotide polymorphism) array for selected immune response genes. The first part of the study compared 62 clinical cases with 101 clinically unaffected dogs that were seronegative for Leishmania antibodies. One SNP in the CIITA gene demonstrated a significantly higher minor allele frequency in the case group, compared with the control group at the individual SNP level after permutation, but was not significant after correction for multiple testing. The second part of the study examined 48 Beagle dogs exposed to L. infantum over two transmission seasons. Twenty-seven dogs with a resistant phenotype (no evidence of clinical disease, seronegative at the end of the study period, negative on lymph node culture and only transiently PCR positive in bone marrow) were compared with 21 dogs demonstrating a susceptible phenotype (clinical disease, seropositive, positive lymph node culture and consistently PCR positive in bone marrow). Three SNPs in TLR3, two SNPs in PTPN22 and one SNP in TLR4 and IL1A were associated with the susceptible phenotype in the Beagle group at the individual SNP level after permutation analysis, but were not significant after correction for multiple testing. Further validation of these SNPs is required in a larger cohort of dogs, ideally with extreme phenotypes to confirm an association with the outcome of L. infantum infection
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