1,472 research outputs found
Modelling cohesive-frictional particulate solids for bulk handling applications
Many powders and particulate solids are cohesive in nature and the strength often exhibits dependence on the consolidation stress. As a result, the stress history in the material leading up to a handling scenario needs to be considered when evaluating its handleability. This paper outlines the development of a DEM contact model accounting for plasticity and adhesion force, which is shown to be suitable for modelling the stress history dependent cohesive strength. The model was used to simulate the confined consolidation and the subsequent unconfined loading of iron ore fines with particle sizes up to 1.18mm. The predicted flow function was found to be comparable to the experimental results
The Supersymmetric Origin of Matter
The Minimal Supersymmetric extension of the Standard Model (MSSM) can provide
the correct neutralino relic abundance and baryon number asymmetry of the
universe. Both may be efficiently generated in the presence of CP violating
phases, light charginos and neutralinos, and a light top squark. Due to the
coannihilation of the neutralino with the light stop, we find a large region of
parameter space in which the neutralino relic density is consistent with WMAP
and SDSS data. We perform a detailed study of the additional constraints
induced when CP violating phases, consistent with the ones required for
baryogenesis, are included. We explore the possible tests of this scenario from
present and future electron Electric Dipole Moment (EDM) measurements, direct
neutralino detection experiments, collider searches and the b -> s gamma decay
rate. We find that the EDM constraints are quite severe and that electron EDM
experiments, together with stop searches at the Tevatron and Higgs searches at
the LHC, will provide a definite test of our scenario of electroweak
baryogenesis in the next few years.Comment: 30 pages, 14 figure
Extinction Corrected Star Formation Rates Empirically Derived from Ultraviolet-Optical Colors
Using a sample of galaxies from the Sloan Digital Sky Survey spectroscopic
catalog with measured star-formation rates (SFRs) and ultraviolet (UV)
photometry from the GALEX Medium Imaging Survey, we derived empirical linear
correlations between the SFR to UV luminosity ratio and the UV-optical colors
of blue sequence galaxies. The relations provide a simple prescription to
correct UV data for dust attenuation that best reconciles the SFRs derived from
UV and emission line data. The method breaks down for the red sequence
population as well as for very blue galaxies such as the local ``supercompact''
UV luminous galaxies and the majority of high redshift Lyman Break Galaxies
which form a low attenuation sequence of their own.Comment: 20 pages, 11 figures, accepted for publication in the ApJS GALEX
special issu
The Star Formation and Extinction Co-Evolution of UV-Selected Galaxies over 0.05<z<1.2
We use a new stacking technique to obtain mean mid IR and far IR to far UV
flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ
COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC
and MIPS Mid IR photometry. This technique permits us to probe infrared excess
(IRX), the ratio of far IR to far UV luminosity, and specific star formation
rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass
and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0)
above which the SSFR drops increase with redshift (downsizing). At any given
epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX
falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases
with time in a fashion consistent with enrichment. We interpret these trends
using a simple model with a Schmidt-Kennicutt law and extinction that tracks
gas density and enrichment. We find that the average IRX and SSFR follows a
galaxy age parameter which is determined mainly by the galaxy mass and time
since formation. We conclude that blue sequence galaxies have properties which
show simple, systematic trends with mass and time such as the steady build-up
of heavy elements in the interstellar media of evolving galaxies and the
exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX
represents a tool for selecting galaxies at various stages of evolution.Comment: Accepted for publication in GALEX Special Ap.J.Suppl., December, 200
Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8
The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained
simultaneous near and far ultraviolet light curves of the ROTSE-I Catalog RR
Lyrae ab-type variable star J143753.84+345924.8. A series of 38 GALEX Deep
Imaging Survey observations well distributed in phase within the star's
0.56432d period shows an AB=4.9mag variation in the far UV (1350-1750A) band
and an AB=1.8mag variation in the near UV (1750-2750A) band, compared with only
a 0.8mag variation in the broad, unfiltered ROTSE-I (4500-10000A) band. These
GALEX UV observations are the first to reveal a large RR Lyrae amplitude
variation at wavelengths below 1800A. We compare the GALEX and ROTSE-I
observations to predictions made by recent Kurucz stellar atmosphere models. We
use published physical parameters for the comparable period (0.57433d),
well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and
ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree
with the Kurucz predictions for [Fe/H]=-1.25 to within AB=0.2mag in the GALEX
NUV and ROTSE-I bands, and within 0.5mag in the FUV. At all metallicities
between solar and one hundredth solar, the Kurucz models predict 6-8mag of
variation at wavelengths between 1000-1700A. Other variable stars with similar
temperature variations, such as Cepheids, should also have large-amplitude FUV
light curves, observable during the ongoing GALEX imaging surveys.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS after
November 22, 200
Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe
We present the first measurements of the angular correlation function of
galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the
GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions.
The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22,
yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm
0.6 Mpc and no significant trend is seen for this value as a function of the
limiting apparent magnitude or between the GALEX bands. This estimate is close
to that found from samples of blue galaxies in the local universe selected in
the visible, and similar to that derived at z ~ 3 for LBGs with similar rest
frame selection criteria. This result supports models that predict anti-biasing
of star forming galaxies at low redshift, and brings an additional clue to the
downsizing of star formation at z<1.Comment: Accepted for publication in GALEX Special ApJs, December 200
Ultraviolet through Infrared Spectral Energy Distributions from 1000 SDSS Galaxies: Dust Attenuation
The meaningful comparison of models of galaxy evolution to observations is
critically dependent on the accurate treatment of dust attenuation. To
investigate dust absorption and emission in galaxies we have assembled a sample
of ~1000 galaxies with ultraviolet (UV) through infrared (IR) photometry from
GALEX, SDSS, and Spitzer and optical spectroscopy from SDSS. The ratio of IR to
UV emission (IRX) is used to constrain the dust attenuation in galaxies. We use
the 4000A break as a robust and useful, although coarse, indicator of star
formation history (SFH). We examine the relationship between IRX and the UV
spectral slope (a common attenuation indicator at high-redshift) and find
little dependence of the scatter on 4000A break strength. We construct average
UV through far-IR spectral energy distributions (SEDs) for different ranges of
IRX, 4000A break strength, and stellar mass (M_*) to show the variation of the
entire SED with these parameters. When binned simultaneously by IRX, 4000A
break strength, and M_* these SEDs allow us to determine a low resolution
average attenuation curve for different ranges of M_*. The attenuation curves
thus derived are consistent with a lambda^{-0.7} attenuation law, and we find
no significant variations with M_*. Finally, we show the relationship between
IRX and the global stellar mass surface density and gas-phase-metallicity.
Among star forming galaxies we find a strong correlation between IRX and
stellar mass surface density, even at constant metallicity, a result that is
closely linked to the well-known correlation between IRX and star-formation
rate.Comment: 12 pages, 8 figures, 2 tables, appearing in the Dec 2007 GALEX
special issue of ApJ Supp (29 papers
Lepton Jets in (Supersymmetric) Electroweak Processes
We consider some of the recent proposals in which weak-scale dark matter is
accompanied by a GeV scale dark sector that could produce spectacular
lepton-rich events at the LHC. Since much of the collider phenomenology is only
weakly model dependent it is possible to arrive at generic predictions for the
discovery potential of future experimental searches. We concentrate on the
production of dark states through bosons and electroweak-inos at the
Tevatron or LHC, which are the cleanest channels for probing the dark sector.
We properly take into account the effects of dark radiation and dark cascades
on the formation of lepton jets. Finally, we present a concrete definition of a
lepton jet and suggest several approaches for inclusive experimental searches.Comment: 23 pages, 13 figures, published version, added section 3.3 expanding
on lepton jet's morpholog
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