60504028
LHCb distributed computing and the grid
The current architecture of the LHCb distributed system for Monte Carlo data production is described, together with details of the implementation and performance of a regional site at INFN-CNAF. An overview is given of the current and planned use of Grid technology from the European Datagrid Project (EDG), and of the development of an experiment-specific user interface to Grid services.
55332577
THE HERA-B ELECTRON PRETRIGGER SYSTEM
In this paper the pretrigger system designed for the electromagnetic calorimeter of the HERA-B experiment is described. This system will provide the electron candidates to the first-level trigger. The first results obtained with a whole readout and pretrigger prototype chain are also described. ( 1998 Elsevier Science B.V. All rights reserved.
202714087
The HERA-B electromagnetic pre-trigger and its possible adaptations to the LHC-B Level-0 calorimeter trigger
In this document the pretrigger system developed for the electromagnetic calorimeter of the HERA-B experiment is described, showing some possible implementations of this system to perform the LHC-B Level-0 Trigger for the Electron and Hadron Trigger Algorithms.
7213389
DIRAC - Distributed Infrastructure with Remote Agent Control
This paper describes DIRAC, the LHCb Monte Carlo production system. DIRAC has a client/server architecture based on: Compute elements distributed among the collaborating institutes; Databases for production management, bookkeeping (the metadata catalogue) and software configuration; Monitoring and cataloguing services for updating and accessing the databases. Locally installed software agents implemented in Python monitor the local batch queue, interrogate the production database for any outstanding production requests using the XML-RPC protocol and initiate the job submission. The agent checks and, if necessary, installs any required software automatically. After the job has processed the events, the agent transfers the output data and updates the metadata catalogue. DIRAC has been successfully installed at 18 collaborating institutes, including the DataGRID, and has been used in recent Physics Data Challenges. In the near to medium term future we must use a mixed environment with different types of grid middleware or no middleware. We describe how this flexibility has been achieved and how ubiquitously available grid middleware would improve DIRAC.
109848273
The high-voltage monitor system for the calorimeter of the OBELIX experiment
Abstract We describe the monitor system developed to manage the high voltage power supply for the electromagnetic calorimeter of the OBELIX experiment, installed on the LEAR facility at CERN. The hardware and software characteristics of the system are discussed, as well as the performance of the same.
121570479
Compatibility and possible implications of the experimental results on solar neutrinos
Abstract The effect of varying the assumed value of the cross-section for the proton-proton fusion weak reaction of the solar cycle on the solar neutrino problem is discussed. It is shown that an increase of about 20% of the relevant astrophysical factor fields compatibility (within the present experimental accuracy) of the available results with theory and among themselves.
18861137
Beryllium abundances and star formation in the halo and in the thick disk
Context. Beryllium is a pure product of cosmic ray spallation. This implies a relatively simple evolution in time of the beryllium abundance and suggests its use as a time-like observable. Aims. Our goal is to derive abundances of Be in a sample of 90 stars, the largest sample of halo and thick disk stars analyzed to date. We study the evolution of Be in the early Galaxy and its dependence on kinematic and orbital parameters, and investigate its use as a cosmochronometer. Abundances of Be, Fe, and α-elements of 73 stars are employed to study the formation of the halo and the thick disk of the Galaxy. Methods. Beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra with spectrum synthesis. Atmospheric parameters and abundances of α-elements are adopted from the literature. Lithium abundances are used to eliminate mixed stars from the sample. The properties of halo and thick disk stars are investigated in diagrams of log(Be/H) vs. [α/H], log(Be/H) vs. [Fe/H], and [α/Fe] vs. log(Be/H) and with orbital and kinematic parameters. Results. We present our observational results in various diagrams. (i) In a log(Be/H) vs. [Fe/H] diagram we find a marginal statistical detection of a real scatter, above what is expected from measurement errors, with a larger scatter among halo stars. The detection of the scatter is further supported by the existence of pairs of stars with identical atmospheric parameters and different Be abundances; (ii) in a log(Be/H) vs. [α/Fe] diagram, the halo stars separate into two components; one is consistent with predictions of evolutionary models, while the other has too high α and Be abundances and is chemically indistinguishable from thick disk stars. This suggests that the halo is not a single uniform population where a clear age-metallicity relation can be defined; (iii) In diagrams of Rmin vs. [α/Fe] and log(Be/H), the thick disk stars show a possible decrease in [α/Fe] with Rmin, whereas no dependence of Be with Rmin is seen. This anticorrelation suggests that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation. The lack of correlation for Be indicates that it is insensitive to the local conditions of star formation.
55648516
The population of planetary nebulae and H II regions in M 81 - A study of radial metallicity gradients and chemical evolution
Context. M81 is an ideal laboratory to investigate the galactic chemical and dynamical evolution through the study of its young and old stellar populations. Aims. We analyze the chemical abundances of planetary nebulae and H ii regions in the M81 disk for insight on galactic evolution, and compare it with that of other galaxies, including the Milky Way. Methods. We acquired Hectospec/MMT spectra of 39 PNe and 20 H ii regions, with 33 spectra viable for temperature and abundance analysis. Our PN observations represent the first PN spectra in M81 ever published, while several H ii region spectra have been published before, although without a direct electron temperature determination. We determine elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon in PNe and H ii regions, and determine their averages and radial gradients. Results. The average O/H ratio of PNe compared to that of the H ii regions indicates a general oxygen enrichment in M81 in the last ∼10 Gyr. The PN metallicity gradient in the disk of M81 is Δlog(O/H)/ΔRG = −0.055 ± 0.02 dex/kpc. Neon and sulfur in PNe have a radial distribution similar to that of oxygen, with similar gradient slopes. If we combine our H ii sample with the one in the literature we find a possible mild evolution of the gradient slope, with results consistent with gradient steepening with time. Additional spectroscopy is needed to confirm this trend. There are no type I PNe in our M81 sample, consistently with the observation of only the brightest bins of the PNLF, the galaxy metallicity, and the evolution of post-AGB shells. Conclusions. Both the young and the old populations of M81 disclose shallow but detectable negative radial metallicity gradient, which could be slightly steeper for the young population, thus not excluding a mild gradients steepening with the time since galaxy formation. During its evolution M81 has been producing oxygen; its total oxygen enrichment exceeds that of other nearby galaxies.
15360960
Inhomogeneous chemical evolution of the Galactic halo
The considerable scatter of heavy element abundances observed in low-metallicity stars, as well as the peculiar abundances observed in some halo stars (e.g. CS 22892-052), support the hypothesis of an inhomogeneous enrichment of the Galactic halo at early times. Moreover the presence of r-process elements in low-metallicity stars is indicative of a prompt enrichment of the Galaxy by early generations of massive stars. In this contribution, we present the results of a Monte Carlo model for the dynamical and chemical Galactic evolution of halo gas, in particular for Fe and elements mostly produced by r-process nucleosynthesis, like Eu.
15997388
Three-fluid plasmas in star formation - I. Magneto-hydrodynamic equations
Context. Interstellar magnetic fields influence all stages of the process of star formation, from the collapse of molecular cloud cores to the formation and evolution of circumstellar disks and protostellar jets. This requires us to have a full understanding of the physical properties of magnetized plasmas of different degrees of ionization for a wide range of densities and temperatures. Aims. We derive general equations governing the magneto-hydrodynamic evolution of a three-fluid medium of arbitrary ionization, also including the possibility of charged dust grains as the main charge carriers. In a companion paper (Pinto & Galli 2008 ,A &A, 484, 17), we complement this analysis computing accurate expressions of the collisional coupling coefficients for a variety of gas mixtures relevant for the process of star formation. Methods. Over spatial and temporal scales larger than the so-called large-scale plasma limit and the collision-dominated plasma limit, and for non-relativistic fluid speeds, the electric field, the electric current and the ion-neutral drift have their instantaneous values determined by the evolution of the magnetic field, which obeys an advection-diffusion equation. The validity of the approximations made is discussed critically. Results. We derive the general expressions for the resistivities, the diffusion timescales and the heating rates in a three-fluid medium and we use them to estimate the evolution of the magnetic field in molecular clouds and protostellar jets. Collisions between charged particles significantly increase the value of the Ohmic resistivity during the process of cloud collapse, possibly affecting the decoupling of matter and magnetic field and enhancing the rate of energy dissipation. The Hall resistivity can take larger values than previously found when the negative charge is mostly carried by dust grains. In weakly- or mildy-ionized protostellar jets, ambipolar diffusion is found to occur on a time scale comparable to the dynamical time scale, limiting the validity of steady-state and nondissipative models to study the jet’s structure.
2902663
Disks around young o-b (proto)stars: observations and theory
Disks are a natural outcome of the star-formation process, in which they play a crucial role. Luminous, massive stars of spectral type earlier than B4 are likely to be those that benefit most from the existence of accretion disks, which may significantly reduce the effect of radiation pressure on the accreting material. The scope of the present contribution is to review the current knowledge about disks in young high-mass (proto)stars and discuss their implications. The issues of disk stability and lifetime are also discussed. We conclude that for protostars of less than ~20 M , disks with mass comparable to that of the central star are common. Above this limit the situation is unclear and there are no good examples of proto O4–O8 stars surrounded by accretion disks: In these objects only huge, massive, toroidal, nonequilibrium rotating structures are seen. It is clear, on the other hand, that the observed disks in stars of 10–20 M are likely to be unstable and with short lifetimes.