8,292 research outputs found
Neutron spin polarization in strong magnetic fields
The effects of strong magnetic fields on the inner crust of neutron stars are
investigated after taking into account the anomalous magnetic moments of
nucleons. Energy spectra and wave functions for protons and neutrons in a
uniform magnetic field are provided. The particle spin polarizations and the
yields of protons and neutrons are calculated in a free Fermi gas model.
Obvious spin polarization occurs when G for protons and
G for neutrons, respectively. It is shown that the neutron spin
polarization depends solely on the magnetic field strength.Comment: Replaced by the revised version; 10 pages, including 3 eps figure
Redshift space 21 cm power spectra from reionization
We construct a simple but self-consistent analytic ionization model for rapid
exploration of 21cm power spectrum observables in redshift space. It is fully
described by the average ionization fraction and HII patch size
and has the flexibility to accommodate various reionization scenarios. The
model associates ionization regions with dark matter halos of the number
density required to recover and treats redshift space distortions
self-consistently with the virial velocity of such halos. Based on this model,
we study the line-of-sight structures in the brightness fluctuations since they
are the most immune to foreground contamination. We explore the degeneracy
between the HII patch size and nonlinear redshift space distortion in the one
dimensional power spectrum. We also discuss the limitations experimental
frequency and angular resolutions place on their distinguishability. Angular
resolution dilutes even the radial signal and will be a serious limitation for
resolving small bubbles before the end of reionization. Nonlinear redshift
space distortions suggest that a resolution of order 1 -- 10\arcsec and a
frequency resolution of 10kHz will ultimately be desirable to extract the full
information in the radial field at . First generation instruments
such as LOFAR and MWA can potentially measure radial HII patches of a few
comoving Mpc and larger at the end of reionization and are unlikely to be
affected by nonlinear redshift space distortions.Comment: 13 pages, 10 figures. Revised version. Includes minor changes. Adds
appendix on accomodating a distribution of radii for the HII regions.
Accepted for publication in Ap
Temporal Regulation of Foregut Development by HTZ-1/H2A.Z and PHA-4/FoxA
The histone variant H2A.Z is evolutionarily conserved and plays an essential role in mice, Drosophila, and Tetrahymena. The essential function of H2A.Z is unknown, with some studies suggesting a role in transcriptional repression and others in activation. Here we show that Caenorhabditis elegans HTZ-1/H2A.Z and the remodeling complex MYS-1/ESA1–SSL-1/SWR1 synergize with the FoxA transcription factor PHA-4 to coordinate temporal gene expression during foregut development. We observe dramatic genetic interactions between pha-4 and htz-1, mys-1, and ssl-1. A survey of transcription factors reveals that this interaction is specific, and thus pha-4 is acutely sensitive to reductions in these three proteins. Using a nuclear spot assay to visualize HTZ-1 in living embryos as organogenesis proceeds, we show that HTZ-1 is recruited to foregut promoters at the time of transcriptional onset, and this recruitment requires PHA-4. Loss of htz-1 by RNAi is lethal and leads to delayed expression of a subset of foregut genes. Thus, the effects of PHA-4 on temporal regulation can be explained in part by recruitment of HTZ-1 to target promoters. We suggest PHA-4 and HTZ-1 coordinate temporal gene expression by modulating the chromatin environment
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Gi- and Gs-coupled GPCRs show different modes of G-protein binding.
More than two decades ago, the activation mechanism for the membrane-bound photoreceptor and prototypical G protein-coupled receptor (GPCR) rhodopsin was uncovered. Upon light-induced changes in ligand-receptor interaction, movement of specific transmembrane helices within the receptor opens a crevice at the cytoplasmic surface, allowing for coupling of heterotrimeric guanine nucleotide-binding proteins (G proteins). The general features of this activation mechanism are conserved across the GPCR superfamily. Nevertheless, GPCRs have selectivity for distinct G-protein family members, but the mechanism of selectivity remains elusive. Structures of GPCRs in complex with the stimulatory G protein, Gs, and an accessory nanobody to stabilize the complex have been reported, providing information on the intermolecular interactions. However, to reveal the structural selectivity filters, it will be necessary to determine GPCR-G protein structures involving other G-protein subtypes. In addition, it is important to obtain structures in the absence of a nanobody that may influence the structure. Here, we present a model for a rhodopsin-G protein complex derived from intermolecular distance constraints between the activated receptor and the inhibitory G protein, Gi, using electron paramagnetic resonance spectroscopy and spin-labeling methodologies. Molecular dynamics simulations demonstrated the overall stability of the modeled complex. In the rhodopsin-Gi complex, Gi engages rhodopsin in a manner distinct from previous GPCR-Gs structures, providing insight into specificity determinants
Plasma Energy Loss into Kaluza-Klein Modes
Recently, Barger {\em et al.} computed energy losses into Kaluza Klein modes
from astrophysical plasmas in the approximation of zero density for the
plasmas. We extend their work by considering the effects of finite density for
two plasmon processes. Our results show that, for fixed temperature, the energy
loss rate per cm is constant up to some critical density and then falls
exponentially. This is true for transverse and longitudinal plasmons in both
the direct and crossed channels over a wide range of temperature and density. A
difficulty in deriving the appropriate covariant interaction energy at finite
density and temperature is addressed. We find that, for the cases considered by
Barger {\em et al.}, the zero density approximation and the neglect of other
plasmon processes is justified to better than an order of magnitude.Comment: 17 pages, LaTeX2e, 4 figures, 11 table
Apollo Lightcraft Project
This second year of the NASA/USRA-sponsored Advanced Aeronautical Design effort focused on systems integration and analysis of the Apollo Lightcraft. This beam-powered, single-stage-to-orbit vehicle is envisioned as the shuttlecraft of the 21st century. The five person vehicle was inspired largely by the Apollo Command Module, then reconfigured to include a new front seat with dual cockpit controls for the pilot and co-pilot, while still retaining the 3-abreast crew accommodations in the rear seat. The gross liftoff mass is 5550 kg, of which 500 kg is the payload and 300 kg is the LH2 propellant. The round trip cost to orbit is projected to be three orders of magnitude lower than the current space shuttle orbiter. The advanced laser-driven 5-speed combined-cycle engine has shiftpoints at Mach 1, 5, 11 and 25+. The Apollo Lightcraft can climb into low Earth orbit in three minutes, or fly to any spot on the globe in less than 45 minutes. Detailed investigations of the Apollo Lightcraft Project this second year further evolved the propulsion system design, while focusing on the following areas: (1) man/machine interface; (2) flight control systems; (3) power beaming system architecture; (4) re-entry aerodynamics; (5) shroud structural dynamics; and (6) optimal trajectory analysis. The principal new findings are documented. Advanced design efforts for the next academic year (1988/1989) will center on a one meter+ diameter spacecraft: the Lightcraft Technology Demonstrator (LTD). Detailed engineering design and analyses, as well as critical proof-of-concept experiments, will be carried out on this small, near-term machine. As presently conceived, the LTD could be constructed using state of the art components derived from existing liquid chemical rocket engine technology, advanced composite materials, and high power laser optics
gamma nu -> gamma gamma nu and crossed processes at energies below m_W
The cross sections for the processes ,
and are
calculated for a range of center of mass energies from below to
considerably above , but much less than . This enables us to treat
the neutrino--electron coupling as a four--Fermi interaction and results in
amplitudes which are electron box diagrams with three real photons and one
virtual photon at their vertices. These calculations extend our previous
low--energy effective interaction results to higher energies and enable us to
determine where the effective theory is reliable.Comment: 12 pages, RevTex, 10 postscript figures include
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