2,086 research outputs found
EPR identification of defects responsible for thermoluminescence in Cu-doped lithium tetraborate (Li2B4O7) crystals
Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200 ◦C in copper-doped lithium tetraborate (Li2B4O7) crystals. As-grown crystals have Cu+ and Cu2+ ions substituting for lithium and have Cu+ ions at interstitial sites. All of the substitutional Cu2+ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu2+ EPR spectrum when a hole is trapped by substitutional Cu+ ions that have no nearby defects. These two Cu2+ trapped-hole centers are referred to as Cu2+-VLi and Cu2+active, respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu0 atoms are produced when interstitial Cu+ ions trap electrons. They are observed with EPR and are labeled Cu0A and Cu0B. When an irradiated crystal is warmed from 25 to 150 ◦C, the Cu2+active centers have a partial decay step that correlates with the TL peak near 100 ◦C. The concentrations of Cu0A and Cu0B centers, however, increase as the crystal is heated through this range. As the crystal is futher warmed between 150 and 250 ◦C, the EPR signals from the Cu2+active hole centers and Cu0A and Cu0B electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200 ◦C
Process to create simulated lunar agglutinate particles
A method of creating simulated agglutinate particles by applying a heat source sufficient to partially melt a raw material is provided. The raw material is preferably any lunar soil simulant, crushed mineral, mixture of crushed minerals, or similar material, and the heat source creates localized heating of the raw material
Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
This paper explores the mapping between the observable properties of a
stellar halo in phase- and abundance-space and the parent galaxy's accretion
history in terms of the characteristic epoch of accretion and mass and orbits
of progenitor objects. The study utilizes a suite of eleven stellar halo models
constructed within the context of a standard LCDM cosmology. The results
demonstrate that coordinate-space studies are sensitive to the recent (0-8
Gyears ago) merger histories of galaxies (this timescale corresponds to the
last few to tens of percent of mass accretion for a Milky-Way-type galaxy).
Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in
substructures in the stellar halo as a function of surface brightness are
indicators of the importance of recent merging and of the luminosity function
of infalling dwarfs. The {\it morphology} of features serves as a guide to the
orbital distribution of those dwarfs. Constraints on the earlier merger history
(> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars:
within our models, dramatic differences in the dominant epoch of accretion or
luminosity function of progenitor objects leave clear signatures in the
[alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by
very early accretion have higher average [alpha/Fe], while those dominated by
high luminosity satellites have higher [Fe/H]. This intuition can be applied to
reconstruct much about the merger histories of nearby galaxies from current and
future data sets.Comment: 21 pages, 20 figures. To appear in the Astrophysical Journa
Probing Very Bright End of Galaxy Luminosity Function at z >~ 7 Using Hubble Space Telescope Pure Parallel Observations
We report the first results from the Hubble Infrared Pure Parallel Imaging
Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble
Space Telescope to do deep imaging along a large number of random sightlines.
To date, our analysis includes 26 widely separated fields observed by the Wide
Field Camera 3, which amounts to 122.8 sq.arcmin in total area. We have found
three bright Y098-dropouts, which are candidate galaxies at z >~ 7.4. One of
these objects shows an indication of peculiar variability and its nature is
uncertain. The other two objects are among the brightest candidate galaxies at
these redshifts known to date L>2L*. Such very luminous objects could be the
progenitors of the high-mass Lyman break galaxis (LBGs) observed at lower
redshifts (up to z~5). While our sample is still limited in size, it is much
less subject to the uncertainty caused by "cosmic variance" than other samples
because it is derived using fields along many random sightlines. We find that
the existence of the brightest candidate at z~7.4 is not well explained by the
current luminosity function (LF) estimates at z~8. However, its inferred
surface density could be explained by the prediction from the LFs at z~7 if it
belongs to the high-redshift tail of the galaxy population at z~7.Comment: ApJL in press (accepted Dec. 27, 2010); minor corrections and one
figure added to address referee's comment
A Physical Model for the Origin of Quasar Lifetimes
We propose a model of quasar lifetimes in which observational quasar
lifetimes and an intrinsic lifetime of rapid accretion are strongly
distinguished by the physics of obscuration by surrounding gas and dust.
Quasars are powered by gas funneled to galaxy centers, but for a large part of
the accretion lifetime are heavily obscured by the large gas densities powering
accretion. In this phase, starbursts and black hole growth are fueled but the
quasar is buried. Eventually, feedback from accretion energy disperses
surrounding gas, creating a window in which the black hole is observable
optically as a quasar, until accretion rates drop below those required to
maintain a quasar luminosity. We model this process and measure the unobscured
and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy
merger. The source luminosity is determined from the black hole accretion rate,
calculated from local gas properties. We calculate the column density of
hydrogen to the source along multiple lines of sight and use these column
densities and gas metallicities to determine B-band attenuation of the source.
Defining the observable quasar lifetime as the total time with an observed
B-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr for
L_B,min=10^11 L_sun (M_B=-23), in good agreement with observationally
determined quasar lifetimes. This is significantly smaller than the intrinsic
lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed
to intrinsic lifetime is also strong function of both the limiting luminosity
and the observed frequency.Comment: 5 pages, 4 figures, submitted to ApJ Letter
Effects of a Sedentary vs. Active Lifestyle on Blood Glucose Uptake
Please see the pdf version of the abstract
Probing the Electronic Structure of Bilayer Graphene by Raman Scattering
The electronic structure of bilayer graphene is investigated from a resonant
Raman study using different laser excitation energies. The values of the
parameters of the Slonczewski-Weiss-McClure model for graphite are measured
experimentally and some of them differ significantly from those reported
previously for graphite, specially that associated with the difference of the
effective mass of electrons and holes. The splitting of the two TO phonon
branches in bilayer graphene is also obtained from the experimental data. Our
results have implications for bilayer graphene electronic devices.Comment: 4 pages, 4 figure
Disk Galaxy Formation in a LambdaCDM Universe
We describe hydrodynamical simulations of galaxy formation in a Lambda cold
dark matter (CDM) cosmology performed using a subresolution model for star
formation and feedback in a multiphase interstellar medium (ISM). In
particular, we demonstrate the formation of a well-resolved disk galaxy. The
surface brightness profile of the galaxy is exponential, with a B-band central
surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1
kpc. We find no evidence for a significant bulge component. The simulated
galaxy falls within the I-band Tully-Fisher relation, with an absolute
magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km
s^-1. While the total specific angular momentum of the stars in the galaxy
agrees with observations, the angular momentum in the inner regions appears to
be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7
M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from
\~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center,
indicating that the disk did not simply form inside-out. The stars exhibit a
metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the
center. Using a suite of idealized galaxy formation simulations with different
models for the ISM, we show that the effective pressure support provided by
star formation and feedback in our multiphase model is instrumental in allowing
the formation of large, stable disk galaxies. If ISM gas is instead modeled
with an isothermal equation of state, or if star formation is suppressed
entirely, growing gaseous disks quickly violate the Toomre stability criterion
and undergo catastrophic fragmentation.Comment: 14 pages, 12 figures, LaTex (emulateapj.cls), submitted to ApJ, high
resolution images available at
http://www-cfa.harvard.edu/~brobertson/papers/galaxy
Small animal disease surveillance: respiratory disease 2017
This report focuses on surveillance for respiratory disease in companion animals. It begins with an analysis of data from 392 veterinary practices contributing to the Small Animal Veterinary Surveillance Network (SAVSNET) between January and December 2017.
The following section describes canine respiratory coronavirus infections in dogs, presenting results from laboratory-confirmed cases across the country between January 2010 and December 2017. This is followed by an update on the temporal trends of three important syndromes in companion animals, namely gastroenteritis, pruritus and respiratory disease, from 2014 to 2017.
A fourth section presents a brief update on Streptococcus equi subspecies zooepidemicus in companion animals. The final section summarises some recent developments pertinent to companion animal health, namely eyeworm (Thelazzia callipaeda) infestations in dogs imported to the UK and canine influenza virus in the USA and Canada
The Evolution of the M-sigma Relation
(Abridged) We examine the evolution of the black hole mass - stellar velocity
dispersion (M-sigma) relation over cosmic time using simulations of galaxy
mergers that include feedback from supermassive black hole growth. We consider
mergers of galaxies varying the properties of the progenitors to match those
expected at redshifts z=0-6. We find that the slope of the resulting M-sigma
relation is the same at all redshifts considered. For the same feedback
efficiency that reproduces the observed amplitude of the M-sigma relation at
z=0, there is a weak redshift-dependence to the normalization that results from
an increasing velocity dispersion for a given galactic stellar mass. We develop
a formalism to connect redshift evolution in the M-sigma relation to the
scatter in the local relation at z=0. We show that the scatter in the local
relation places severe constraints on the redshift evolution of both the
normalization and slope of the M-sigma relation. Furthermore, we demonstrate
that cosmic downsizing introduces a black hole mass-dependent dispersion in the
M-sigma relation and that the skewness of the distribution about the locally
observed M-sigma relation is sensitive to redshift evolution in the
normalization and slope. In principle, these various diagnostics provide a
method for differentiating between theories for producing the M-sigma relation.
In agreement with existing constraints, our simulations imply that hierarchical
structure formation should produce the relation with small intrinsic scatter.Comment: 12 pages, 6 figures, version accepted by Ap
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