The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon

Spatial inhomogeneities in the spectral shape of the ultra-violet background (UVB) at the tail-end of HeII reionisation are thought to be the primary cause of the large fluctuations observed in the HeII to HI Ly-a forest optical depth ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will also influence the ionisation balance of intergalactic metals; we extract realistic quasar absorption spectra from a large hydrodynamical simulation to examine their impact on intergalactic SiIV and CIV absorbers. Using a variety of toy UVB models, we find that while the predicted spatial inhomogeneities in spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean free path for photons with frequencies above and below the HeII ionisation edge means these fluctuations have less effect on the SiIV and CIV ionisation balance. Furthermore, UVB models which produce the largest fluctuations in specific intensity at the HeII ionisation edge also have the softest ionising spectra, and thus result in photo-ionisation rates which are too low to produce significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find spatial variations in the IGM metallicity will dominate any scatter in tau_SiIV/tau_CIV. Our results suggest that observational evidence for homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the possibility of significant fluctuations in the UVB spectral shape at z~2-3. On the other hand, the scatter in metallicity inferred from observations of intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform ionisation corrections is likely intrinsic, and therefore provides a valuable constraint on intergalactic metal enrichment scenarios at these redshifts.Comment: 13 pages, 7 figures, accepted to MNRA

The implications of resonant x-ray scattering data on the physics of the insulating phase of V_2O_3

We have performed a quantitative analysis of recent resonant x-ray scattering experiments carried out in the antiferromagnetic phase of V_2O_3 by means of numerical ab-initio simulations. In order to treat magnetic effects, we have developed a method based on multiple scattering theory (MST) and a relativistic extension of the Schr\"{o}dinger Equation, thereby working with the usual non relativistic set of quantum numbers $l,m,\sigma$ for angular and spin momenta. Electric dipole-dipole (E1-E1), dipole-quadrupole (E1-E2) and quadrupole-quadrupole (E2-E2) transition were considered altogether. We obtain satisfactory agreement with experiments, both in energy and azimuthal scans. All the main features of the V K edge Bragg-forbidden reflections with $h+k+l=$odd can be interpreted in terms of the antiferromagnetic ordering only, {\it ie}, they are of magnetic origin. In particular the ab-initio simulation of the energy scan around the (1,1,1)-monoclinic reflection excludes the possibility of any symmetry reduction due to a time-reversal breaking induced by orbital ordering.Comment: 11 pages, 6 figure

On the nature of the magnetic ground-state wave function of V_2O_3

After a brief historical introduction, we dwell on two recent experiments in the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray scattering and non-reciprocal linear dichroism, whose interpretations are in conflict, as they require incompatible magnetic space groups. Such a conflict is critically reviewed, in the light of the present literature, and new experimental tests are suggested, in order to determine unambiguously the magnetic group. We then focus on the correlated, non-local nature of the ground-state wave function, that is at the basis of some drawbacks of the LDA+U approach: we singled out the physical mechanism that makes LDA+U unreliable, and indicate the way out for a possible remedy. Finally we explain, by means of a symmetry argument related to the molecular wave function, why the magnetic moment lies in the glide plane, even in the absence of any local symmetry at vanadium sites.Comment: 7 pages, 1 figur

Rapid, Specific Determination of Iodine and Iodide by Combined Solid-Phase Extraction/Diffuse Reflectance Spectroscopy

A new, rapid methodology for trace analysis using solid-phase extraction is described. The two-step methodology is based on the concentration of an analyte onto a membrane disk and on the determination by diffuse reflectance spectroscopy of the amount of analyte extracted on the disk surface. This method, which is adaptable to a wide range of analytes, has been used for monitoring ppm levels of iodine and iodide in spacecraft water. Iodine is used as a biocide in spacecraft water. For these determinations, a water sample is passed through a membrane disk by means of a 10-mL syringe that is attached to a disk holder assembly. The disk, which is a polystyrene−divinylbenzene composite, is impregnated with poly(vinylpyrrolidone) (PVP), which exhaustively concentrates iodine as a yellow iodine−PVP complex. The amount of concentrated iodine is then determined in only 2 s by using a hand-held diffuse reflectance spectrometer by comparing the result with a calibration curve based on the Kubelka−Munk function. The same general procedure can be used to determine iodide levels after its facile and exhaustive oxidation to iodine by peroxymonosulfate (i.e., Oxone reagent). For samples containing both analytes, a two-step procedure can be used in which the iodide concentration is calculated from the difference in iodine levels before and after treatment of the sample with peroxymonosulfate. With this methodology, iodine and iodide levels in the 0.1−5.0 ppm range can be determined with a total workup time of ∼60 s with a RSD of ∼6%

Galactic Centre stellar winds and Sgr A* accretion

(ABRIDGED) We present in detail our new 3D numerical models for the accretion of stellar winds on to Sgr A*. In our most sophisticated models, we put stars on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account for radiative cooling. We first model only one phase `fast' stellar winds (1000 km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5 Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models. In contrast, Mdot decreases by an order of magnitude for stars following circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively, forming cold clumps immersed into the X-ray emitting gas. We test two orbital configurations for the stars in this scenario, an isotropic distribution and two rotating discs with perpendicular orientation. The morphology of cold gas is quite sensitive to the orbits. In both cases, however, most of the accreted gas is hot, with an almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The cold gas accretes in intermittent, short but powerful episodes which may give rise to large amplitude variability in the luminosity of Sgr A* on time scales of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and 1e4 Rsch, for the one and two-phase wind simulations, respectively, never forming the quasi-spherical accretion flows suggested in some previous work. Our work suggests that, averaged over time scales of 100s to 1000s of years, the radiative and mechanical luminosity of Sgr A* may be substantially higher than it is in its current state. Further improvements of the wind accretion modelling of Sgr A* will rely on improved observational constraints for the wind properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2: minor changes

Rapid, Low Level Determination of Silver(I) in Drinking Water by Colorimetric–solid-phase Extraction

A rapid, highly sensitive two-step procedure for the trace analysis of silver(I) is described. The method is based on: (1) the solid-phase extraction (SPE) of silver(I) from a water sample onto a disk impregnated with a silver-selective colorimetric reagent, and (2) the determination of the amount of complexed analyte extracted by the disk by diffuse reflectance spectroscopy (DRS). This method, called colorimetric–solid-phase extraction (C–SPE), was recently shown effective in determining low concentrations (0.1–5.0 mg/ml) of iodine and iodide in drinking water. This report extends C–SPE to the trace (∼4 μg/l) level monitoring of silver(I) which is a biocide used on the International Space Station (ISS). The determination relies on the manually driven passage of a water sample through a polystyrene–divinylbenzene disk that has been impregnated with the colorimetric reagent 5-(p-dimethylaminobenzylidene) rhodanine (DMABR) and with an additive such as a semi-volatile alcohol (1,2-decanediol) or nonionic surfactant (Brij 30). The amount of concentrated silver(I) is then determined in a few seconds by using a hand-held diffuse reflectance spectrometer, with a total sample workup and readout time of ∼60 s. Importantly, the additive induces the uptake of water by the disk, which creates a local environment conducive to silver(I) complexation at an extremely high concentration factor (∼800). There is no detectable reaction between silver(I) and impregnated DMABR in the absence of the additive. This strategy represents an intriguing new dimension for C–SPE in which additives, directly loaded in the disk material, provide a means to manipulate the reactivity of the impregnated reagent

Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V_2O_3

Motivated by recent neutron and x-ray observations in V_2O_3, we derive the effective Hamiltonian in the strong coupling limit of an Hubbard model with three degenerate t_{2g} states containing two electrons coupled to spin S = 1, and use it to re-examine the low-temperature ground-state properties of this compound. An axial trigonal distortion of the cubic states is also taken into account. Since there are no assumptions about the symmetry properties of the hopping integrals involved, the resulting spin-orbital Hamiltonian can be generally applied to any crystallographic configuration of the transition metal ion giving rise to degenerate t_{2g} orbitals. Specializing to the case of V_2O_3 we consider the antiferromagnetic insulating phase. We find two variational regimes, depending on the relative size of the correlation energy of the vertical pairs and the in-plane interaction energy. The former favors the formation of stable molecules throughout the crystal, while the latter tends to break this correlated state. We determine in both cases the minimizing orbital solutions for various spin configurations, and draw the corresponding phase diagrams. We find that none of the symmetry-breaking stable phases with the real spin structure presents an orbital ordering compatible with the magnetic space group indicated by very recent observations of non-reciprocal x-ray gyrotropy in V_2O_3. We do however find a compatible solution with very small excitation energy in two distinct regions of the phase space, which might turn into the true ground state of V_2O_3 due to the favorable coupling with the lattice. We illustrate merits and drawbacks of the various solutions and discuss them in relation to the present experimental evidence.Comment: 36 pages, 19 figure

PKN: The Challenge of Building a Virtual Knowledge Network to Address Food and Agribusiness Management Research, Service and Training Needs

The Parma Agribusiness Research & Management Knowledge Network, PARMaKN (PKN in the article) is an innovative public-private partnership, focused on the development of advanced food and agribusiness management research and service activities. PKN was launched in Parma in July 2007 by Societa’ Parmense per gli Insediamenti Produttivi (SPIP), the economic development corporation of the City of Parma. This paper presents the vision, mission and medium term activity plan of PKN.Agribusiness, Agricultural and Food Policy, Food Consumption/Nutrition/Food Safety, Industrial Organization,