A First Step Towards Automatically Building Network Representations

To fully harness Grids, users or middlewares must have some knowledge on the topology of the platform interconnection network. As such knowledge is usually not available, one must uses tools which automatically build a topological network model through some measurements. In this article, we define a methodology to assess the quality of these network model building tools, and we apply this methodology to representatives of the main classes of model builders and to two new algorithms. We show that none of the main existing techniques build models that enable to accurately predict the running time of simple application kernels for actual platforms. However some of the new algorithms we propose give excellent results in a wide range of situations

Factors controlling nitrate in ice cores: Evidence from the Dome C deep ice core

In order to estimate past changes in atmospheric NOx concentration, nitrate, an oxidation product of NOx, has often been measured in polar ice cores. In the frame of the European Project for Ice Coring in Antarctica (EPICA), a high-resolution nitrate record was obtained by continuous flow analysis (CFA) of a new deep ice core drilled at Dome C. This record allows a detailed comparison of nitrate with other chemical trace substances in polar snow under different climatic regimes. Previous studies showed that it would be difficult to make firm conclusions about atmospheric NOx concentrations based on ice core nitrate without a better understanding of the factors controlling NO3− deposition and preservation. At Dome C, initially high nitrate concentrations (over 500 ppb) decrease within the top meter to steady low values around 15 ppb that are maintained throughout the Holocene ice. Much higher concentrations (averaging 53 ppb) are found in ice from the Last Glacial Maximum (LGM). Combining this information with data from previous sampling elsewhere in Antarctica, it seems that under climatic conditions of the Holocene, temperature and accumulation rate are the key factors determining the NO3− concentration in the ice. Furthermore, ice layers with high acidity show a depletion of NO3−, but higher concentrations are found before and after the acidity layer, indicating that NO3− has been redistributed after deposition. Under glacial conditions, where NO3− shows a higher concentration level and also a larger variability, non-sea-salt calcium seems to act as a stabilizer, preventing volatilization of NO3− from the surface snow layers

An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277

We present a spectrophotometric study based on VLT/FORS I observations of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol 1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity of the BCD originates from a bright and compact starburst region located at the northeastern tip of a faint dwarf galaxy with cometary appearance. The starburst has ignited less than 4 Myr ago and its emission is powered by several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars located within a compact region with < 500 pc in diameter. For the first time in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together with intense He II 4686 emission indicates the presence of a very hard radiation field in Tol 1214-277. We argue that this extraordinarily hard radiation originates from both Wolf--Rayet stars and radiative shocks in the starburst region. The structural properties of the low-surface-brightness (LSB) component underlying the starburst have been investigated by means of surface photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides an adequate approximation to its radial intensity distribution. The broad-band colors in the outskirts of the LSB component of Tol 1214-277 are nearly constant and are consistent with an age below one Gyr. This conclusion is supported by the comparison of the observed spectral energy distribution (SED) of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in Astronomical Journa

Hydrogenation of CO on a silica surface: an embedded cluster approach

The sequential addition of H atoms to CO adsorbed on a siliceous edingtonite surface is studied with an embedded cluster approach, using density functional theory for the quantum mechanical (QM) cluster and a molecular force field for the molecular mechanical (MM) cluster. With this setup, calculated QM/MM adsorption energies are in agreement with previous calculations employing periodic boundary conditions. The catalytic effect of the siliceous edingtonite (100) surface on CO hydrogenation is assessed because of its relevance to astrochemistry. While adsorption of CO on a silanol group on the hydroxylated surface did not reduce the activation energy for the reaction with a H atom, a negatively charged defect on the surface is found to reduce the gas phase barriers for the hydrogenation of both CO and H2C = O. The embedded cluster approach is shown to be a useful and flexible tool for studying reactions on (semi-)ionic surfaces and specific defects thereon. The methodology presented here could easily be applied to study reactions on silica surfaces that are of relevance to other scientific areas, such as biotoxicity of silica dust and geochemistry

The Carrington event not observed in most ice core nitrate records

The Carrington Event of 1859 is considered to be among the largest space weather events of the last 150 years. We show that only one out of 14 well-resolved ice core records from Greenland and Antarctica has a nitrate spike dated to 1859. No sharp spikes are observed in the Antarctic cores studied here. In Greenland numerous spikes are observed in the 40 years surrounding 1859, but where other chemistry was measured, all large spikes have the unequivocal signal, including co-located spikes in ammonium, formate, black carbon and vanillic acid, of biomass burning plumes. It seems certain that most spikes in an earlier core, including that claimed for 1859, are also due to biomass burning plumes, and not to solar energetic particle (SEP) events. We conclude that an event as large as the Carrington Event did not leave an observable, widespread imprint in nitrate in polar ice. Nitrate spikes cannot be used to derive the statistics of SEPs

Atmospheric nitrogen oxides (NO and NO2) at Dome C, East Antarctica, during the OPALE campaign

Mixing ratios of the atmospheric nitrogen oxides NO and NO2 were measured as part of the OPALE (Oxidant Production in Antarctic Lands & Export) campaign at Dome C, East Antarctica (75.1 degrees S, 123.3 degrees E, 3233 m), during December 2011 to January 2012. Profiles of NOx mixing ratios of the lower 100m of the atmosphere confirm that, in contrast to the South Pole, air chemistry at Dome C is strongly influenced by large diurnal cycles in solar irradiance and a sudden collapse of the atmospheric boundary layer in the early evening. Depth profiles of mixing ratios in firn air suggest that the upper snowpack at Dome C holds a significant reservoir of photolytically produced NO2 and is a sink of gas-phase ozone (O-3). First-time observations of bromine oxide (BrO) at Dome C show that mixing ratios of BrO near the ground are low, certainly less than 5 pptv, with higher levels in the free troposphere. Assuming steady state, observed mixing ratios of BrO and RO2 radicals are too low to explain the large NO2 : NO ratios found in ambient air, possibly indicating the existence of an unknown process contributing to the atmospheric chemistry of reactive nitrogen above the Antarctic Plateau. During 2011-2012, NOx mixing ratios and flux were larger than in 2009-2010, consistent with also larger surface O-3 mixing ratios resulting from increased net O-3 production. Large NOx mixing ratios at Dome C arise from a combination of continuous sunlight, shallow mixing height and significant NOx emissions by surface snow (F-NOx). During 23 December 2011-12 January 2012, median F-NOx was twice that during the same period in 20092010 due to significantly larger atmospheric turbulence and a slightly stronger snowpack source. A tripling of F-NOx in December 2011 was largely due to changes in snowpack source strength caused primarily by changes in NO3- concentrations in the snow skin layer, and only to a secondary order by decrease of total column O-3 and associated increase in NO3- photolysis rates. A source of uncertainty in model estimates of F-NOx is the quantum yield of NO3- photolysis in natural snow, which may change over time as the snow ages

The clone size of peripheral CD8 T cells is regulated by TCR promiscuity

Positive selection in the thymus and peripheral T cell survival depend on T cell receptor (TCR)–major histocompatibility complex (MHC) interactions, but it is not yet clear if both events follow exactly the same rules. We studied peripheral T cell survival and clone sizes in conditions of progressive reduction of restricting MHC-bearing cells or progressive ablation of different MHC molecules. Different CD8+ T cell clones/polyclonal populations showed different survival and/or lymphopenia-driven proliferation requirements. We could correlate clone sizes to the capacity of each TCR to interact with different types of MHC complexes. Thus, although repertoire selection in the thymus is mainly conditioned by the affinity of TCR–MHC interactions, peripheral selection is determined by TCR cross-reactivity to environmental ligands