860 research outputs found
Analysis of surface waves generated on subwavelength-structured silver films
Using transmission electron microscopy (TEM) to analyse the physical-chemical
surface properties of subwavlength structured silver films and
finite-difference time-domain (FDTD) numerical simulations of the optical
response of these structures to plane-wave excitation, we report on the origin
and nature of the persistent surface waves generated by a single slit-groove
motif and recently measured by far-field optical interferometry. The surface
analysis shows that the silver films are free of detectable oxide or sulfide
contaminants, and the numerical simulations show very good agreement with the
results previously reported.Comment: 9 Figure
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
Charge injection instability in perfect insulators
We show that in a macroscopic perfect insulator, charge injection at a
field-enhancing defect is associated with an instability of the insulating
state or with bistability of the insulating and the charged state. The effect
of a nonlinear carrier mobility is emphasized. The formation of the charged
state is governed by two different processes with clearly separated time
scales. First, due to a fast growth of a charge-injection mode, a localized
charge cloud forms near the injecting defect (or contact). Charge injection
stops when the field enhancement is screened below criticality. Secondly, the
charge slowly redistributes in the bulk. The linear instability mechanism and
the final charged steady state are discussed for a simple model and for
cylindrical and spherical geometries. The theory explains an experimentally
observed increase of the critical electric field with decreasing size of the
injecting contact. Numerical results are presented for dc and ac biased
insulators.Comment: Revtex, 7pages, 4 ps figure
Estimation of Polarized Power Spectra by Gibbs sampling
Earlier papers introduced a method of accurately estimating the angular
cosmic microwave background (CMB) temperature power spectrum based on Gibbs
sampling. Here we extend this framework to polarized data. All advantages of
the Gibbs sampler still apply, and exact analysis of mega-pixel polarized data
sets is thus feasible. These advantages may be even more important for
polarization measurements than for temperature measurements. While approximate
methods can alias power from the larger E-mode spectrum into the weaker B-mode
spectrum, the Gibbs sampler (or equivalently, exact likelihood evaluations)
allows for a statistically optimal separation of these modes in terms of power
spectra. To demonstrate the method, we analyze two simulated data sets: 1) a
hypothetical future CMBPol mission, with the focus on B-mode estimation; and 2)
a Planck-like mission, to highlight the computational feasibility of the
method.Comment: 8 pages, 5 figures. High-resolution version available from
http://www.astro.uio.no/~hke/docs/larson_et_al_2006.ps.gz; accepted for
publication in Ap
Off-limb (spicule) DEM distribution from SoHO/SUMER observations
In the present work we derive a Differential Emission Measure (DEM) dis-
tribution from a region dominated by spicules. We use spectral data from the
Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer
on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER
wavelength range taken off-limb in the Northern polar coronal hole to construct
this DEM distribution using the CHIANTI atomic database. This distribution is
then used to study the thermal properties of the emission contributing to the
171 {\AA} channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar
Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in
the AIA 171 {\AA} channel is dominated by emission from the Fe ix 171.07 {\AA}
line and has sparingly little contribution from other lines. The product of the
Fe ix 171.07 {\AA} line contribution function with the off-limb DEM was found
to have a maximum at logTmax (K) = 5.8 indicating that during spicule
observations the emission in this line comes from plasma at transition region
temperatures rather than coronal. For comparison, the same product with a quiet
Sun and prominence DEM were found to have a maximum at logT max (K) = 5.9 and
logTmax (K) = 5.7, respectively. We point out that the interpretation of data
obtained from the AIA 171 {\AA} filter should be done with foreknowledge of the
thermal nature of the observed phenomenon. For example, with an off-limb DEM we
find that only 3.6% of the plasma is above a million degrees, whereas using a
quiet Sun DEM, this contribution rises to 15%.Comment: 12 pages, 6 figures accepted by Solar Physic
Is there only one species of Hepatozoon infecting Brazilian caimans? Integrative taxonomy unveiling the parasite’s diversity
Abstract Hepatozoon spp. are the most common haemoparasites reported from reptiles around the world, however, only six species have been described infecting crocodilians. In Brazil, Hepatozoon caimani Carini, 1909 is currently the only recognized species from the caiman hosts. This study provides new data on the diversity of species of Hepatozoon infecting Caiman crocodilus (Linnaeus) using molecular data and phylogenetic analysis, with additional support of morphological data of developmental stages from host blood and tissue. Forty-four individuals were collected and screened for haemogregarines, and blood and tissue samples were analysed by light microscopy with 31 (70.45%) infected. Hepatozoon spp. blood developmental stages included immature and mature gamonts with or without cytoplasmic vacuoles and free gamonts. Additionally, merogonic developmental stages were found in the liver and spleen of infected hosts. Based on the morphological and molecular data, this study identified two possible different species of Hepatozoon, being one of them the H. caimani with intragenotypic divergence
Herschel/HIFI observations of O-rich AGB stars : molecular inventory
Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel
Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form
part of the guaranteed time key program HIFISTARS are presented. The aim of
this program is to study the dynamical structure, mass-loss driving mechanism,
and chemistry of the outflows from AGB stars as a function of chemical
composition and initial mass.
We used the HIFI instrument to observe nine AGB stars, mainly in the H2O and
high rotational CO lines We investigate the correlation between line
luminosity, line ratio and mass-loss rate, line width and excitation energy.
A total of nine different molecules, along with some of their isotopologues
have been identified, covering a wide range of excitation temperature. Maser
emission is detected in both the ortho- and para-H2O molecules. The line
luminosities of ground state lines of ortho- and para-H2O, the high-J CO and
NH3 lines show a clear correlation with mass-loss rate. The line ratios of H2O
and NH3 relative to CO J=6-5 correlate with the mass-loss rate while ratios of
higher CO lines to the 6-5 is independent of it. In most cases, the expansion
velocity derived from the observed line width of highly excited transitions
formed relatively close to the stellar photosphere is lower than that of lower
excitation transitions, formed farther out, pointing to an accelerated outflow.
In some objects, the vibrationally excited H2O and SiO which probe the
acceleration zone suggests the wind reaches its terminal velocity already in
the innermost part of the envelope, i.e., the acceleration is rapid.
Interestingly, for R Dor we find indications of a deceleration of the outflow
in the region where the material has already escaped from the star.Comment: 6 Figures in the main paper + 12 further figures in the appendix (to
be printed in electronic form) Accepted for publication by A&
Cosmological Parameter Constraints as Derived from the Wilkinson Microwave Anisotropy Probe Data via Gibbs Sampling and the Blackwell-Rao Estimator
We study the Blackwell-Rao (BR) estimator of the probability distribution of
the angular power spectrum, P(C_l|d), by applying it to samples of full-sky
no-noise CMB maps generated via Gibbs sampling. We find the estimator, given a
set of samples, to be very fast and also highly accurate, as determined by
tests with simulated data. We also find that the number of samples required for
convergence of the BR estimate rises rapidly with increasing l, at least at low
l. Our existing sample chains are only long enough to achieve convergence at l
less than about 40. In comparison with P(C_l|d) as reported by the WMAP team we
find significant differences at these low l values. These differences lead to
up to approximately 0.5 sigma shifts in the estimates of parameters in a
7-parameter Lambda CDM model with non-zero dn_s/dln(k). Fixing dn_s/dln(k)= 0
makes these shifts much less significant.
Unlike existing analytic approximations, the BR estimator can be
straightforwardly extended for the case of power spectra from correlated
fields, such as temperature and polarization. We discuss challenges to
extending the procedure to higher l and provide some solutions.Comment: 11 pages, 8 figures, to be submitted to Phys. Rev.
Spatially resolved observations of a split-band coronal type-II radio burst
Context. The origin of coronal type-II radio bursts and of their
band-splitting are still not fully understood. Aims. To make progress in
solving this problem on the basis of one extremely well observed solar eruptive
event. Methods. The relative dynamics of multi-thermal eruptive plasmas,
observed in detail by the SDO/AIA and of the harmonic type-II burst sources,
observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the
partially behind the limb event on 3 November 2010. Special attention is given
to the band-splitting of the burst. Analysis is supplemented by investigation
of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is
found that the flare impulsive phase was accompanied by the formation of a
double coronal HXR source, whose upper part coincided with the hot (T~10 MK)
eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK)
moved upward from the flare region with the speed of v=900-1400 km/s. The type
II burst source initially appeared just above the LE apex and moved with the
same speed and in the same direction. After about 20 s it started to move about
twice faster, but still in the same direction. At any given moment the low
frequency component (LFC) source of the splitted type-II burst was situated
above the high frequency component (HFC) source, which in turn was situated
above the LE. It is also found that at a given frequency the HFC source was
located slightly closer to the photosphere than the LFC source. Conclusions.
The shock wave, which could be responsible for the observed type-II radio
burst, was initially driven by the multi-temperature eruptive plasmas, but
later transformed to a freely propagating blast shock wave. The most preferable
interpretation of the type-II burst splitting is that its LFC was emitted from
the upstream region of the shock, whereas the HFC - from the downstream region.Comment: 14 pages, 10 figure
Hard X-ray Emission Associated with White Dwarfs
We have used the WGACAT to search for hard X-ray sources associated with
white dwarfs (WDs) from the catalog of McCook & Sion (1999). We find 17 X-ray
sources coincident with WDs showing significant hard X-ray emission at energies
>0.5 keV. Twelve of these WDs are in known binary systems, in two of which the
accretion of the close companion's material onto the white dwarf produces the
hard X-ray emission, and in the other ten of which the late-type companions'
coronal activity emits hard X-rays. One WD is projected near an AGN which is
responsible for the hard X-ray emission. The remaining four WDs and two
additional white dwarfs with hard X-ray emission appear single. The lack of
near-IR excess from the apparently single WDs suggests that either X-ray
observations are more effective than near-IR photometry in diagnosing faint
companions or a different emission mechanism is needed. It is intriguing that
50% of the six apparently single WDs with hard X-ray emission are among the
hottest WDs. We have compared X-ray properties of 11 hot WDs with different
spectral types, and conclude that stellar pulsation and fast stellar winds are
not likely the origin of the hard X-ray emission, but a leakage of the
high-energy Wien tail of emission from deep in the stellar atmosphere remains a
tantalizing source of hard X-ray emission from hot DO and DQZO WDs. (This
abstract is an abridged version.)Comment: 35 pages, 8 figures, 4 tables, accepted for publication in AJ, April
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