502 research outputs found
Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer
Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer
TectonoâStratigraphic Evolution of the Kerguelen Large Igneous Province: The Conjugate Williamâs RidgeâBroken Ridge Rifted Margins
AbstractExtensive investigation of continental rift systems has been fundamental for advancing the understanding of extensional tectonics and modes of formation of new ocean basins. However, current rift classification schemes do not account for conjugate end members formed by Large Igneous Province crust, referring to thick mafic crust, sometimes including continental fragments. Here, we investigate the rifting of William's Ridge (Kerguelen Plateau) and Broken Ridge, components of the Kerguelen Large Igneous Province now situated in the Southeast Indian Ocean, and incorporate these end members into the deformation migration concept for rifted margins. We use multichannel seismic reflection profiles and data from scientific drill cores acquired on both conjugate margins to propose, for the first time, a combined tectonoâstratigraphic framework. We interpret seismic patterns, tectonic features, and magnetic anomaly picks to determine an acrossâstrike structural domain classification. This interpretation considers the rift system overall to be âmagmaâpoorâ despite being located proximal to the Kerguelen plume but suggests that synârift interaction between the Kerguelen mantle plume and the lithospheric structure of William's Ridge and Broken Ridge has controlled the alongâstrike segmentation of both conjugates. We integrate seismic reflection and bathymetric data to test the hypothesis of predominantly transform motion, between the Australian and Antarctic plates, in Late Cretaceous and Paleogene time.</jats:p
CâH Bond Activation by Iridium(III) and Iridium(IV) Oxo Complexes
Oxidation of an iridium(III) oxo precursor enabled the structural, spectroscopic, and quantum-chemical characterization of the first well-defined iridium(IV) oxo complex. Side-by-side examination of the proton-coupled electron transfer thermochemistry revealed similar driving forces for the isostructural oxo complexes in two redox states due to compensating contributions from H+ and eâ transfer. However, CâH activation of dihydroanthracene revealed significant hydrogen tunneling for the distinctly more basic iridium(III) oxo complex. Our findings complement the growing body of data that relate tunneling to ground state properties as predictors for the selectivity of CâH bond activation.</p
The Hydrogen Epoch of Reionization Array Dish II: Characterization of Spectral Structure with Electromagnetic Simulations and its science Implications
We use time-domain electromagnetic simulations to determine the spectral
characteristics of the Hydrogen Epoch of Reionization Arrays (HERA) antenna.
These simulations are part of a multi-faceted campaign to determine the
effectiveness of the dish's design for obtaining a detection of redshifted 21
cm emission from the epoch of reionization. Our simulations show the existence
of reflections between HERA's suspended feed and its parabolic dish reflector
that fall below -40 dB at 150 ns and, for reasonable impedance matches, have a
negligible impact on HERA's ability to constrain EoR parameters. It follows
that despite the reflections they introduce, dishes are effective for
increasing the sensitivity of EoR experiments at relatively low cost. We find
that electromagnetic resonances in the HERA feed's cylindrical skirt, which is
intended to reduce cross coupling and beam ellipticity, introduces significant
power at large delays ( dB at 200 ns) which can lead to some loss of
measurable Fourier modes and a modest reduction in sensitivity. Even in the
presence of this structure, we find that the spectral response of the antenna
is sufficiently smooth for delay filtering to contain foreground emission at
line-of-sight wave numbers below Mpc, in
the region where the current PAPER experiment operates. Incorporating these
results into a Fisher Matrix analysis, we find that the spectral structure
observed in our simulations has only a small effect on the tight constraints
HERA can achieve on parameters associated with the astrophysics of
reionization.Comment: Accepted to ApJ, 18 pages, 17 Figures. Replacement matches accepted
manuscrip
Mapmaking for Precision 21 cm Cosmology
In order to study the "Cosmic Dawn" and the Epoch of Reionization with 21 cm
tomography, we need to statistically separate the cosmological signal from
foregrounds known to be orders of magnitude brighter. Over the last few years,
we have learned much about the role our telescopes play in creating a
putatively foreground-free region called the "EoR window." In this work, we
examine how an interferometer's effects can be taken into account in a way that
allows for the rigorous estimation of 21 cm power spectra from interferometric
maps while mitigating foreground contamination and thus increasing sensitivity.
This requires a precise understanding of the statistical relationship between
the maps we make and the underlying true sky. While some of these calculations
would be computationally infeasible if performed exactly, we explore several
well-controlled approximations that make mapmaking and the calculation of map
statistics much faster, especially for compact and highly redundant
interferometers designed specifically for 21 cm cosmology. We demonstrate the
utility of these methods and the parametrized trade-offs between accuracy and
speed using one such telescope, the upcoming Hydrogen Epoch of Reionization
Array, as a case study.Comment: 28 pages, 14 figures. Slightly revised to match published Physical
Review D versio
The Hydrogen Epoch of Reionization Array Dish I: Beam Pattern Measurements and Science Implications
The Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer
aiming to detect the power spectrum of 21 cm fluctuations from neutral hydrogen
from the Epoch of Reionization (EOR). Drawing on lessons from the Murchison
Widefield Array (MWA) and the Precision Array for Probing the Epoch of
Reionization (PAPER), HERA is a hexagonal array of large (14 m diameter) dishes
with suspended dipole feeds. Not only does the dish determine overall
sensitivity, it affects the observed frequency structure of foregrounds in the
interferometer. This is the first of a series of four papers characterizing the
frequency and angular response of the dish with simulations and measurements.
We focus in this paper on the angular response (i.e., power pattern), which
sets the relative weighting between sky regions of high and low delay, and
thus, apparent source frequency structure. We measure the angular response at
137 MHz using the ORBCOMM beam mapping system of Neben et al. We measure a
collecting area of 93 m^2 in the optimal dish/feed configuration, implying
HERA-320 should detect the EOR power spectrum at z~9 with a signal-to-noise
ratio of 12.7 using a foreground avoidance approach with a single season of
observations, and 74.3 using a foreground subtraction approach. Lastly we study
the impact of these beam measurements on the distribution of foregrounds in
Fourier space.Comment: 13 pages, 9 figures. Replaced to match accepted ApJ versio
Mapping our Universe in 3D with MITEoR
Mapping our universe in 3D by imaging the redshifted 21 cm line from neutral
hydrogen has the potential to overtake the cosmic microwave background as our
most powerful cosmological probe, because it can map a much larger volume of
our Universe, shedding new light on the epoch of reionization, inflation, dark
matter, dark energy, and neutrino masses. We report on MITEoR, a pathfinder
low-frequency radio interferometer whose goal is to test technologies that
greatly reduce the cost of such 3D mapping for a given sensitivity. MITEoR
accomplishes this by using massive baseline redundancy both to enable automated
precision calibration and to cut the correlator cost scaling from N^2 to NlogN,
where N is the number of antennas. The success of MITEoR with its 64
dual-polarization elements bodes well for the more ambitious HERA project,
which would incorporate many identical or similar technologies using an order
of magnitude more antennas, each with dramatically larger collecting area.Comment: To be published in proceedings of 2013 IEEE International Symposium
on Phased Array Systems & Technolog
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