5,432 research outputs found
Molar Entropy and Enthalpy of CO Adsorbed in Zeolites as Derived from VTIR Data: Role of Intermolecular Modes
Detailed analysis of recently reported variable-temperature IR (VTIR) spectra of carbon monoxide adsorbed in alkaline zeolites shows how, not only the corresponding values of standard adsorption enthalpy ((Formula presented.)) and entropy ((Formula presented.)) can be obtained, but also the thermodynamic values of molar entropy and enthalpy which characterize the adsorbed gas phase. In addition, it is shown that the so obtained molar entropy data can lead to new insights into soft molecular modes, which would be hardly accessible by conventional IR spectroscopic techniques
Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope
We use nine years of gamma-ray data provided by the Fermi Large Area
Telescope (LAT) to systematically study the light curves of more than two
thousand active galactic nuclei (AGN) included in recent Fermi-LAT catalogs.
Ten different techniques are used, which are organized in an automatic
periodicity-search pipeline, in order to search for evidence of periodic
emission in gamma rays. Understanding the processes behind this puzzling
phenomenon will provide a better view about the astrophysical nature of these
extragalactic sources. However, the observation of temporal patterns in
gamma-ray light curves of AGN is still challenging. Despite the fact that there
have been efforts on characterizing the temporal emission of some individual
sources, a systematic search for periodicities by means of a full likelihood
analysis applied to large samples of sources was missing. Our analysis finds 11
AGN, of which 9 are identified for the first time, showing periodicity at more
than 4sigma in at least four algorithms. These findings will help in solving
questions related to the astrophysical origin of this periodic behavior.Comment: 16 pages, 5 figures, 4 tables. Accepted by Ap
Infrared Excess in the Be Star Delta Scorpii
We present infrared photometric observations of the Be binary system delta
Scorpii obtained in 2006. The J,H and K magnitudes are the same within the
errors compared to observations taken 10 months earlier. We derive the infrared
excess from the observation and compare this to the color excess predicted by a
radiative equilibrium model of the primary star and its circumstellar disk. We
use a non-LTE computational code to model the gaseous envelope concentrated in
the star's equatorial plane and calculate the expected spectral energy
distribution and Halpha emission profile of the star with its circumstellar
disk. Using the observed infrared excess of delta Sco, as well as Halpha
spectroscopy bracketing the IR observations in time, we place constraints on
the radial density distribution in the circumstellar disk. Because the disk
exhibits variability in its density distribution, this work will be helpful in
understanding its dynamics.Comment: 12 pages, 14 figures, to be published in PASP May 200
Signature of ionospheric irregularities under different geophysical conditions on SBAS performance in the western African low-latitude region
Abstract. Rate of change of TEC (ROT) and its index (ROTI) are considered a good proxy to characterize the occurrence of ionospheric plasma irregularities like those observed after sunset at low latitudes. SBASs (satellite-based augmentation systems) are civil aviation systems that provide wide-area or regional improvement to single-frequency satellite navigation using GNSS (Global Navigation Satellite System) constellations. Plasma irregularities in the path of the GNSS signal after sunset cause severe phase fluctuations and loss of locks of the signals in GNSS receiver at low-latitude regions. ROTI is used in this paper to characterize plasma density ionospheric irregularities in central–western Africa under nominal and disturbed conditions and identified some days of irregularity inhibition. A specific low-latitude algorithm is used to emulate potential possible SBAS message using real GNSS data in the western African low-latitude region. The performance of a possible SBAS operation in the region under different ionospheric conditions is analysed. These conditions include effects of geomagnetic disturbed periods when SBAS performance appears to be enhanced due to ionospheric irregularity inhibition. The results of this paper could contribute to a feasibility assessment of a European Geostationary Navigation Overlay System-based SBAS in the sub-Saharan African region
Suppression of Ferromagnetic Double Exchange by Vibronic Phase Segregation
From Raman spectroscopy, magnetization, and thermal-expansion on the system
La2/3(Ca1-xSrx)1/3MnO3, we have been able to provide a quantitative basis for
the heterogeneous electronic model for manganites exhibiting colossal
magnetoresistance (CMR). We construct a mean-field model that accounts
quantitatively for the measured deviation of TC(x) from the TC predicted by de
Gennes double exchange in the adiabatic approximation, and predicts the
occurrence of a first order transition for a strong coupling regime, in
accordance with the experiments. The existence of a temperature interval
TC<T<T* where CMR may be found is discussed, in connection with the occurrence
of an idealized Griffiths phase.Comment: Replaced with the final version accepted in Physical Review Letter
The inner flow geometry in MAXI J1820+070 during hard and hard-intermediate states
[Abridged] Context: We present a systematic X-ray spectral-timing study of
the recently discovered, exceptionally bright black hole X-ray binary system
MAXI J1820+070. Our analysis focuses on the first part of the 2018 outburst,
covering the rise throughout the hard state, the bright hard and
hard-intermediate states, and the transition to the soft-intermediate state.
Aims: We address the issue of constraining the geometry of the innermost
accretion flow and its evolution throughout an outburst. Methods: We employed
two independent X-ray spectral-timing methods applied to the NICER data of MAXI
J1820+070. We first identified and tracked the evolution of a characteristic
frequency of soft X-ray reverberation lags. Then, we studied the spectral
evolution of the quasi-thermal component responsible for the observed thermal
reverberation lags. Results: The frequency of thermal reverberation lags
steadily increases throughout most of the outburst, implying that the relative
distance between the X-ray source and the disc decreases as the source softens.
However, near transition this evolution breaks, showing a sudden increase
(decrease) of lag amplitude (frequency). The evolution of the quasi-thermal
component in high-frequency covariance spectra is consistent with a steady
decrease of the inner radius of the disc as the source softens. Conclusions:
The behaviour of thermal reverberation lags near transition might be related to
relativistic plasma ejections detected at radio wavelengths later in the
outburst, possibly representing the precursor to such events. Throughout most
of the hard and hard-intermediate states the disc is consistent with being
truncated (with an inner radius ), reaching close
to the innermost stable circular orbit only near transition.Comment: Submitted for publication in Astronomy & Astrophysic
A Nonzero Gap Two-Dimensional Carbon Allotrope from Porous Graphene
Graphene is considered one of the most promising materials for future
electronic. However, in its pristine form graphene is a gapless material, which
imposes limitations to its use in some electronic applications. In order to
solve this problem many approaches have been tried, such as, physical and
chemical functionalizations. These processes compromise some of the desirable
graphene properties. In this work, based on ab initio quantum molecular
dynamics, we showed that a two-dimensional carbon allotrope, named biphenylene
carbon (BPC) can be obtained from selective dehydrogenation of porous graphene.
BPC presents a nonzero bandgap and well-delocalized frontier orbitals.
Synthetic routes to BPC are also addressed.Comment: Published on J. Phys. Chem. C, 2012, 116 (23), pp 12810-1281
Major role of nutrient supply in the control of picophytoplankton community structure.
abstractThe Margalef´s mandala (1978) is a simplified bottom-up control model that explains how mixing and
nutrient concentration determine the composition of marine phytoplankton communities. Due to the
difficulties of measuring turbulence in the field, previous attempts to verify this model have applied
different proxies for nutrient supply, and very often used interchangeably the terms mixing and
stratification. Moreover, because the mandala was conceived before the discovery of smaller
phytoplankton groups (picoplankton <2 μm), it describes only the succession of vegetative phases of
microplankton. In order to test the applicability of the classical mandala to picoplankton groups, we
used a multidisciplinary approach including specifically designed field observations supported by
remote sensing, database analyses, and modeling and laboratory chemostat experiments.
Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and
picoplankton abundance collected in more than 200 stations, spanning widely different hydrographic
regimes, showed that the contribution of eukaryotes to picoautotrophic biomass increases with
nutrient supply, whereas that of picocyanobacteria shows the opposite trend. These findings were
supported by laboratory and modeling chemostat experiments that reproduced the competitive
dynamics between picoeukaryote sand picocyanobacteria as a function of changing nutrient supply.
Our results indicate that nutrient supply controls the distribution of picoplankton functional groups in
the ocean, further supporting the model proposed by Margalef.RADIALES (IEO
Vulvar cancer staging: guidelines of the European Society of Urogenital Radiology (ESUR)
Objective The aim of the Female Pelvic Imaging Working Group of the European Society of Urogenital Radiology (ESUR) was to develop imaging staging guidelines for vulvar cancer and to propose standardised MRI protocols and reporting. Methods The guidelines recommended from the ESUR in this article resulted from a questionnaire analysis regarding imaging staging of vulvar cancer that was answered by all members of the Female Pelvic Imaging Working Group. Only the answers with an agreement equal to or more than 80% were considered. Additionally, the literature was reviewed to complement and further support our conclusions. Results The critical review of the literature and consensus obtained among experts allows for recommendations regarding imaging staging guidelines, patient preparation, MRI protocol, and a structured MRI report. Conclusions Standardising image acquisition techniques and MRI interpretation reduces ambiguity and ultimately improves the contribution of radiology to the staging and management of patients with vulvar cancer. Moreover, structured reporting assists with the communication of clinically relevant information to the referring physician
SARS-CoV-2 outbreak on a Spanish mink farm: epidemiological, molecular, and pathological studies
Farmed minks have been reported to be highly susceptible to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and may represent a risk to humans. In this study, we describe the first outbreak of SARS-CoV-2 occurred on a mink farm in Spain, between June and July 2020, involving 92,700 animals. The outbreak started shortly after some farm workers became seropositive for SARS-CoV-2. Minks showed no clinical signs compatible with SARS-CoV-2 infection throughout the outbreak. Samples from 98 minks were collected for histopathological, serological, and molecular studies. Twenty out of 98 (20.4%) minks were positive by RT-qPCR and 82 out 92 (89%) seroconverted. This finding may reflect a rapid spread of the virus at the farm with most of the animals overcoming the infection. Additionally, SARS-CoV-2 was detected by RT-qPCR in 30% of brain samples from positive minks. Sequencing analysis showed that the mink sequences were not closely related with the other mink SARS-CoV-2 sequences available, and that this mink outbreak has its probable origin in one of the genetic variants that were prevalent in Spain during the first COVID-19 epidemic wave. Histological studies revealed bronchointerstitial pneumonia in some animals. Immunostaining of viral nucleocapsid was also observed in nasal turbinate tissue. Farmed minks could therefore constitute an important SARS-CoV-2 reservoir, contributing to virus spread among minks and humans. Consequently, continuous surveillance of mink farms is needed
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