3,824 research outputs found
Bromophenyl functionalization of carbon nanotubes : an ab initio study
We study the thermodynamics of bromophenyl functionalization of carbon
nanotubes with respect to diameter and metallic/insulating character using
density-functional theory (DFT). On one hand, we show that the activation
energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag
nanotube ranges from 0.73 eV to 0.76 eV without any clear trend with respect to
diameter within numerical accuracy. On the other hand, the binding energy of a
single bromophenyl molecule shows a clear diameter dependence and ranges from
1.51 eV for a (8,0) zigzag nanotube to 0.83 eV for a (20,0) zigzag nanotube.
This is in part explained by the transition from sp2 to sp3 bonding occurring
to a carbon atom of a nanotube when a phenyl is grafted to it and the fact that
smaller nanotubes are closer to a sp3 hybridization than larger ones due to
increased curvature. Since a second bromophenyl unit can attach without energy
barrier next to an isolated grafted unit, they are assumed to exist in pairs.
The para configuration is found to be favored for the pairs and their binding
energy decreases with increasing diameter, ranging from 4.34 eV for a (7,0)
nanotube to 2.27 eV for a (29,0) nanotube. An analytic form for this radius
dependence is derived using a tight binding hamiltonian and first order
perturbation theory. The 1/R^2 dependance obtained (where R is the nanotube
radius) is verified by our DFT results within numerical accuracy. Finally,
metallic nanotubes are found to be more reactive than semiconducting nanotubes,
a feature that can be explained by a non-zero density of states at the Fermi
level for metallic nanotubes.Comment: 7 pages, 5 figures and 3 table
Ground-based optical transmission spectrum of the hot Jupiter HAT-P-1b
Time-series spectrophotometric studies of exoplanets during transit using
ground-based facilities are a promising approach to characterize their
atmospheric compositions. We aim to investigate the transit spectrum of the hot
Jupiter HAT-P-1b. We compare our results to those obtained at similar
wavelengths by previous space-based observations. We observed two transits of
HAT-P-1b with the Gemini Multi-Object Spectrograph (GMOS) instrument on the
Gemini North telescope using two instrument modes covering the 320 - 800 nm and
520 - 950 nm wavelength ranges. We used time-series spectrophotometry to
construct transit light curves in individual wavelength bins and measure the
transit depths in each bin. We accounted for systematic effects. We addressed
potential photometric variability due to magnetic spots in the planet's host
star with long-term photometric monitoring. We find that the resulting transit
spectrum is consistent with previous Hubble Space Telescope (HST) observations.
We compare our observations to transit spectroscopy models that marginally
favor a clear atmosphere. However, the observations are also consistent with a
flat spectrum, indicating high-altitude clouds. We do not detect the Na
resonance absorption line (589 nm), and our observations do not have sufficient
precision to study the resonance line of K at 770 nm. We show that even a
single Gemini/GMOS transit can provide constraining power on the properties of
the atmosphere of HAT-P-1b to a level comparable to that of HST transit studies
in the optical when the observing conditions and target and reference star
combination are suitable. Our 520 - 950 nm observations reach a precision
comparable to that of HST transit spectra in a similar wavelength range of the
same hot Jupiter, HAT-P-1b. However, our GMOS transit between 320 - 800 nm
suffers from strong systematic effects and yields larger uncertainties.Comment: A&A, accepted, 16 pages, 8 figures, 5 table
Planar Feeding Circuit Integrated With a Compact Dielectric Resonator for Polarization Diversity
Two-Timescale Learning Using Idiotypic Behaviour Mediation For A Navigating Mobile Robot
A combined Short-Term Learning (STL) and Long-Term Learning (LTL) approach to
solving mobile-robot navigation problems is presented and tested in both the
real and virtual domains. The LTL phase consists of rapid simulations that use
a Genetic Algorithm to derive diverse sets of behaviours, encoded as variable
sets of attributes, and the STL phase is an idiotypic Artificial Immune System.
Results from the LTL phase show that sets of behaviours develop very rapidly,
and significantly greater diversity is obtained when multiple autonomous
populations are used, rather than a single one. The architecture is assessed
under various scenarios, including removal of the LTL phase and switching off
the idiotypic mechanism in the STL phase. The comparisons provide substantial
evidence that the best option is the inclusion of both the LTL phase and the
idiotypic system. In addition, this paper shows that structurally different
environments can be used for the two phases without compromising
transferability.Comment: 40 pages, 12 tables, Journal of Applied Soft Computin
Right vs. left ventricular longitudinal strain for mortality prediction after transcatheter aortic valve implantation
INTRODUCTION
This study aims at exploring biventricular remodelling and its implications for outcome in a representative patient cohort with severe aortic stenosis (AS) undergoing transcatheter aortic valve implantation (TAVI).
METHODS AND RESULTS
Pre-interventional echocardiographic examinations of 100 patients with severe AS undergoing TAVI were assessed by speckle tracking echocardiography of both ventricles. Association with mortality was determined for right ventricular global longitudinal strain (RVGLS), RV free wall strain (RVFWS) and left ventricular global longitudinal strain (LVGLS). During a median follow-up of 1,367 [959-2,123] days, 33 patients (33%) died. RVGLS was lower in non-survivors [-13.9% (-16.4 to -12.9)] than survivors [-17.1% (-20.2 to -15.2); P = 0.001]. In contrast, LVGLS as well as the conventional parameters LV ejection fraction (LVEF) and RV fractional area change (RVFAC) did not differ (P = ns). Kaplan-Meier analyses indicated a reduced survival probability when RVGLS was below the -14.6% cutpoint (P < 0.001). Lower RVGLS was associated with higher mortality [HR 1.13 (95% CI 1.04-1.23); P = 0.003] independent of LVGLS, LVEF, RVFAC, and EuroSCORE II. Addition of RVGLS clearly improved the fitness of bivariable and multivariable models including LVGLS, LVEF, RVFAC, and EuroSCORE II with potential incremental value for mortality prediction. In contrast, LVGLS, LVEF, and RVFAC were not associated with mortality.
DISCUSSION
In patients with severe AS undergoing TAVI, RVGLS but not LVGLS was reduced in non-survivors compared to survivors, differentiated non-survivors from survivors, was independently associated with mortality, and exhibited potential incremental value for outcome prediction. RVGLS appears to be more suitable than LVGLS for risk stratification in AS and timely valve replacement
Upconversion channels in Er3+:ZBLALiP fluoride glass microspheres
We present results on the realization of a multicolour microspherical glass light source fabricated from the erbium doped fluoride glass ZBLALiP. Whispering gallery mode lasing and upconversion processes give rise to laser and fluorescent emissions at multiple wavelengths from the ultraviolet to the infrared. Thirteen discrete emissions ranging from 320 to 849 nm have been observed in the upconversion spectrum. A Judd-Ofelt analysis was performed to calculate the radiative properties of Er3+:ZBLALiP microspheres, including the radiative transition probabilities, the electric dipole strengths, the branching ratios and the radiative lifetimes of the transitions involved. We have also identified the primary processes responsible for the generation of the observed wavelengths and have shown that this material has an improved range of emissions over other erbium doped fluoride glasses
Climate Change and Biosphere Response: Unlocking the Collections Vault
Natural history collections (NHCs) are an important source of the long-term data needed to understand how biota respond to ongoing anthropogenic climate change. These include taxon occurrence data for ecological modeling, as well as information that can be used to reconstruct mechanisms through which biota respond to changing climates. The full potential of NHCs for climate change research cannot be fully realized until high-quality data sets are conveniently accessible for research, but this requires that higher priority be placed on digitizing the holdings most useful for climate change research (e.g., whole-biota studies, time series, records of intensively sampled common taxa). Natural history collections must not neglect the proliferation of new information from efforts to understand how present-day ecosystems are responding to environmental change. These new directions require a strategic realignment for many NHC holders to complement their existing focus on taxonomy and systematics. To set these new priorities, we need strong partnerships between NHC holders and global change biologists
Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b
The helium absorption triplet at a wavelength of 10,833 \AA\ has been
proposed as a way to probe the escaping atmospheres of exoplanets. Recently
this feature was detected for the first time using Hubble Space Telescope (HST)
WFC3 observations of the hot Jupiter WASP-107b. We use similar HST/WFC3
observations to detect helium in the atmosphere of the hot Neptune HAT-P-11b at
the confidence level. We compare our observations to a grid of 1D
models of hydrodynamic escape to constrain the thermospheric temperatures and
mass loss rate. We find that our data are best fit by models with high mass
loss rates of - g s. Although we do
not detect the planetary wind directly, our data are consistent with the
prediction that HAT-P-11b is experiencing hydrodynamic atmospheric escape.
Nevertheless, the mass loss rate is low enough that the planet has only lost up
to a few percent of its mass over its history, leaving its bulk composition
largely unaffected. This matches the expectation from population statistics,
which indicate that close-in planets with radii greater than 2 R
form and retain H/He-dominated atmospheres. We also confirm the independent
detection of helium in HAT-P-11b obtained with the CARMENES instrument, making
this the first exoplanet with the detection of the same signature of
photoevaporation from both ground- and space-based facilities.Comment: 12 pages, 9 figures, accepted for publication in ApJ
Low False-Positive Rate of Kepler Candidates Estimated From A Combination Of Spitzer And Follow-Up Observations
(Abridged) NASA's Kepler mission has provided several thousand transiting
planet candidates, yet only a small subset have been confirmed as true planets.
Therefore, the most fundamental question about these candidates is the fraction
of bona fide planets. Estimating the rate of false positives of the overall
Kepler sample is necessary to derive the planet occurrence rate. We present the
results from two large observational campaigns that were conducted with the
Spitzer telescope during the the Kepler mission. These observations are
dedicated to estimating the false positive rate (FPR) amongst the Kepler
candidates. We select a sub-sample of 51 candidates, spanning wide ranges in
stellar, orbital and planetary parameter space, and we observe their transits
with Spitzer at 4.5 microns. We use these observations to measures the
candidate's transit depths and infrared magnitudes. A bandpass-dependent depth
alerts us to the potential presence of a blending star that could be the source
of the observed eclipse: a false-positive scenario. For most of the candidates
(85%), the transit depths measured with Kepler are consistent with the depths
measured with Spitzer as expected for planetary objects, while we find that the
most discrepant measurements are due to the presence of unresolved stars that
dilute the photometry. The Spitzer constraints on their own yield FPRs between
5-40%, depending on the KOIs. By considering the population of the Kepler field
stars, and by combining follow-up observations (imaging) when available, we
find that the overall FPR of our sample is low. The measured upper limit on the
FPR of our sample is 8.8% at a confidence level of 3 sigma. This observational
result, which uses the achromatic property of planetary transit signals that is
not investigated by the Kepler observations, provides an independent indication
that Kepler's false positive rate is low.Comment: 33 pages, 16 figures, 3 tables; accepted for publication in ApJ on
February 7, 201
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