234 research outputs found
UV-visible Spectra and Gas-Phase Rate Coefficients for the Reaction of 2,3 pentanedione and 2,4-pentanedione with OH Radicals
International audienceThe UV absorption cross-sections of 2,3-pentanedione and 2,4-pentanedione were measured and their reactions with OH were investigated in the gas-phase using a relative rate method. A temperature dependence of the rate coefficients was observed for both reactions over the temperature range 298–338 K. This work provides the first UV cross-section in the gas phase for 2,4-pentanedione and the first kinetic data for the reaction of 2,3-pentanedione with OH radicals as a function of temperature. The tropospheric lifetimes obtained in this work suggest that once emitted into the atmosphere, these species could be quickly degraded close to their emission sources
Reactivity of OH and CH3OH between 22 and 64 K: Modelling the gas phase production of CH3O in Barnard 1b
In the last years, ultra-low temperature chemical kinetic experiments have
demonstrated that some gas-phase reactions are much faster than previously
thought. One example is the reaction between OH and CH3OH, which has been
recently found to be accelerated at low temperatures yielding CH3O as main
product. This finding opened the question of whether the CH3O observed in the
dense core Barnard 1b could be formed by the gas-phase reaction of CH3OH and
OH. Several chemical models including this reaction and grain-surface processes
have been developed to explain the observed abundance of CHO with little
success. Here we report for the first time rate coefficients for the gas-phase
reaction of OH and CH3OH down to a temperature of 22 K, very close to those in
cold interstellar clouds. Two independent experimental set-ups based on the
supersonic gas expansion technique coupled to the pulsed laser photolysis-laser
induced fluorescence technique were used to determine rate coefficients in the
temperature range 22-64 K. The temperature dependence obtained in this work can
be expressed as k(22-64 K) = (3.6+/-0.1)e-12 (T/ 300)^(-1.0+/-0.2) cm3
molecule-1 s-1. Implementing this expression in a chemical model of a cold
dense cloud results in CH3O/CH3OH abundance ratios similar or slightly lower
than the value of 3e-3 observed in Barnard 1b. This finding confirms that the
gas-phase reaction between OH and CH3OH is an important contributor to the
formation of interstellar CH3O. The role of grain-surface processes in the
formation of CH3O, although it cannot be fully neglected, remains
controversial.Comment: Accepted for publication in The Astrophysical Journa
Decay constants, semi-leptonic and non-leptonic decays in a Bethe-Salpeter Model
We evaluate the decay constants for the B and mesons and the form factors
for the semileptonic decays of the B meson to and mesons in a
Bethe-Salpeter model. From data we extract from and from decays. The form factors are then used to obtain non-leptonic
decay partial widths for and in the
factorization approximation.Comment: 15 Pages, 3 Postscript figures (available also from [email protected]
High-Redshift Star-Forming Galaxies: Angular Momentum and Baryon Fraction, Turbulent Pressure Effects and the Origin of Turbulence
The structure of a sample of high-redshift (z=2), rotating galaxies with high
star formation rates and turbulent gas velocities of sigma=40-80 km/s is
investigated. Fitting the observed disk rotational velocities and radii with a
Mo, Mao, White (1998) (MMW) model requires unusually large disk spin parameters
lambda_d>0.1 and disk-to-dark halo mass fraction m_d=0.2, close to the cosmic
baryon fraction. The galaxies segregate into dispersion-dominated systems with
1<vmax/sigma<3, maximum rotational velocities vmax<200 km/s and disk half-light
radii rd=1-3 kpc and rotation-dominated systems with vmax>200 km/s,
vmax/sigma>3 and rd=4-8 kpc. For the dispersion-dominated sample, radial
pressure gradients partly compensate the gravitational force, reducing the
rotational velocities. Including this pressure effect in the MMW model,
dispersion-dominated galaxies can be fitted well with spin parameters lf
lambda_d=0.03-0.05 for high disk mass fractions of m_d=0.2 and with
lambda_d=0.01-0.03 for m_d=0.05. These values are in good agreement with
cosmological expectations. For the rotation-dominated sample however pressure
effects are small and better agreement with theoretically expected disk spin
parameters can only be achieved if the dark halo mass contribution in the
visible disk regime (2-3*rd) is smaller than predicted by the MMW model. We
argue that these galaxies can still be embedded in standard cold dark matter
halos if the halos did not contract adiabatically in response to disk
formation. It is shown that the observed high turbulent gas motions of the
galaxies are consistent with a Toomre instability parameter Q=1 which is equal
to the critical value, expected for gravitational disk instability to be the
major driver of turbulence. The dominant energy source of turbulence is then
the potential energy of the gas in the disk.Comment: 23 pages, 4 figures, ApJ, in pres
Neuromonitoring in neonatal critical care part II: extremely premature infants and critically ill neonates
Abstract: Neonatal intensive care has expanded from cardiorespiratory care to a holistic approach emphasizing brain health. To best understand and monitor brain function and physiology in the neonatal intensive care unit (NICU), the most commonly used tools are amplitude-integrated EEG, full multichannel continuous EEG, and near-infrared spectroscopy. Each of these modalities has unique characteristics and functions. While some of these tools have been the subject of expert consensus statements or guidelines, there is no overarching agreement on the optimal approach to neuromonitoring in the NICU. This work reviews current evidence to assist decision making for the best utilization of these neuromonitoring tools to promote neuroprotective care in extremely premature infants and in critically ill neonates. Neuromonitoring approaches in neonatal encephalopathy and neonates with possible seizures are discussed separately in the companion paper. Impact: For extremely premature infants, NIRS monitoring has a potential role in individualized brain-oriented care, and selective use of aEEG and cEEG can assist in seizure detection and prognostication.For critically ill neonates, NIRS can monitor cerebral perfusion, oxygen delivery, and extraction associated with disease processes as well as respiratory and hypodynamic management. Selective use of aEEG and cEEG is important in those with a high risk of seizures and brain injury.Continuous multimodal monitoring as well as monitoring of sleep, sleep–wake cycling, and autonomic nervous system have a promising role in neonatal neurocritical care
A facility to Search for Hidden Particles (SHiP) at the CERN SPS
A new general purpose fixed target facility is proposed at the CERN SPS
accelerator which is aimed at exploring the domain of hidden particles and make
measurements with tau neutrinos. Hidden particles are predicted by a large
number of models beyond the Standard Model. The high intensity of the SPS
400~GeV beam allows probing a wide variety of models containing light
long-lived exotic particles with masses below (10)~GeV/c,
including very weakly interacting low-energy SUSY states. The experimental
programme of the proposed facility is capable of being extended in the future,
e.g. to include direct searches for Dark Matter and Lepton Flavour Violation.Comment: Technical Proposa
Comment on “Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature” by W. Siebrand, Z. Smedarchina, E. Martínez-Núñez and A. Fernández-Ramos, Phys. Chem. Chem. Phys., 2016, 18, 22712
The article “Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature” proposes a dimer mediated mechanism in order to explain the large low temperature rate coefficients for the OH + methanol reaction measured by several groups. It is demonstrated here theoretically that under the conditions of these low temperature experiments, there are insufficient dimers formed for the proposed new mechanism to apply. Experimental evidence is also presented to show that dimerization of the methanol reagent does not influence the rate coefficients reported under the conditions of methanol concentration used for the kinetics studies. It is also emphasised that the low temperature experiments have been performed using both the Laval nozzle expansion and flow-tube methods, with good agreement found for the rate coefficients measured using these two distinct techniques
Neuroprotective therapies in the NICU in term infants: present and future
Outcomes of neonatal encephalopathy (NE) have improved since the widespread implementation of therapeutic hypothermia (TH) in high-resource settings. While TH for NE in term and near-term infants has proven beneficial, 30–50% of infants with moderate-to-severe NE treated with TH still suffer death or significant impairments. There is therefore a critical need to find additional pharmacological and non-pharmacological interventions that improve the outcomes for these children. There are many potential candidates; however, it is unclear whether these interventions have additional benefits when used with TH. Although primary and delayed (secondary) brain injury starting in the latent phase after HI are major contributors to neurodisability, the very late evolving effects of tertiary brain injury likely require different interventions targeting neurorestoration. Clinical trials of seizure management and neuroprotection bundles are needed, in addition to current trials combining erythropoietin, stem cells, and melatonin with TH
B --> D(D^*) Form Factors in a Bethe-Salpeter model
We calculate the form factors for the semileptonic decays of the meson to
and mesons in a Bethe-Salpeter model. We show that our model is
consistent with the constraints of Heavy Quark Effective Theory (HQET) and we
extract the matrix elements that represent the corrections to the form
factors in HQET. With available data, we obtain =.Comment: 15 pages, 5 figure
Acellular Bone Marrow Extracts Significantly Enhance Engraftment Levels of Human Hematopoietic Stem Cells in Mouse Xeno-Transplantation Models
Hematopoietic stem cells (HSC) derived from cord blood (CB), bone marrow (BM), or mobilized peripheral blood (PBSC) can differentiate into multiple lineages such as lymphoid, myeloid, erythroid cells and platelets. The local microenvironment is critical to the differentiation of HSCs and to the preservation of their phenotype in vivo. This microenvironment comprises a physical support supplied by the organ matrix as well as tissue specific cytokines, chemokines and growth factors. We investigated the effects of acellular bovine bone marrow extracts (BME) on HSC in vitro and in vivo. We observed a significant increase in the number of myeloid and erythroid colonies in CB mononuclear cells (MNC) or CB CD34+ cells cultured in methylcellulose media supplemented with BME. Similarly, in xeno-transplantation experiments, pretreatment with BME during ex-vivo culture of HSCs induced a significant increase in HSC engraftment in vivo. Indeed, we observed both an increase in the number of differentiated myeloid, lymphoid and erythroid cells and an acceleration of engraftment. These results were obtained using CB MNCs, BM MNCs or CD34+ cells, transplanted in immuno-compromised mice (NOD/SCID or NSG). These findings establish the basis for exploring the use of BME in the expansion of CB HSC prior to HSC Transplantation. This study stresses the importance of the mechanical structure and soluble mediators present in the surrounding niche for the proper activity and differentiation of stem cells
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