Time evolution of MX-80 bentonite geochemistry under thermo-hydraulic gradients

Indexación: Web of ScienceTwo 20-cm long columns of MX-80 bentonite compacted at a nominal dry density of 1.7 g/cm(3) with a water content of 17% were tested in thermo-hydraulic (TH) cells with the aim of simulating the conditions of a sealing material in a nuclear waste repository. On top of the columns a hydration surface simulated the host rock supplying groundwater and at the bottom a heater simulated the waste canister. The tests comprised two phases: a heating phase and a 'heating + hydration' phase. The temperatures at the ends of the columns were set during the last phase to 30 degrees C at the top and 140 degrees C at the bottom, respectively. The thermo-hydraulic treatment resulted in major changes along the bentonite columns. These changes led to significant gradients along the column with respect to the physical state (water content, dry density) and geochemistry of the bentonite. Smectite dissolution processes occurred. As a result, colloids were probably produced, particularly in the more hydrated areas. In the warmest part of the columns precipitation of carbonates took place, caused by their solubility decrease with temperature and the evaporation. The increase in water content reduced the ionic strength of the pore water in the more hydrated areas where species such as gypsum were dissolved. The solubilized ions were transported towards the bottom of the columns; Na+, Ca+, Mg2+ and SO42- moved at a similar rate and K+ and Cl- moved farther. These solubilized ions precipitated in the form of salts farther away along the columns as the test was longer. The TH treatment implied the loss of exchangeable positions in the smectite, particularly towards the heater. The cation exchange complex was also modified.http://www.ingentaconnect.com/content/minsoc/cm/2016/00000051/00000002/art0000

Photoionization models for extreme Lyα\alpha λ\lambda1216 and HeII λ\lambda1640 ratios in quasar halos, and PopIII vs AGN diagnostics

We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or to enhance the observed number of Ly-alpha photons per incident ionizing photon, in extended AGN-photoionized nebulae at high-redshift. Using photoionization models, we explore the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the observed number of Ly-alpha photons per incident ionizing photon. We find that low ionization parameter, a relatively soft or filtered ionizing spectrum, low gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha photons by HI can all result in significantly enhanced Ly-alpha relative to other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing spectrum, the observed number of Ly-alpha photons per incident ionizing photon is itself significantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values up to 5.3 in our 'extreme case' model. At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is predominantly via collisional excitation rather than recombination. In addition, we find that collisional excitation of Ly-alpha becomes more efficient if the ionizing continuum is pre-filtered through an optically thin screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of the z~3.5 quasars studied by Borisova et al. (2016) are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization/excitation mechanisms such as strong line-transfer effects. We also present UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and AGN photoionization.Comment: Accepted for publication in A&A. 14 pages, 9 figures. Abstract slightly shortened to meet arxiv character limi

Checklist of freshwater Cladocera from Perú

Seventy six species of Cladocera are recorded in the present study, of which, 31 are new to the Peruvian fauna. Five species, Alonella nana, Ephemeroporus acanthodes, Alona cf. karelica, Indialona ganapati, and Daphniopsis sp., has never been recorded before in South America. The distribution of the various species in the different water bodies of Peru is given

Unconventional antiferromagnetic correlations of the doped Haldane gap system Y2_2BaNi1−x_{1-x}Znx_xO5_5

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y$_2$BaNi$_{1-x}$Zn$_x$O$_5$. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility $\chi(T) \sim C / (\Theta+T)$ as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the ``impurity'' susceptibility $\chi_{imp}(T)$ by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by $T \chi_{imp}(T) = C_{imp} (1 + T_{imp}/T )^{-\gamma}$. In the temperature range [100 mK, 1 K] the experimental data are well fitted by $T \chi_{imp}(T) = A \ln{(T/T_c)}$, where $T_c$ increases with $x$. This fit suggests the existence of a finite N\'eel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility $\chi'(T)$ which suggests the existence of antiferromagnetic correlations at very low temperature.Comment: 19 pages, 17 figures, revised version (minor modifications

Interactions, star formation and extended nebulae in SDSS type 2 quasars at 0.3<~ z <~ 0.6

We present long-slit spectroscopy and imaging data obtained with FORS2 on the Very Large Telescope of 13 optically selected type 2 quasars at z~0.3-0.6 from the original sample of Zakamska et al. (2003). The sample is likely to be affected by different selection biases. We investigate the evidence for: a) mergers/interactions b) star formation activity in the neighborhood of the quasars and c) extended emission line regions and their nature. Evidence for mergers/interactions is found in 5/13 objects. This is a lower limit for our sample, given the shallowness of most of our continuum images. Although AGN photoionization cannot be totally discarded, line ratios consistent with stellar photoionization are found in general in companion galaxies/knots/nuclei near these same objects. On the contrary, the gas in the neighborhood of the quasar nucleus shows line ratios inconsistent with HII galaxies and typical of AGN photoionized nebulae. A natural scenario to explain the observations is that star formation is ongoing in companion galaxies/knots/nuclei, possibly triggered by the interactions. These systems are, therefore, composite in their emission line properties showing a combination of AGN and star formation features. Extended emission line regions (EELRs) have been found in 7/13 objects, although this fraction might be higher if a complete spatial coverage around the quasars was performed. The sizes vary between few and up to 64 kpc. In general, the EELRs apparently consist of an extended nebula associated with the quasar. In at least one case the EELR is associated with ionized tidal features.Comment: Accepted for publication in MNRAS. 19 pages, 30 figure

Generation of Kerr non-Gaussian motional states of trapped ions

Non-Gaussian states represent a powerful resource for quantum information protocols in the continuous variables regime. Cat states, in particular, have been produced in the motional degree of freedom of trapped ions by controlled displacements dependent on the ionic internal state. An alternative method harnesses the Kerr nonlinearity naturally existent in this kind of system. We present detailed calculations confirming its feasibility for typical experimental conditions. Additionally, this method permits the generation of complex non-Gaussian states with negative Wigner functions. Especially, superpositions of many coherent states are achieved at a fraction of the time necessary to produce the cat state.Comment: 6 pages, 5 figure

Jet-gas interactions in z~2.5 radio galaxies: evolution of the ultraviolet line and continuum emission with radio morphology

We present an investigation into the nature of the jet-gas interactions in a sample of 10 radio galaxies at 2.3<z<2.9 using deep spectroscopy of the UV line and continuum emission obtained at Keck II and the Very Large Telescope. Kinematically perturbed gas, which we have shown to be within the radio structure in previous publications, is always blueshifted with respect to the kinematically quiescent gas, is usually spatially extended, and is usually detected on both sides of the nucleus. In the three objects from this sample for which we are able to measure line ratios for both the perturbed and quiescent gases, we suggest that the former has a lower ionization state than the latter. We propose that the perturbed gas is part of a jet-induced outflow, with dust obscuring the outflowing gas that lies on the far side of the object. The spatial extent of the blueshifted perturbed gas, typically ~35 kpc, implies that the dust is spatially extended at least on similar spatial scales. We also find interesting interrelationships between UV line, UV continuum and radio continuum properties of this sample.Comment: Accepted for publication in MNRA

Ly-alpha excess in high redshift radio galaxies: a signature of star formation

About 54% of radio galaxies at z>3 and 8% of radio galaxies at 2<z<3 show unusually strong Ly-alpha emission, compared with the general population of high redshift (z>2) radio galaxies. These Ly-alpha excess objects (LAEs) show Ly-alpha/HeII values consistent with or above standard photoionization model predictions. We show that the most successful explanation is the presence of a young stellar population which provides the extra supply of ionizing photons required to explain the Ly-alpha excess in at least the most extreme LAEs (probably in all of them). The measurement of unusually high Ly-alpha ratios in the extended gas of some high redshift radio galaxies suggests that star formation activity occurs in spatial scales of tens of kpc. We argue that, although the fraction of LAEs may be incompletely determined, both at 23, the much larger fraction of LAEs found at z>3 is a genuine redshift evolution and not due to selection effects. Therefore, our results suggest that the radio galaxy phenomenon is more often associated with a massive starburst at z>3 than at z<3.Comment: 14 pages, 5 figures, accepted for publication in MNRA