Correlations in a confined magnetized free-electron gas

Equilibrium quantum statistical methods are used to study the pair correlation function for a magnetized free-electron gas in the presence of a hard wall that is parallel to the field. With the help of a path-integral technique and a Green function representation the modifications in the correlation function caused by the wall are determined both for a non-degenerate and for a completely degenerate gas. In the latter case the asymptotic behaviour of the correlation function for large position differences in the direction parallel to the wall and perpendicular to the field, is found to change from Gaussian in the bulk to algebraic near the wall.Comment: 24 pages, 10 figures, submitted to J. Phys. A: Math. Ge

Steinberg modules and Donkin pairs

We prove that in positive characteristic a module with good filtration for a group of type E6 restricts to a module with good filtration for a subgroup of type F4. (Recall that a filtration of a module for a semisimple algebraic group is called good if its layers are dual Weyl modules.) Our result confirms a conjecture of Brundan for one more case. The method relies on the canonical Frobenius splittings of Mathieu. Next we settle the remaining cases, in characteristic not 2, with a computer-aided variation on the old method of Donkin.Comment: 16 pages; proof of Brundan's conjecture adde

Electrochemical methods for speciation of trace elements in marine waters. Dynamic aspects

The contribution of electrochemical methods to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the determination of trace elements. Competitive ligand ex- change-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is in fluenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning

[BMIm][BARF] imidazolium salt solutions in alkyl carbonate solvents: Structure and interactions

Solutions of weakly coordinating ionic liquids (ILs) in alkyl carbonates are gaining growing attention, as the latter are "green" solvents with high solvation power, but the phase behavior and structure of ILs in organic polar solvents are still poorly understood. Here, we study the interactions and nanoscale structure of 1-butyl-3-methylimidazolium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [BMIm][BARF], in three symmetrical alkyl carbonate solvents with increasing alkyl chain-length. Electrical conductivity and nuclear magnetic resonance measurements showed that [BMIm][BARF] was mostly undissociated in these solvents, especially at lower IL concentration. Small angle X-ray scattering patterns evidenced the presence of rod-like nanostructures in the IL/solvent mixtures. At higher IL concentration, [BMIm][BARF] is increasingly more dissociated in solvents with lower dielectric constant, as confirmed by analysis of the solvents' carbonyl stretching band via Fourier transform infrared spectroscopy. This trend is opposite to that exhibited by BMIm ILs with less bulky counterions. The bulky BARF(-) is weakly coordinating and has no ability to give strong H-bonding, thus short-range anisotropic van der Waals forces are likely key in the interaction of the ion pairs. The slower self-diffusion of the ions in alkyl carbonates with lower dielectric constants might partially hinder close contact needed for self-assembly into local nano-sized structures. Overall, our results shed light on interactions and self-organization in imidazolium salt-alkyl carbonate mixtures, with potential impact in applicative fields spanning from batteries, catalysis and extraction, up to bio-applications (antimicrobial and bioengineering)

“SPLIT” Pancreaticojejunostomy in the Surgical Treatment of Chronic Pancreatitis

“Split” pancreaticojejunostomy is a procedure consisting of vertical transection of the pancreas and anastomosis of both sides of the cut pancreatic duct with an interposed, Roux-en-Y jejunal loop. In this paper we report the long term results of this procedure in the treatment of eight patients with chronic pancreatitis (CP)

Modelling CO emission from Mira's wind

We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new observational constraints. These are obtained from photospheric light scattered in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6 micron: absolute fluxes, the radial dependence of the scattered intensity, and two line ratios. Further observational constraints are provided by ISO observations of far-IR emission lines from highly excited rotational states of the ground vibrational state of CO, and radio observations of lines from rotational levels of low excitation of CO. A code based on the Monte-Carlo technique is used to model the circumstellar line emission. We find that it is possible to model the radio and ISO fluxes, as well as the highly asymmetric radio-line profiles, reasonably well with a spherically symmetric and smooth stellar wind model. However, it is not possible to reproduce the observed NIR line fluxes consistently with a `standard model' of the stellar wind. This is probably due to incorrectly specified conditions of the inner regions of the wind model, since the stellar flux needs to be larger than what is obtained from the standard model at the point of scattering, i.e., the intermediate regions at approximately 100-400 stellar radii (2"-7") away from the star. Thus, the optical depth in the vibrational-rotational lines from the star to the point of scattering has to be decreased. This can be accomplished in several ways. For instance, the gas close to the star (within approximately 2") could be in such a form that light is able to pass through, either due to the medium being clumpy or by the matter being in radial structures (which, further out, developes into more smooth or shell-like structures).Comment: 18 pages, 3 figures, accepted for publication in Ap

Why are the K dwarfs in the Pleiades so Blue?

The K dwarfs in the Pleiades fall nearly one half magnitude below a main sequence isochrone when plotted in a color-magnitude diagram utilizing V magnitude as the luminosity index and B-V as the color index. This peculiarity has been known for forty years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using B-V as the color index. However, using V-I as the color index, stars in the two clusters are coincident to M_V ~ 10; using V-K as the color index, Pleiades late K and M stars fall above the main sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (=> 50%). Weak-lined T Tauri stars have similar color anomalies, and we suspect this is a common feature of all very young K dwarfs (sp. type > K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre-main sequence isochrone-fitting ages for clusters like the Pleiades since the age derived will depend on the temperature index used.Comment: 41 pages, 15 figures, AASTeX5.0. Accepted 05 May 2003; Scheduled for publication in the Astronomical Journal (August 2003

Pulsars with the Australian Square Kilometre Array Pathfinder

The Australian Square Kilometre Array Pathfinder (ASKAP) is a 36-element array with a 30-square-degree field of view being built at the proposed SKA site in Western Australia. We are conducting a Design Study for pulsar observations with ASKAP, planning both timing and search observations. We provide an overview of the ASKAP telescope and an update on pulsar-related progress.Comment: To appear in proceedings of "Radio Pulsars: An astrophysical key to unlock the secrets of the Universe