61,195 research outputs found
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 Ly 1216 and HeII 1640 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 YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. 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 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 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 . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. 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 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
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