340 research outputs found
Three-Dimensional Phthalocyanine Metal-Catecholates for High Electrochemical Carbon Dioxide Reduction.
The synthesis of a new anionic 3D metal-catecholate framework, termed MOF-1992, is achieved by linking tetratopic cobalt phthalocyanin-2,3,9,10,16,17,23,24-octaol linkers with Fe3(-C2O2-)6(OH2)2 trimers into an extended framework of roc topology. MOF-1992 exhibits sterically accessible Co active sites together with charge transfer properties. Cathodes based on MOF-1992 and carbon black (CB) display a high coverage of electroactive sites (270 nmol cm-2) and a high current density (-16.5 mA cm-2; overpotential, -0.52 V) for the CO2 to CO reduction reaction in water (faradaic efficiency, 80%). Over the 6 h experiment, MOF-1992/CB cathodes reach turnover numbers of 5800 with turnover frequencies of 0.20 s-1 per active site
Rapid Cycling and Exceptional Yield in a Metal-Organic Framework Water Harvester.
Sorbent-assisted water harvesting from air represents an attractive way to address water scarcity in arid climates. Hitherto, sorbents developed for this technology have exclusively been designed to perform one water harvesting cycle (WHC) per day, but the productivities attained with this approach cannot reasonably meet the rising demand for drinking water. This work shows that a microporous aluminum-based metal-organic framework, MOF-303, can perform an adsorption-desorption cycle within minutes under a mild temperature swing, which opens the way for high-productivity water harvesting through rapid, continuous WHCs. Additionally, the favorable dynamic water sorption properties of MOF-303 allow it to outperform other commercial sorbents displaying excellent steady-state characteristics under similar experimental conditions. Finally, these findings are implemented in a new water harvester capable of generating 1.3 L kgMOF -1 day-1 in an indoor arid environment (32% relative humidity, 27 °C) and 0.7 L kgMOF -1 day-1 in the Mojave Desert (in conditions as extreme as 10% RH, 27 °C), representing an improvement by 1 order of magnitude over previously reported devices. This study demonstrates that creating sorbents capable of rapid water sorption dynamics, rather than merely focusing on high water capacities, is crucial to reach water production on a scale matching human consumption
Fast-neutron induced background in LaBr3:Ce detectors
The response of a scintillation detector with a cylindrical 1.5-inch LaBr3:Ce
crystal to incident neutrons has been measured in the energy range En = 2-12
MeV. Neutrons were produced by proton irradiation of a Li target at Ep = 5-14.6
MeV with pulsed proton beams. Using the time-of-flight information between
target and detector, energy spectra of the LaBr3:Ce detector resulting from
fast neutron interactions have been obtained at 4 different neutron energies.
Neutron-induced gamma rays emitted by the LaBr3:Ce crystal were also measured
in a nearby Ge detector at the lowest proton beam energy. In addition, we
obtained data for neutron irradiation of a large-volume high-purity Ge detector
and of a NE-213 liquid scintillator detector, both serving as monitor detectors
in the experiment. Monte-Carlo type simulations for neutron interactions in the
liquid scintillator, the Ge and LaBr3:Ce crystals have been performed and
compared with measured data. Good agreement being obtained with the data, we
present the results of simulations to predict the response of LaBr3:Ce
detectors for a range of crystal sizes to neutron irradiation in the energy
range En = 0.5-10 MeVComment: 28 pages, 10 figures, 4 Table
Absolute magnitude and kinematics of Barium stars
The absolute magnitude of barium stars has been obtained from kinematical data using a new algorithm based on the maximum-likelihood principle. The method allows to separate a sample into groups characterized by different mean absolute magnitudes, kinematics and z-scale heights. It also takes into account, simultaneously, the censorship in the sample and the errors on the observables. The method has been applied to a sample of 318 barium stars. Four groups have been detected. Three of them show a kinematical behaviour corresponding to disk population stars. The fourth group contains stars with halo kinematics. The luminosities of the disk population groups spread a large range. The intrinsically brightest one (M_v_=-1.5mag, σ_M_=0.5mag) seems to be an inhomogeneous group containing barium binaries as well as AGB single stars. The most numerous group (about 150 stars) has a mean absolute magnitude corresponding to stars in the red giant branch (M_v_=0.9mag, σ_M_=0.8mag). The third group contains barium dwarfs, the obtained mean absolute magnitude is characteristic of stars on the main sequence or on the subgiant branch (M_v_=3.3mag, σ_M_=0.5mag). The obtained mean luminosities as well as the kinematical results are compatible with an evolutionary link between barium dwarfs and classical barium giants. The highly luminous group is not linked with these last two groups. More high-resolution spectroscopic data will be necessary in order to better discriminate between barium and non-barium stars
EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
We report the discovery of eleven beat Cepheids in the Small Magellanic
Cloud, using data obtained by the EROS microlensing survey. Four stars are
beating in the fundamental and first overtone mode (F/1OT), seven are beating
in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is
systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the
SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A
A Letter
The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant () determination
Recent HST determinations of the expansion's rate of the Universe (the Hubble
constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I
are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996,
Tanvir et al., 1995). The three groups obtain different vales for H_0. We note
that most of this discrepancy stems from the asumption (by both groups) that
the Period-Luminosity relation is independent of metallicity. We come to this
conclusion as a result of our study of the Period-Luminosity relation of 481
Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud
and the Small Magellanic Cloud obtained as a by-product of the EROS
microlensing survey. We find that the derived interstellar absorption
corrections are particularly sensitive to the metallicity and when our result
is applied to recent estimates based on HST Cepheids observations it makes the
low-H_0 values higher and the high-H_0 value lower, bringing those discrepant
estimates into agrement around .Comment: 4 pages, Latex, with 2 .ps accepted for publication astronomy and
astrophysics Letter
The short-duration GRB 050724 host galaxy in the context of the long-duration GRB hosts
We report optical and near-infrared broad band observations of the
short-duration GRB 050724 host galaxy, used to construct its spectral energy
distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs),
which show younger stellar populations, the SED of the GRB 050724 host galaxy
is optimally fitted with a synthetic elliptical galaxy template based on an
evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to
reproduce with non-evolving metallicity templates. In contrast, if the short
GRB host galaxy metallicity enrichment is considered, the synthetic templates
fit the observed SED satisfactorily. The internal host extinction is low (A_v
\~< 0.4 mag) so it cannot explain the faintness of the afterglow. This short
GRB host galaxy is more massive (~5x10^10 Mo) and luminous (~1.1 L*) than most
of the long-duration GRB hosts. A statistical comparison based on the ages of
short- and long-duration GRB host galaxies strongly suggests that
short-duration GRB hosts contain, on average, older progenitors. These findings
support a different origin for short- and long-duration GRBs.Comment: A&A in pres
Observation of periodic variable stars towards the galactic spiral arms by EROS II
We present the results of a massive variability search based on a photometric
survey of a six square degree region along the Galactic plane at (, ) and (, ). This
survey was performed in the framework of the EROS II (Exp\'erience de Recherche
d'Objets Sombres) microlensing program. The variable stars were found among
1,913,576 stars that were monitored between April and June 1998 in two
passbands, with an average of 60 measurements. A new period-search technique is
proposed which makes use of a statistical variable that characterizes the
overall regularity of the flux versus phase diagram. This method is well suited
when the photometric data are unevenly distributed in time, as is our case.
1,362 objects whose luminosity varies were selected. Among them we identified 9
Cepheids, 19 RR Lyrae, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular
stars. Most of them are newly identified objects. The cross-identification with
known catalogues has been performed. The mean distance of the RR Lyrae is
estimated to be kpc undergoing an average absorption of
magnitudes. This distance is in good agreement with the one
of disc stars which contribute to the microlensing source star population.Our
catalogue and light curves are available electronically from the CDS,
Strasbourg and from our Web site http://eros.in2p3.fr.Comment: 15 pages, 11 figures, accepted in A&A (april 2002
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