Adaptation de la mesure potentiométrique à l'estimation en continu de la teneur en cuivre de solutions aqueuses présentant de fortes variations physico-chimiques

La mesure en continu, sans prélèvement et sous conditions physico-chimiques variables, de la teneur en cuivre d'une solution aqueuse, constitue le cadre de ce travail. La méthode d'estimation proposée est basée sur la potentiométrie sélective à l'aide d'une électrode spécifique au cuivre. Le comportement de l'électrode est modélisé par une loi non linéaire s'inspirant de la relation de Nernst généralisée. Le modèle intègre les paramètres température, conductivité et pH de la solution. Il rend possible une estimation de la concentration totale de cuivre même en présence de complexations hydroxyles. La phase de modélisation est présentée en détail. Les performances de l'estimateur sont évaluées puis discutées.Ion Selective electrodes (ISEs) offer an attractive solution for continuously evaluating the content of certain ionic species in aqueous media. Manufacturers propose a wide range of electrodes specific to heavy metals (Cu2+, Pb2+ …). Because they eliminate the need for sampling, are of reasonable size and have few electronic parts, ISEs seem highly appropriate for continuous monitoring in urban purification systems. Measurements obtained by these sensors in controlled media in the laboratory are usually precise, reliable and reproducible. However, it is not so with complex and uncontrolled media. This work falls within the general scope of the continuous measurement of heavy metals in wastewater. More particularly, it is devoted to the description of the behaviour of a copper-selective electrode (ISECu) in a medium presenting wide physicochemical variations.Experimental set-upIn order to study ISE behaviour, we developed an experimental platform that allowed us to reproduce in a reactor the physicochemical variations observed in wastewater, particularly with regards to salinity and acidity. The reactor was fitted with a measuring set consisting of five electrodes that measured the following parameters: pH (ref. integrated Ag/Agcl), redox (red), ISECu (ECu), temperature (T) and conductivity (s). A computer system carried out the acquisition of the five signals with a 10-second sampling period. The species concentration in the reactor was determined by calculating the weight of the solutions extracted from or injected into the reactor. Controlling the temperature of the system was undertaken using a cryostat. Sequential tests allowed the pH, redox potential and conductivity of the medium to be varied and were carried out by successive injections of different chemical products. The response times of the conductivity probe and of the pH and redox electrodes are shown here; the short response time of the sensors (20 to 30 s) and the strong correlation between the measured pH and redox are noted. ISE modellingThe model used to explain the ISE response is based on a generalization of Nernst's Law that takes into account the temperature and the activity of the free ions (Cu2+). Taking into consideration chemical equilibria and mass equations allowed us to link the activity of the free copper ions to the total injected copper concentration |Cu2+|tot and to the pH. Redox, strongly correlated to pH, was ignored in the mathematical model. Since hydroxyl complexation is the major complexation reaction (compared to other copper-binding ligands), the potential measured with the ISE took the following form:ECu=b0+b1T.log[(ϒ2|Cu2+|tot) / (1+b2ϒ210pH+b3ϒ2102pH) + b4]The activity coefficient ϒ2 of the Cu2+ ions was calculated from the ionic strength (I) of the solution, using the Debye-Hückel approximation. Ionic strength was derived from conductivity corrected to 25 °C. In wastewater, the ranges of the physicochemical parameters were as follows: T from 5 to 35°C; pH from 4 to 9; Omega from 500 to 2000 mS/cm; redox from 400 to -400 mV/ENH; and copper concentrations 10-3 mol/dm3.In order to identify the bi coefficients of the model, we established an experimental plan comprising 108 measurement points that covered, with a minimal number of experiments, the ranges of variations of the parameters of influence. A dispersion diagram of measured and modelled values gave a linear adjustment coefficient close to 0.99 and a standard deviation of 8.8 mV, which corresponds to a 0.34 decadal standard error in the concentration estimate. With a temperature of 25 °C, the model has a sensitivity of -26.4 mV/decade, very close to the theoretical slope of an electrode sensitive to divalent ions.ISE measurement of the copper concentration with large pH variations pH is the parameter which exerts the greatest influence on ISE response, which is why tests simulating copper pollution with large variations of pH were carried out. These tests enabled us to evaluate the performances of the model in terms of the estimation of copper content. Four solutions of total copper concentration equal to 10-6, 10-5, 10-4, 10-3 mol/dm3 respectively, were used. Their temperature was 25 °C and their conductivity was fixed at approximately 500 mS/cm. We varied the pH of each solution between 4 and 10. For the four tests, we show the estimate of the copper concentration obtained with our model starting from the potential measured by the ISE.In the case of strong copper pollution (10-3 mol/dm3), the model yields an overestimated concentration below pH 7 with a decadal error of less than 0.5. Above pH 7, the concentration is underestimated while maintaining a decadal error of less than 0.5. At pH 7, a 0.04-decade minimal error is found. For pollution equal to or less than 10-4 mol/dm3, the model gives good results in an acid or neutral medium with a decadal error usually less than 0.3. In an alkaline medium, concentration is overestimated. In this case the error increases in a roughly linear manner with the pH and the co-logarithm of copper concentration. From the results of these tests, we defined a valid domain of ISE copper concentration measurement using our model. In conclusion, the suggested method, although not very accurate, could be used as an indicator of the copper concentration level in wastewater. The ISE-response correction model is currently being tested under operational conditions at a water treatment plant in Nancy-Maxéville (France)

Beyond Gravitoelectromagnetism: Critical Speed in Gravitational Motion

A null ray approaching a distant astronomical source appears to slow down, while a massive particle speeds up in accordance with Newtonian gravitation. The integration of these apparently incompatible aspects of motion in general relativity is due to the existence of a critical speed. Dynamics of particles moving faster than the critical speed could then be contrary to Newtonian expectations. Working within the framework of gravitoelectromagnetism, the implications of the existence of a critical speed are explored. The results are expected to be significant for high energy astrophysics.Comment: 13 pages, to appear in the Special December 2005 Issue of Int. J. Mod. Phys.

Secular interactions between inclined planets and a gaseous disk

In a planetary system, a secular particle resonance occurs at a location where the precession rate of a test particle (e.g. an asteroid) matches the frequency of one of the precessional modes of the planetary system. We investigate the secular interactions of a system of mutually inclined planets with a gaseous protostellar disk that may contain a secular nodal particle resonance. We determine the normal modes of some mutually inclined planet-disk systems. The planets and disk interact gravitationally, and the disk is internally subject to the effects of gas pressure, self-gravity, and turbulent viscosity. The behavior of the disk at a secular resonance is radically different from that of a particle, owing mainly to the effects of gas pressure. The resonance is typically broadened by gas pressure to the extent that global effects, including large-scale warps, dominate. The standard resonant torque formula is invalid in this regime. Secular interactions cause a decay of the inclination at a rate that depends on the disk properties, including its mass, turbulent viscosity, and sound speed. For a Jupiter-mass planet embedded within a minimum-mass solar nebula having typical parameters, dissipation within the disk is sufficient to stabilize the system against tilt growth caused by mean-motion resonances.Comment: 30 pages, 6 figures, to be published in The Astrophysical Journa

Ultra--cold gases and the detection of the Earth's rotation: Bogoliubov space and gravitomagnetism

The present work analyzes the consequences of the gravitomagnetic effect of the Earth upon a bosonic gas in which the corresponding atoms have a non--vanishing orbital angular momentum. Concerning the ground state of the Bogoliubov space of this system we deduce the consequences, on the pressure and on the speed of sound, of the gravitomagnetic effect. We prove that the effect on a single atom is very small, but we also show that for some thermodynamical properties the consequences scale as a non--trivial function of the number of particles.Comment: 4 page

Peculiar Hydrogen-deficient Carbon Stars: Strontium-Rich Stars and the s-Process

Context: R Coronae Borealis (RCB) variables and their non-variable counterparts, the dustless Hydrogen-Deficient Carbon (dLHdC) stars have been known to exhibit enhanced s-processed material on their surfaces, especially Sr, Y, and Ba. No comprehensive work has been done to explore the s-process in these types of stars, however one particular RCB star, U Aqr, has been under scrutiny for its extraordinary Sr enhancement. Aims: We aim to identify RCB and dLHdC stars that have significantly enhanced Sr abundances, such as U Aqr, and use stellar evolution models to begin to estimate the type of neutron exposure that occurs in a typical HdC star. Methods: We compare the strength of the Sr II 4077 $\AA$ spectral line to Ca II H to identify the new subclass of Sr-rich HdCs. We additionally use the structural and abundance information from existing RCB MESA models to calculate the neutron exposure parameter, $\tau$ Results: We identify six stars in the Sr-rich class. Two are RCBs, and four are dLHdCs. We additionally find that the preferred RCB MESA model has a neutron exposure $\tau$ ~ 0.1 mb$^{-1}$, which is lower than the estimated $\tau$ between 0.15 and 0.6 mb$^{-1}$ for the Sr-rich star U Aqr found in the literature. We find trends in the neutron exposure corresponding to He-burning shell temperature, metallicity, and assumed s-processing site. Conclusions: We have found a sub-class of 6 HdCs known as the Sr-rich class, which tend to lie in the halo, outside the typical distribution of RCBs and dLHdCs. We find that dLHdC stars are more likely to be Sr-rich than RCBs, with an occurrence rate of ~13\% for dLHdCs and ~2\% for RCBs. This is one of the first potential spectroscopic differences between RCBs and dLHdCs, along with dLHdCs having stronger surface abundances of $^{18}$O.Comment: 8 pages, submitted to A&

The ongoing pursuit of R Coronae Borealis stars: the ASAS-3 survey strikes again

CONTEXT: R Coronae Borealis stars( RCBs) are rare, hydrogen-deficient, carbon rich super giant variable stars that are likely the evolved merger products of pairs of CO and He white dwarfs. Only 55 RCB stars have been found in our galaxy and their distribution on the sky is weighted heavily by microlensing survey field positions. A less biased wide-area survey would enable us to test competing evolutionary scenarios, understand the population or populations that produce RCBs, and constrain their formation rate. AIMS: The ASAS-3 survey monitored the sky south of declination +28deg between 2000 and 2010 to a limiting magnitude of V = 14. We searched ASAS-3 for RCB variables using several different methods to ensure that the probability of RCB detection was as high as possible and to reduce selection biases based on luminosity, temperature, dust production activity and shell brightness. METHODS: Candidates whose light curves were visually inspected were pre-selected based on their infrared (IR) excesses due to warm dust in their circumstellar shells using the WISE and/or 2MASS catalogues. Criteria on light curve variability were also applied when necessary to minimise the number of objects. Initially, we searched for RCB stars among the ASAS-3 ACVS1.1 variable star catalogue, then among the entire ASAS-3 south source catalogue, and finally directly interrogated the light curve database for objects that were not catalogued in either of those. We then acquired spectra of 104 stars to determine their real nature using the SSO/WiFeS spectrograph. RESULTS: We report 21 newly discovered RCB stars and 2 new DY Per stars. Two previously suspected RCB candidates were also spectroscopically confirmed. Our methods allowed us to extend our detection efficiency to fainter magnitudes that would not have been easily accessible to discovery techniques based onlight curve variability. The overall detection efficiencyis about 90% for RCBs with maximum light brighter than V ∼13. CONCLUSIONS: With these new discoveries, 76 RCBs are now known in our Galaxy and 22 in the Magellanic Clouds. This growing sample is of great value to constrain the peculiar and disparate atmosphere composition of RCBs. Most importantly, we show that the spatial distribution and apparent magnitudes of Galactic RCB stars is consistent with RCBs being part of the Galactic bulge population.Department of HE and Training approved lis

Photon deflection and precession of the periastron in terms of spatial gravitational fields

We show that a Maxwell-like system of equations for spatial gravitational fields $\bf g$ and $\bf B$ (latter being the analogy of a magnetic field), modified to include an extra term for the $\bf B$ field in the expression for force, leads to the correct values for the photon deflection angle and for the precession of the periastron

The DICE calibration project: design, characterization, and first results

We describe the design, operation, and first results of a photometric calibration project, called DICE (Direct Illumination Calibration Experiment), aiming at achieving precise instrumental calibration of optical telescopes. The heart of DICE is an illumination device composed of 24 narrow-spectrum, high-intensity, light-emitting diodes (LED) chosen to cover the ultraviolet-to-near-infrared spectral range. It implements a point-like source placed at a finite distance from the telescope entrance pupil, yielding a flat field illumination that covers the entire field of view of the imager. The purpose of this system is to perform a lightweight routine monitoring of the imager passbands with a precision better than 5 per-mil on the relative passband normalisations and about 3{\AA} on the filter cutoff positions. The light source is calibrated on a spectrophotometric bench. As our fundamental metrology standard, we use a photodiode calibrated at NIST. The radiant intensity of each beam is mapped, and spectra are measured for each LED. All measurements are conducted at temperatures ranging from 0{\deg}C to 25{\deg}C in order to study the temperature dependence of the system. The photometric and spectroscopic measurements are combined into a model that predicts the spectral intensity of the source as a function of temperature. We find that the calibration beams are stable at the $10^{-4}$ level -- after taking the slight temperature dependence of the LED emission properties into account. We show that the spectral intensity of the source can be characterised with a precision of 3{\AA} in wavelength. In flux, we reach an accuracy of about 0.2-0.5% depending on how we understand the off-diagonal terms of the error budget affecting the calibration of the NIST photodiode. With a routine 60-mn calibration program, the apparatus is able to constrain the passbands at the targeted precision levels.Comment: 25 pages, 27 figures, accepted for publication in A&

An Atlantic-Pacific ventilation seesaw across the last deglaciation

It has been proposed that the rapid rise of atmospheric CO2across the last deglaciation was driven by the release of carbon from an extremely radiocarbon-depleted abyssal ocean reservoir that was ‘vented’ to the atmosphere primarily via the deep-and intermediate overturning loops in the Southern Ocean. While some radiocarbon observations from the intermediate ocean appear to confirm this hypothesis, others appear to refute it. Here we use radiocarbon measurements in paired benthic-and planktonic foraminifera to reconstruct the benthic–planktonic14C age offset (i.e. ‘ventilation age’) of intermediate waters in the western equatorial Atlantic. Our results show clear increases in local radiocarbon-based ventilation ages during Heinrich-Stadial 1 (HS1) and the Younger Dryas (YD). These are found to coincide with opposite changes of similar magnitude observed in the Pacific, demonstrating a ‘seesaw’ in the ventilation of the intermediate Atlantic and Pacific Oceans that numerical model simulations of North Atlantic overturning collapse indicate was primarily driven by North Pacific overturning. We propose that this Atlantic–Pacific ventilation seesaw would have combined with a previously identified North Atlantic–Southern Ocean ventilation seesaw to enhance ocean–atmosphere CO2exchange during a ‘collapse’ of the North Atlantic deep overturning limb. Whereas previous work has emphasized a more passive role for intermediate waters in deglacial climate change (merely conveying changes originating in the Southern Ocean) we suggest instead that the intermediate water seesaw played a more active role via relatively subtle but globally coordinated changes in ocean dynamics that may have further influenced ocean–atmosphere carbon exchange.We are grateful to Adam Scrivner for technical assistance in the laboratory, as well as the Royal Society and NERC grant NE/L006421/1 for research support. The UVic ESCM numerical ex-periments were performed on a computational cluster from the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme sup-ported by the Australian Government. A.T. and T.F. acknowledge support from the US NSF grants 1341311, 1400914. L.M. is sup-ported by the Australian Research Council grant DE150100107.This is the final version. It was first published by Elsevier at http://www.sciencedirect.com/science/article/pii/S0012821X15003301