6,363 research outputs found
Swift Identification of Dark Gamma-Ray Bursts
We present an optical flux vs. X-ray flux diagram for all known gamma-ray
bursts (GRBs) for which an X-ray afterglow has been detected. We propose an
operational definition of dark bursts as those bursts that are optically
subluminous with respect to the fireball model, i.e., which have an
optical-to-X-ray spectral index beta_OX < 0.5. Out of a sample of 52 GRBs we
identify 5 dark bursts. The definition and diagram serve as a simple and quick
diagnostic tool for identifying dark GRBs based on limited information,
particularly useful for early and objective identification of dark GRBs
observed with the Swift satellite.Comment: 4 pages, 1 figure. ApJ Letters, in pres
ISO far-infrared observations of rich galaxy clusters II. Sersic 159-03
The far-infrared emission from rich galaxy clusters is investigated. Maps
have been obtained by ISO at 60, 100, 135, and 200 microns using the PHT-C
camera. Ground based imaging and spectroscopy were also acquired. Here we
present the results for the cooling flow cluster Sersic 159-03. An infrared
source coincident with the dominant cD galaxy is found. Some off-center sources
are also present, but without any obvious counterparts.Comment: 6 pages, 4 postscript figures, accepted for publication in `Astronomy
and Astrophysics
Volume-energy correlations in the slow degrees of freedom of computer-simulated phospholipid membranes
Constant-pressure molecular-dynamics simulations of phospholipid membranes in
the fluid phase reveal strong correlations between equilibrium fluctuations of
volume and energy on the nanosecond time-scale. The existence of strong
volume-energy correlations was previously deduced indirectly by Heimburg from
experiments focusing on the phase transition between the fluid and the ordered
gel phases. The correlations, which are reported here for three different
membranes (DMPC, DMPS-Na, and DMPSH), have volume-energy correlation
coefficients ranging from 0.81 to 0.89. The DMPC membrane was studied at two
temperatures showing that the correlation coefficient increases as the phase
transition is approached
Isomorphs in model molecular liquids
Isomorphs are curves in the phase diagram along which a number of static and
dynamic quantities are invariant in reduced units. A liquid has good isomorphs
if and only if it is strongly correlating, i.e., the equilibrium
virial/potential energy fluctuations are more than 90% correlated in the NVT
ensemble. This paper generalizes isomorphs to liquids composed of rigid
molecules and study the isomorphs of two systems of small rigid molecules, the
asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for
both systems we find that the isochoric heat capacity, the excess entropy, the
reduced molecular center-of-mass self part of the intermediate scattering
function, the reduced molecular center-of-mass radial distribution function to
a good approximation are invariant along an isomorph. In agreement with theory,
we also find that an instantaneous change of temperature and density from an
equilibrated state point to another isomorphic state point leads to no
relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be
more approximative than those of the asymmetric dumbbell model, which is
consistent with the OTP model being less strongly correlating. For both models
we find "master isomorphs", i.e., isomorphs have identical shape in the
virial/potential energy phase diagram.Comment: 20 page
On the organic carbon maximum on the continental slope of the eastern Arabian Sea
The sedimentary organic carbon maximum on the continental slope off western India is widely believed to be due to the preferential preservation of deposited organic matter at water depths where the intense oxygen minimum intersects the sea floor. This region is considered to constitute one of the modern analogues for the environment of formation of organic-rich sedimentary facies that are common in the geological record. We critically examine the hypothesis that the oxygen minimum in the eastern Arabian Sea is the site of enhanced organic matter accumulation and preservation using analyses of suites of samples with wide geographical coverage along this margin. Organic carbon and nitrogen reach maximum concentrations between 200 and 1600 m depth, whereas the lowest dissolved oxygen contents in the oxygen minimum lie between 200 and 800 m depth. The Corganic/N ratios and the δ13Corganic values show that the organic matter is overwhelmingly marine, and Rock-Eval pyrolysis data demonstrate that the hydrogen indices of the sediments are similar in the sediments accumulating within and outside the oxygen minimum. Thus, the organic carbon maximum extends over a larger depth range than the oxygen minimum (as is also evident on some other slopes), and there is no evidence for preferential preservation of the organic matter within the oxygen minimum. The distribution of organic matter on the western Indian continental margin is controlled by (1) variations in supply (decreasing westward away from the centers of coastal upwelling and also decreasing with increasing water depth), (2) dilution by other sedimentary components, and (3) the texture of the sediments (coarser-grained sediments having lower carbon contents), which is controlled in turn by sediment supply and reworking. The evidence available suggests that the organic carbon maximum on this slope is not related to the position of the oxygen minimum and, consequently, that oxygen minima cannot be used to explain the distribution of organic carbon at intermediate palaeodepths in the geological record
The Rigidly Rotating Magnetosphere of Sigma Ori E
We attempt to characterize the observed variability of the magnetic
helium-strong star sigma Ori E in terms of a recently developed rigidly
rotating magnetosphere model. This model predicts the accumulation of
circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic
equilibrium at the intersection between magnetic and rotational equators. We
find that the model can reproduce well the periodic modulations observed in the
star's light curve, H alpha emission-line profile, and longitudinal field
strength, confirming that it furnishes an essentially correct, quantitative
description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap
Measurement-based modeling of bromine chemistry at the Dead Sea boundary layer ? Part 2: The influence of NO<sub>2</sub> on bromine chemistry at mid-latitude areas
International audienceUnderstanding the interaction between anthropogenic air pollution and Reactive Halogen Species (RHS) activity has had only limited support of direct field measurements, due to the fact that past field measurements of RHS have been mainly performed in Polar Regions. The present paper investigates the interaction between NO2 and Reactive Bromine Species (RBS) activity by model simulations based on extensive field measurements performed in the Dead Sea area, as described in a companion paper (Tas et al., 2006). The Dead Sea is an excellent natural laboratory for this investigation since elevated concentrations of BrO (up to more than 150 pptv) are frequently observed, while the average levels of NO2 are around several ppb. The results of the present study show that under the chemical mechanisms that occur at the Dead Sea, higher levels of NO2 lead to higher daily average concentrations of BrOX, as a result of an increase in the rate of the heterogeneous decomposition of BrONO2 that in turn causes an increase in the rate of the "Bromine Explosion" mechanism. The present study has shown that the influence of NO2 on BrOX production clearly reflects an enhancement of RBS activity caused by anthropogenic activity. However, above a certain threshold level of NO2 (daily average mixing ratios of 0.2 ppbv during RBS activity), the daily average concentrations of BrOX decrease for a further increase in the NO2 concentrations
Measurement-based modeling of bromine chemistry in the boundary layer: 1. Bromine chemistry at the Dead Sea
International audienceThe Dead Sea is an excellent natural laboratory for the investigation of Reactive Bromine Species (RBS) chemistry, due to the high RBS levels observed in this area, combined with anthropogenic air pollutants up to several ppb. The present study investigated the basic chemical mechanism of RBS at the Dead Sea using a numerical one-dimensional chemical model. Simulations were based on data obtained from comprehensive measurements performed at sites along the Dead Sea. The simulations showed that the high BrO levels measured frequently at the Dead Sea could only partially be attributed to the highly concentrated Br? present in the Dead Sea water. Furthermore, the RBS activity at the Dead Sea cannot solely be explained by a pure gas phase mechanism. This paper presents a chemical mechanism which can account for the observed chemical activity at the Dead Sea, with the addition of only two heterogeneous processes: the "Bromine Explosion" mechanism and the heterogeneous decomposition of BrONO2. Ozone frequently dropped below a threshold value of ~1 to 2 ppbv at the Dead Sea evaporation ponds, and in such cases, O3 became a limiting factor for the production of BrOx (BrO+Br). The entrainment of O3 fluxes into the evaporation ponds was found to be essential for the continuation of RBS activity, and to be the main reason for the jagged diurnal pattern of BrO observed in the Dead Sea area, and for the positive correlation observed between BrO and O3 at low O3 concentrations. The present study has shown that the heterogeneous decomposition of BrONO2 has a great potential to affect the RBS activity in areas influenced by anthropogenic emissions, mainly due to the positive correlation between the rate of this process and the levels of NO2. Further investigation of the influence of the decomposition of BrONO2 may be especially important in understanding the RBS activity at mid-latitudes
One Dimensional Kondo Lattice Model Studied by the Density Matrix Renormalization Group Method
Recent developments of the theoretical investigations on the one-dimensional
Kondo lattice model by using the density matrix renormalization group (DMRG)
method are discussed in this review. Short summaries are given for the
zero-temperature DMRG, the finite-temperature DMRG, and also its application to
dynamic quantities. Away from half-filling, the paramagnetic metallic state is
shown to be a Tomonaga-Luttinger liquid with the large Fermi surface. For the
large Fermi surface its size is determined by the sum of the densities of the
conduction electrons and the localized spins. The correlation exponent K_rho of
this metallic phase is smaller than 1/2. At half-filling the ground state is
insulating. Excitation gaps are different depending on channels, the spin gap,
the charge gap and the quasiparticle gap. Temperature dependence of the spin
and charge susceptibilities and specific heat are discussed. Particularly
interesting is the temperature dependence of various excitation spectra, which
show unusual properties of the Kondo insulators.Comment: 18 pages, 23 Postscript figures, REVTe
Development of a standardised cup anemometer suited to wind energy applications.:Publishable final report
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