Cosmological radiative transfer codes comparison project – I. The static density field tests

Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H  ii region expansion; (2) H  ii region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form.     

An investigation about natural radioactivity,hydrochemistry, and metal pollution in groundwater from Calabrian selected areas,southern Italy

The increasing interest in radioactivity has brought about the need for an assessment of human exposure to radiation. It is, therefore, necessary to examine naturally occurring radioactivity in the environment, especially its occurrence in groundwater. The aim of this work was then to study the levels and behavior of the most significant natural radionuclides, also in order to improve the knowledge of the hydrochemical processes involved in the selected groundwater systems. Natural radioactivity in fifteen Calabrian groundwaters for human use was investigated through high-resolution gamma spectrometry (with a negative-biased Ortec HPGe detector) and liquid scintillation measurements. Particular attention was given on those radionuclides (³H, ²³⁸U, ²²⁶Ra, and ²²⁸Ra), which contribute in a significant way to the overall effective dose received by members of the public due to the intake of drinking water originating from groundwater systems. The activity concentration of ²³⁸U varied from 1 to 51 mBq/L, as a result of the geology of the investigated area and of the oxidizing conditions that favored U dissolution. ²²⁶Ra presented a broad range of activity concentrations (0.011–0.14 mBq/L), lower than uranium ones because radium occurs in groundwater under reducing conditions. Some heavy metals (Cd, Pb, Be, Hg, Ag, As, Tl, Sb, Se, and Ni) were also investigated through ICP-MS measurements and compared with the limits set by the Italian Legislation. Metals are released into the environment by both natural and anthropogenic sources; they leach into underground waters, moving along water pathways and eventually depositing in the aquifer.     

The Hubble Deep Field North reveals a supernova at z∼0.95

We report the discovery of a variable object in the Hubble Deep Field North (HDF-N) which brightened, during the 8.5 d sampled by the data, by more than 0.9 mag in I ₈₁₄ and about 0.7 mag in V ₆₀₆, remaining stable in B ₄₅₀. Subsequent observations of the HDF-N show that two years later this object has dimmed to about its original brightness in I ₈₁₄. The colours of this object, its brightness, its time behaviour in the various filters and the evolution of its morphology are consistent with it being a Type Ib supernova in a faint galaxy at z .     

An operational model to estimate hourly and daily crop evapotranspiration in hilly terrain: validation on wheat and oat crops

In this paper, we present an operational model to estimate the actual evapotranspiration (ET) of crops cultivated on hilly terrains. This new model has the following three characteristics: (1) ET modelling is based on a Penman?CMonteith (PM) type equation (Monteith 1965) where canopy resistance is simulated by following an approach already illustrated by Katerji and Perrier (Agronomie 3(6):513?C521, 1983); (2) the estimation of ET, by means of the PM equation, is made by using meteorological variables simulated on sloped sites as input; (3) these variables are simulated by using simple relationships linking the variables measured at a reference site on plane to the topographic characteristics of the site (slope, orientation, altitude as difference between reference, and sloped sites). This approach presents two advantages if compared with previously proposed models: Not only computation steps are greatly simplified but also error sources due to the simulation of climatic variables in sloped sites and the ET estimation are well distinguished. This model was validated at hourly and daily scales at four sites cultivated with wheat and oats offering a wide range of slope and orientation values: a reference site on plane, site 1 (9° sloping, NW orientation, 7 m from the reference site in plane), site 2 (6°, SE, 12 m) and site 3 (1°, SE, 18 m). At hourly scale, the new model performed well at all sites studied. The observed slope of the linear relationships between estimated and measured ET values ranged between 0.93 and 1.03, with coefficients of determination, r ², between 0.80 and 0.98. At daily scale, the slopes of the linear relationships between measured and estimated ET for the sites on plane and the sloped sites were practically the same (0.98?±?0.01); however, the coefficient of determination r ² observed in the site on plane was clearly greater (0.98) than that observed in the sloped sites (0.83). The presented analysis does not show any significant systematic effect of topography (slope and orientation of the plots) on the good performance of the proposed model for the ET estimation. Furthermore, we observed that coefficients of determination tend to decrease with the increase in the slope of the site, which translates into increased inaccuracy of the climatic variables simulation, in particular the net radiation, as the slope of the site increases. The proposed model allows to verify the different steps for calculating the fluxes, to identify the eventual sources of error and to make the needed corrections. For this reasons, the proposed model seems to be particularly ??operational??, i.e. a useful tool for estimating fluxes on hilly terrains.     

Cosmological reionization around the first stars: Monte Carlo radiative transfer

We study the evolution of ionization fronts around the first protogalaxies by using high-resolution numerical cosmological (Λ+ cold dark matter, CDM, model) simulations and Monte Carlo radiative transfer methods. We present the numerical scheme in detail and show the results of test runs from which we conclude that the scheme is both fast and accurate. As an example of interesting cosmological application, we study the reionization produced by a stellar source of total mass M =2×10⁸ M_⊙ turning on at z ≈12, located at a node of the cosmic web. The study includes a spectral energy distribution of a zero-metallicity stellar population, and two initial mass functions (IMFs; Salpeter/Larson). The expansion of the ionization front (I-front) is followed as it breaks out from the galaxy and is channelled by the filaments into the voids, assuming (in a 2D representation) a characteristic butterfly shape. The ionization evolution is very well tracked by our scheme, as realized by the correct treatment of the channelling and shadowing effects resulting from overdensities. We confirm previous claims that both the shape of the IMF and the ionizing power metallicity dependence are important to correctly determine the reionization of the Universe.     

Limitations of an Eddy-Covariance System in Measuring Low Ammonia Fluxes

Green manuring of legume crops can improve soil fertility and sustainability. To evaluate its agronomic and environmental effectiveness, gaseous losses of ammonia (NH₃) in the surface layer need to be quantified by direct measurements in the field. However, the application of the eddy-covariance technique to atmospheric NH₃ is challenging: its high reactivity, water solubility, and low background concentrations all hinder the response time of closed-path sensors for fast measurements of NH₃ concentration. Ammonia emissions following green manuring were measured for 21 days using a flux system equipped with a fast-pulsed quantum-cascade tunable-infrared-laser spectrometer. The noisy cross-covariance function for this configuration indicates flux measurements are close to the limit of detection; the low signal-to-noise ratio further increases the uncertainties, introducing a mirroring effect on the fluxes, which results in the rapid alternation between emission and deposition, within the limit of detection (around 13 and 20 ng m^?2 s^?1, at the 95 and 99% confidence limits, respectively). An evaluation of the measurement errors is presented, focussing on three technical aspects of the eddy-covariance system: (1) time lag, (2) random error, and (3) limit of detection. The NH₃ fluxes measured by the spectrometer are close to its limit of detection, with a random error of the same order as the flux.