19,784 research outputs found
Prospects for Redshifted 21-cm observations of quasar HII regions
The introduction of low-frequency radio arrays over the coming decade is
expected to revolutionize the study of the reionization epoch. Observation of
the contrast in redshifted 21cm emission between a large HII region and the
surrounding neutral IGM will be the simplest and most easily interpreted
signature. We find that an instrument like the planned Mileura Widefield Array
Low-Frequency Demonstrator (LFD) will be able to obtain good signal to noise on
HII regions around the most luminous quasars, and determine some gross
geometric properties, e.g. whether the HII region is spherical or conical. A
hypothetical follow-up instrument with 10 times the collecting area of the LFD
(MWA-5000) will be capable of mapping the detailed geometry of HII regions,
while SKA will be capable of detecting very narrow spectral features as well as
the sharpness of the HII region boundary. The MWA-5000 will discover
serendipitous HII regions in widefield observations. We estimate the number of
HII regions which are expected to be generated by quasars. Assuming a late
reionization at z~6 we find that there should be several tens of quasar HII
regions larger than 4Mpc at z~6-8 per field of view. Identification of HII
regions in forthcoming 21cm surveys can guide a search for bright galaxies in
the middle of these regions. Most of the discovered galaxies would be the
massive hosts of dormant quasars that left behind fossil HII cavities that
persisted long after the quasar emission ended, owing to the long recombination
time of intergalactic hydrogen. A snap-shot survey of candidate HII regions
selected in redshifted 21cm image cubes may prove to be the most efficient
method for finding very high redshift quasars and galaxies.Comment: 14 pages, 8 figures. Submitted to Ap
Transfer of organic carbon through marine water columns to sediments â insights from stable and radiocarbon isotopes of lipid biomarkers
© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 11 (2014): 6895-6914, doi:10.5194/bg-11-6895-2014.Compound-specific 13C and 14C compositions of diverse lipid biomarkers (fatty acids, alkenones, hydrocarbons, sterols and fatty alcohols) were measured in sinking particulate matter collected in sediment traps and from underlying surface sediments in the Black Sea, the Arabian Sea and the Ross Sea. The goal was to develop a multiparameter approach to constrain relative inputs of organic carbon (OC) from marine biomass, terrigenous vascular-plant and relict-kerogen sources. Using an isotope mass balance, we calculate that marine biomass in sediment trap material from the Black Sea and Arabian Sea accounted for 66â100% of OC, with lower terrigenous (3â8%) and relict (4â16%) contributions. Marine biomass in sediments constituted lower proportions of OC (66â90%), with consequentially higher proportions of terrigenous and relict carbon (3â17 and 7â13%, respectively). Ross Sea data were insufficient to allow similar mass balance calculations. These results suggest that, whereas particulate organic carbon is overwhelmingly marine in origin, pre-aged allochthonous terrigenous and relict OC become proportionally more important in sediments, consistent with pre-aged OC being better preserved during vertical transport to and burial at the seafloor than the upper-ocean-derived marine OC.Grants OCE-9310364
and OCE-9911678 from the US National Science Foundation
(NSF) and the NSF Cooperative Agreement for the Operation of a
National Ocean Sciences Accelerator Mass Spectrometry Facility
(OCE-0753487 and OCE-123966) supported this research. S. G.
Wakeham acknowledges the Hanse Wissenschaftskolleg (Hanse
Institute for Advanced Studies), Delmenhorst, Germany, for a
fellowship that supported the writing of this manuscript
Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading
We thank the Associate Editor, Michelle Cooke, and the reviewers, Ze'ev Reches and Yves Guéguen, for useful comments which helped to improve the manuscript. We thank J.G. Van Munster for providing access to the true triaxial apparatus at KSEPL and for technical support during the experimental program. We thank R. Pricci for assistance with technical drawings of the apparatus. This work was partly funded by NERC award NE/N002938/1 and by a NERC Doctoral Studentship, which we gratefully acknowledge. Supporting data are included in a supporting information file; any additional data may be obtained from J.B. (e-mail: [email protected]).Peer reviewedPublisher PD
Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking Interferometry
We have used the Palomar 200" Adaptive Optics (AO) system to directly detect
the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This
observation is achieved with a novel combination of aperture masking
interferometry and AO. The dynamical masses are 0.1750.021 M and
0.0640.032 M for the primary and secondary respectively. The
inferred absolute H band magnitude of GJ 802B is M=12.8 resulting in a
model-dependent T of 1850 50K and mass range of
0.057--0.074 M.Comment: 4 Pages, 5 figures, emulateapj format, submitted to ApJ
International Workshop on Sustainable Sanitation: Resource Papers
Domestic wastewater reuse is currently not permitted anywhere in Australia but is widely supported by the community, promoted by researchers, and improvised by up to 20% of householders. Its widespread implementation will make an enormous contribution to the sustainability of water resources. Integrated with other strategies in the outdoor living environment of settlements in arid lands great benefit will be derived. This paper describes six options for wastewater reuse under research by the Remote Area Developments Group (RADG) at Murdoch University and case studies are given where productive use is being made for revegetation and food production strategies at household and community scales. Pollution control techniques, public health precautions and maintenance requirements are described. The special case of remote Aboriginal communities is explained where prototype systems have been installed by RADG to generate windbreaks and orchards. New Australian design standards and draft guidelines for domestic greywater reuse produced by the Western Australian state government agencies for mainstream communities are evaluated. It is recommended that dry composting toilets be coupled with domestic greywater reuse and the various types available in Australia are described. For situations where only the flushing toilet will suffice the unique âwet compostingâ system can be used and this also is described. A vision for household and community-scale on-site application is presented
Higher-Order Topological Insulators
Three-dimensional topological (crystalline) insulators are materials with an
insulating bulk, but conducting surface states which are topologically
protected by time-reversal (or spatial) symmetries. Here, we extend the notion
of three-dimensional topological insulators to systems that host no gapless
surface states, but exhibit topologically protected gapless hinge states. Their
topological character is protected by spatio-temporal symmetries, of which we
present two cases: (1) Chiral higher-order topological insulators protected by
the combination of time-reversal and a four-fold rotation symmetry. Their hinge
states are chiral modes and the bulk topology is -classified. (2)
Helical higher-order topological insulators protected by time-reversal and
mirror symmetries. Their hinge states come in Kramers pairs and the bulk
topology is -classified. We provide the topological invariants for
both cases. Furthermore we show that SnTe as well as surface-modified
BiTeI, BiSe, and BiTe are helical higher-order topological insulators and
propose a realistic experimental setup to detect the hinge states.Comment: 8 pages (4 figures) and 16 pages supplemental material (7 figures
Non-Fermi Liquid Quantum Impurity Physics from non-Abelian Quantum Hall States
We study the physics of electron tunneling between multiple quantum dots and
the edge of a quantum Hall state. Our results generalize earlier work [G. A.
Fiete, W. Bishara, C. Nayak, Phys. Rev. Lett. 101, 176801 (2008)] in which it
was shown that a single quantum dot tunnel coupled to a non-Abelian quantum
Hall state can realize a stable multi-channel Kondo fixed point at low-energy.
In this work, we investigate the physics of multiple dots and find that a rich
set of possible low-energy fixed points arises, including those with non-Fermi
liquid properties. Previously unidentified fixed points may also be among the
possibilities. We examine both the situation where the dots are spatially
separated and where they are in close proximity. We discuss the relation to
previous work on two-impurity Kondo models in Fermi liquids and highlight new
research directions in multiple quantum impurity problems.Comment: 12 pages, 2 figure
Width and Partial Widths of Unstable Particles in the Light of the Nielsen Identities
Fundamental properties of unstable particles, including mass, width, and
partial widths, are examined on the basis of the Nielsen identities (NI) that
describe the gauge dependence of Green functions. In particular, we prove that
the pole residues and associated definitions of branching ratios and partial
widths are gauge independent to all orders. A simpler, previously discussed
definition of branching ratios and partial widths is found to be gauge
independent through next-to-next-to-leading order. It is then explained how it
may be modified in order to extend the gauge independence to all orders. We
also show that the physical scattering amplitude is the most general
combination of self-energy, vertex, and box contributions that is gauge
independent for arbitrary s, discuss the analytical properties of the NI
functions, and exhibit explicitly their one-loop expressions in the Z-gamma
sector of the Standard Model.Comment: 20 pages (Latex); minor changes included, accepted for publication in
Phys. Rev.
On the Solution of Convection-Diffusion Boundary Value Problems Using Equidistributed Grids
The effect of using grid adaptation on the numerical solution of model convection-diffusion equations with a conservation form is studied. The grid adaptation technique studied is based on moving a fixed number of mesh points to equidistribute a generalization of the arc-length of the solution. In particular, a parameter-dependent monitor function is introduced which incorporates fixed meshes, approximate arc-length equidistribution, and equidistribution of the absolute value of the solution, in a single framework. Thus the resulting numerical method is a coupled nonlinear system of equations for the mesh spacings and the nodal values. A class of singularly perturbed problems, including Burgers's equation in the limit of small viscosity, is studied. Singular perturbation and bifurcation techniques are used to analyze the solution of the discretized equations, and numerical results are compared with the results from the analysis. Computation of the bifurcation diagram of the system is performed numerically using a continuation method and the results are used to illustrate the theory. It is shown that equidistribution does not remove spurious solutions present on a fixed mesh and that, furthermore, the spurious solutions can be stable for an appropriate moving mesh method
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