123031545
Three-fluid plasmas in star formation II. Momentum transfer rate coefficients
Context. The charged component of the insterstellar medium consists of atomic and molecular ions, electrons, and charged dust grains, coupled to the local Galactic magnetic field. Collisions between neutral particles (mostly atomic or molecular hydrogen) and charged species, and between the charged species themselves, affect the magnetohydrodynamical behaviour of the medium and the dissipation of electric currents. Aims. The friction force due to elastic collisions between particles of different species in the multi-component interstellar plasma is a nonlinear function of the temperature of each species and the Mach number of the relative drift velocity. The aim of this paper is to provide an accurate and, as far as possible, complete set of momentum transfer rate coefficients for magnetohydrodynamical studies of the interstellar medium. Methods. Momentum transfer rates are derived from available experimental data and theoretical calculations of cross sections within the classic approach developed by Boltzmann and Langevin for a wide range of values of the temperature and the drift velocity. Results. Accurate numerical values for momentum transfer rates are obtained and fitted to simple analytical formulae expressing the dependence of the results on the gas temperature and the relative drift velocity. The often used polarization approximation is in satisfactory agreement with our results only for collisions between H2 and molecular ions (HCO + ,H + ). For other kinds of collisions, the polarization approximation fails by large factors, and must be replaced by more accurate expressions.
91183333
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that 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 is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.
14748555
Possible signature of hypernova nucleosynthesis in a beryllium‐rich halo dwarf★
As part of a large survey of halo and thick disc stars, we found one halo star, HD 106038, exceptionally overabundant in beryllium. In spite of its low metallicity, [Fe/H]=−1.26, the star has log(Be/H) =−10.60, which is similar to the solar meteoritic abundance, log(Be/H) =−10.58. This abundance is more than 10 times higher the abundance of stars with similar metallicity and cannot be explained by models of chemical evolution of the Galaxy that include the standard theory of cosmic ray spallation. No other halo star exhibiting such a beryllium overabundance is known. In addition, overabundances of Li, Si, Ni, Y and Ba are also observed. We suggest that all these chemical peculiarities, excepting the Ba abundance, can be simultaneously explained if the star was formed in the vicinity of a hypernova.
14004400
Galactic chemical evolution of heavy elements: from barium to europium
We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable for reproducing a large set of Galactic (local and nonlocal) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(α, n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(α, n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, there is a sharp production of the Ba-peak elements around Z Z☉/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution, given by the Galactic chemical evolution model at the epoch of the formation of the solar system. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars), showing that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.
18035202
Dust emissivity in the submm/mm SCUBA and SIMBA observations of Barnard 68
We have observed the dark cloud Barnard 68 with SCUBA at 850  μ m and with SIMBA at 1.2 mm. The submillimetre and millimetre dust emission correlate well with the extinction map of Alves et al. (2001). The A V /850  μ m correlation is clearly not linear and suggests lower temperatures for the dust in the inner core of the cloud. Assuming a model for the temperature gradient, we derive the cloud-averaged dust emissivities (normalised to the V -band extinction efficiency) at 850  μ m and 1.2 mm. We find $\kappa_{850~\mu{\rm m}}/\kappa_{\rm V}=4.0\pm 1.0\times 10^{-5}$ and $\kappa_{1.2~{\rm mm}}/\kappa_{\rm V}=9.0\pm 3.0\times 10^{-6}$. These values are compared with other determinations in this wavelength regime and with expectations for models of diffuse dust and grain growth in dense clouds.
10085260
Mean Field Magnetohydrodynamics of Accretion Disks
We consider the accretion process in a disk with magnetic fields that are dragged in from the interstellar medium by gravitational collapse. Two diffusive processes are at work in the system: (1) viscous torques exerted by turbulent and magnetic stresses, and (2) resistive redistribution of mass with respect to the magnetic flux arising from the imperfect conduction of current. In steady state, self-consistency between the two rates of drift requires that a relationship exists between the coefficients of turbulent viscosity and turbulent resistivity. Ignoring any interactions with a stellar magnetosphere, we solve the steady-state equations for a magnetized disk under the gravitational attraction of a mass point and threaded by an amount of magnetic flux consistent with calculations of magnetized gravitational collapse in star formation. Our model mean field equations have an exact analytical solution that corresponds to magnetically diluted Keplerian rotation about the central mass point. The solution yields the strength of the magnetic field and the surface density as functions of radial position in the disk and their connection with the departure from pure Keplerian rotation in representative cases. We compare the predictions of the theory with the available observations concerning T Tauri stars, FU Orionis stars, and low- and high-mass protostars. Finally, we speculate on the physical causes for high and low states of the accretion disks that surround young stellar objects. One of the more important results of this study is the physical derivation of analytic expressions for the turbulent viscosity and turbulent resistivity.
122515004
The Electromagnetic calorimeter for the OBELIX experiment
Abstract We describe the electromagnetic calorimeter of the OBELIX detector installed on the LEAR facility at CERN: the criteria followed to design its structure and the tests and preparation work performed on the different components are discussed, to conclude with the description of the main control procedures operating on the assembled detector during the first tests of the apparatus on the LEAR beam.
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.
27136629
A Comparison of Data-Access Platforms for the Computing of Large Hadron Collider Experiments
Performance, reliability and scalability in data-access are key issues in the context of the computing Grid and High Energy Physics data processing and analysis applications, in particular considering the large data size and I/O load that a Large Hadron Collider data centre has to support. In this paper we present the technical details and the results of a large scale validation and performance measurement employing different data-access platforms-namely CASTOR, dCache, GPFS and Scalla/Xrootd. The tests have been performed at the CNAF Tier-1, the central computing facility of the Italian National Institute for Nuclear Research (INFN). Our storage back-end was based on Fibre Channel disk-servers organized in a Storage Area Network, being the disk-servers connected to the computing farm via Gigabit LAN. We used 24 disk-servers, 260 TB of raw-disk space and 280 worker nodes as computing clients, able to run concurrently up to about 1100 jobs. The aim of the test was to perform sequential and random read/write accesses to the data, as well as more realistic access patterns, in order to evaluate efficiency, availability, robustness and performance of the various data-access solutions.
2567762
The LHCb Readout System and Real-Time Event Management
The LHCb Experiment is a hadronic precision experiment at the LHC accelerator aimed at mainly studying b-physics by profiting from the large b-anti-b-production at LHC. The challenge of high trigger efficiency has driven the choice of a readout architecture allowing the main event filtering to be performed by a software trigger with access to all detector information on a processing farm based on commercial multi-core PCs. The readout architecture therefore features only a relatively relaxed hardware trigger with a fixed and short latency accepting events at 1 MHz out of a nominal proton collision rate of 30 MHz, and high bandwidth with event fragment assembly over Gigabit Ethernet. A fast central system performs the entire synchronization, event labelling and control of the readout, as well as event management including destination control, dynamic load balancing of the readout network and the farm, and handling of special events for calibrations and luminosity measurements. The event filter farm processes the events in parallel and reduces the physics event rate to about 2 kHz which are formatted and written to disk before transfer to the offline processing. A spy mechanism allows processing and reconstructing a fraction of the events for online quality checking. In addition a 5 Hz subset of the events are sent as express stream to offline for checking calibrations and software before launching the full offline processing on the main event stream.
35523068
The LHCb Farm Monitoring and Control System
The LHCb experiment at CERN will have an online trigger farm composed of up to 2000 PCs. In order to monitor and control each PC and to supervise the overall status of the farm, a Farm Monitoring and Control System (FMC) was developed. The FMC is based on DIM (Distributed Information Management System) as network communication layer, it is accessible both through a command line interface and through the PVSS (Prozessvisualisierung und Steuerung) graphical interface, and it is interfaced to the Finite State Machine (FSM) of the LHCb Experiment Control System (ECS) in order to manage anomalous farm conditions. The FMC is an integral part of the ECS, which is in charge of monitoring and controlling all on-line components; it uses the same tools (DIM, PVSS, FSM, etc.) to guarantee its complete integration and a coherent look and feel throughout the whole control system.