5,593 research outputs found
Orbits of Exceptional Groups, Duality and BPS States in String Theory
We give an invariant classification of orbits of the fundamental
representations of exceptional groups and which classify
BPS states in string and M theories toroidally compactified to d=4 and d=5. The
exceptional Jordan algebra and the exceptional Freudenthal triple system and
their cubic and quartic invariants play a major role in this classification.
The cubic and quartic invariants correspond to the black hole entropy in d=5
and d=4, respectively. The classification of BPS states preserving different
numbers of supersymmetries is in close parallel to the classification of the
little groups and the orbits of timelike, lightlike and space-like vectors in
Minkowski space. The orbits of BPS black holes in N=2 Maxwell-Einstein
supergravity theories in d=4 and d=5 with symmetric space geometries are also
classified including the exceptional N=2 theory that has and
as its symmety in the respective dimensions.Comment: New references and two tables added, a new section on the orbits of
N=2 Maxwell-Einstein supergravity theories in d=4 and d=5 included and some
minor changes were made in other sections. 17 pages. Latex fil
Plant yourself where language blooms: Direct experience of nature changes how parents and children talk about nature.
The current study investigated the affordances of direct and indirect experience of nature on parent-child talk. Parents and children produced a wider range of nature words when exploring a park (direct experience) than when exploring a thematically matched indoor visitor center (indirect experience). Parents and children also produced more plant-related nature word types when exploring the park compared to the visitor center. Findings suggest that direct experience of nature increases the diversity and specificity of parent-child talk about nature, and mitigates the phenomenon of âplant blindnessâ (cf. Wandersee & Schussler, 1999). Direct experience of nature provides an optimal context for children to learn the language of nature and consequently to cultivate children's status as custodians of the natural world
Virtual-crystal approximation that works: Locating a composition phase boundary in Pb(Zr_{1-x}Ti_3)O_3
We present a new method for modeling disordered solid solutions, based on the
virtual crystal approximation (VCA). The VCA is a tractable way of studying
configurationally disordered systems; traditionally, the potentials which
represent atoms of two or more elements are averaged into a composite atomic
potential. We have overcome significant shortcomings of the standard VCA by
developing a potential which yields averaged atomic properties. We perform the
VCA on a ferroelectric oxide, determining the energy differences between the
high-temperature rhombohedral, low-temperature rhombohedral and tetragonal
phases of Pb(Zr_{1-x}Ti_x)O_3 at x=0.5 and comparing these results to
superlattice calculations and experiment. We then use our new method to
determine the preferred structural phase at x=0.4. We find that the
low-temperature rhombohedral phase becomes the ground state at x=0.4, in
agreement with experimental findings.Comment: 5 pages, no figure
Using binary stars to bound the mass of the graviton
Interacting white dwarf binary star systems, including helium cataclysmic
variable (HeCV) systems, are expected to be strong sources of gravitational
radiation, and should be detectable by proposed space-based laser
interferometer gravitational wave observatories such as LISA. Several HeCV star
systems are presently known and can be studied optically, which will allow
electromagnetic and gravitational wave observations to be correlated.
Comparisons of the phases of a gravitational wave signal and the orbital light
curve from an interacting binary white dwarf star system can be used to bound
the mass of the graviton. Observations of typical HeCV systems by LISA could
potentially yield an upper bound on the inverse mass of the graviton as strong
as km (
eV), more than two orders of magnitude better than present solar system derived
bounds.Comment: 21 pages plus 4 figures; ReVTe
Acoustic cues to tonal contrasts in Mandarin: Implications for cochlear implants
The present study systematically manipulated three acoustic cues-fundamental frequency (f0), amplitude envelope, and duration-to investigate their contributions to tonal contrasts in Mandarin. Simplified stimuli with all possible combinations of these three cues were presented for identification to eight normal-hearing listeners, all native speakers of Mandarin from Taiwan. The f0 information was conveyed either by an f0-controlled sawtooth carrier or a modulated noise so as to compare the performance achievable by a clear indication of voice f0 and what is possible with purely temporal coding of f0. Tone recognition performance with explicit f0 was much better than that with any combination of other acoustic cues (consistently greater than 90% correct compared to 33%-65%; chance is 25%). In the absence of explicit f0, the temporal coding of f0 and amplitude envelope both contributed somewhat to tone recognition, while duration had only a marginal effect. Performance based on these secondary cues varied greatly across listeners. These results explain the relatively poor perception of tone in cochlear implant users, given that cochlear implants currently provide only weak cues to f0, so that users must rely upon the purely temporal (and secondary) features for the perception of tone. (c) 2008 Acoustical Society of America
The Hot Inner Disk of FU Ori
We have constructed a detailed radiative transfer disk model which reproduces
the main features of the spectrum of the outbursting young stellar object FU
Orionis from ~ 4000 angstrom, to ~ 8 micron. Using an estimated visual
extinction Av~1.5, a steady disk model with a central star mass ~0.3 Msun and a
mass accretion rate ~ 2e-4 Msun/yr, we can reproduce the spectral energy
distribution of FU Ori quite well. With the mid-infrared spectrum obtained by
the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we
estimate that the outer radius of the hot, rapidly accreting inner disk is ~ 1
AU using disk models truncated at this outer radius. Inclusion of radiation
from a cooler irradiated outer disk might reduce the outer limit of the hot
inner disk to ~ 0.5 AU. In either case, the radius is inconsistent with a pure
thermal instability model for the outburst. Our radiative transfer model
implies that the central disk temperature Tc > 1000 K out to ~ 0.5 - 1 AU,
suggesting that the magnetorotational instability (MRI) can be supported out to
that distance. Assuming that the ~ 100 yr decay timescale in brightness of FU
Ori represents the viscous timescale of the hot inner disk, we estimate the
viscosity parameter (alpha) to be ~ 0.2 - 0.02 in the outburst state,
consistent with numerical simulations of MRI in disks. The radial extent of the
high mass accretion region is inconsistent with the model of Bell & Lin, but
may be consistent with theories incorporating both gravitational instability
and MRI.Comment: 32 pages, 10 figures, to appear in the Astrophysical Journa
PSR J1723-2837: An Eclipsing Binary Radio Millisecond Pulsar
We present a study of PSR J1723-2837, an eclipsing, 1.86 ms millisecond
binary radio pulsar discovered in the Parkes Multibeam survey. Radio timing
indicates that the pulsar has a circular orbit with a 15 hr orbital period, a
low-mass companion, and a measurable orbital period derivative. The eclipse
fraction of ~15% during the pulsar's orbit is twice the Roche lobe size
inferred for the companion. The timing behavior is significantly affected by
unmodeled systematics of astrophysical origin, and higher-order orbital period
derivatives are needed in the timing solution to account for these variations.
We have identified the pulsar's (non-degenerate) companion using archival
ultraviolet, optical, and infrared survey data and new optical photometry.
Doppler shifts from optical spectroscopy confirm the star's association with
the pulsar and indicate a pulsar-to-companion mass ratio of 3.3 +/- 0.5,
corresponding to a companion mass range of 0.4 to 0.7 Msun and an orbital
inclination angle range of between 30 and 41 degrees, assuming a pulsar mass
range of 1.4-2.0 Msun. Spectroscopy indicates a spectral type of G for the
companion and an inferred Roche-lobe-filling distance that is consistent with
the distance estimated from radio dispersion. The features of PSR J1723-2837
indicate that it is likely a "redback" system. Unlike the five other Galactic
redbacks discovered to date, PSR J1723-2837 has not been detected as a
gamma-ray source with Fermi. This may be due to an intrinsic spin-down
luminosity that is much smaller than the measured value if the unmeasured
contribution from proper motion is large.Comment: 11 pages, including 8 figures and 5 tables. Accepted by the
Astrophysical Journa
Direct characterization of the native structure and mechanics of cyanobacterial carboxysomes
Carboxysomes are proteinaceous organelles that play essential roles in enhancing carbon fixation in cyanobacteria and some proteobacteria. These self-assembling organelles encapsulate Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and carbonic anhydrase using a protein shell structurally resembling an icosahedral viral capsid. The protein shell serves as a physical barrier to protect enzymes from the cytosol and a selectively permeable membrane to mediate transport of enzyme substrates and products. The structural and mechanical nature of native carboxysomes remain unclear. Here, we isolate functional β-carboxysomes from the cyanobacterium Synechococcus elongatus PCC7942 and perform the first characterization of the macromolecular architecture and inherent physical mechanics of single β-carboxysomes using electron microscopy, atomic force microscopy (AFM) and proteomics. Our results illustrate that the intact β-carboxysome comprises three structural domains, a single-layered icosahedral shell, an inner layer and paracrystalline arrays of interior Rubisco. We also observe the protein organization of the shell and partial β-carboxysomes that likely serve as the β-carboxysome assembly intermediates. Furthermore, the topography and intrinsic mechanics of functional β-carboxysomes are determined in native conditions using AFM and AFM-based nanoindentation, revealing the flexible organization and soft mechanical properties of β-carboxysomes compared to rigid viruses. Our study provides new insights into the natural characteristics of β-carboxysome organization and nanomechanics, which can be extended to diverse bacterial microcompartments and are important considerations for the design and engineering of functional carboxysomes in other organisms to supercharge photosynthesis. It offers an approach for inspecting the structural and mechanical features of synthetic metabolic organelles and protein scaffolds in bioengineering
Holographic fermions in charged Gauss-Bonnet black hole
We study the properties of the Green's functions of the fermions in charged
Gauss-Bonnet black hole. What we want to do is to investigate how the presence
of Gauss-Bonnet coupling constant affects the dispersion relation,
which is a characteristic of Fermi or non-Fermi liquid, as well as what
properties such a system has, for instance, the Particle-hole (a)symmetry. One
important result of this research is that we find for , the behavior of
this system is different from that of the Landau Fermi liquid and so the system
can be candidates for holographic dual of generalized non-Fermi liquids. More
importantly, the behavior of this system increasingly similar to that of the
Landau Fermi liquid when is approaching its lower bound. Also we find
that this system possesses the Particle-hole asymmetry when , another
important characteristic of this system. In addition, we also investigate
briefly the cases of the charge dependence.Comment: 22 pages, 6 figures; version published in JHE
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