15769380
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Context. Magnetic fields are believed to play a crucial role in the process of star formation. Aims. We compare high-angular resolution observations of the submillimeter polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a low-mass protostar, with models of the collapse of magnetized molecular cloud cores. Methods. Assuming a uniform dust alignment efficiency, we computed the Stokes parameters and synthetic polarization maps from the model density and magnetic field distribution by integrations along the line-of-sight and convolution with the interferometric response. Results. The synthetic maps are in good agreement with the data. The best-fitting models were obtained for a protostellar mass of 0.8 M� ,o f age 9× 10 4 yr, formed in a cloud with an initial mass-to-flux ratio ∼2 times the critical value. Conclusions. The magnetic field morphology in NGC 1333 IRAS 4A is consistent with the standard theoretical scenario for the formation of solar-type stars, where well-ordered, large-scale, rather than turbulent, magnetic fields control the evolution and collapse of the molecular cloud cores from which stars form.
10126720
Monte Carlo radiative transfer in molecular cloud cores
We present the results of a three-dimensional Monte Carlo radiative transfer code for starless molecular cloud cores heated by an external isotropic or non-isotropic interstellar radiation field. The code computes the dust temperature distribution inside model clouds with specified but arbitrary density profiles. In particular we examine in detail spherical (Bonnor-Ebert) clouds, axisymmetric and non-axisymmetric toroids, and clouds heated by an external stellar source in addition to the general interstellar field. For these configurations, the code also computes maps of the emergent intensity at different wavelengths and arbitrary viewing angle, that can be compared directly with continuum maps of prestellar cores. In the approximation where the dust temperature is independent of interactions with the gas and where the gas is heated both by collisions with dust grains and ionization by cosmic rays, the temperature distribution of the gas is also calculated. For cloud models with parameters typical of dense cores, the results show that the dust temperature decreases monotonically from a maximum value near the cloud's edge (14–15 K) to a minimum value at the cloud's center (6–7 K). Conversely, the gas temperature varies in a similar range, but, due to efficient dust-gas coupling in the inner regions and inefficient cosmic-ray heating in the outer regions, the gradient is non-monotonic and the gas temperature reaches a maximum value at intermediate radii. The emission computed for these models (at 350  μ m and 1.3 mm) shows that deviations from spherical symmetry in the density and/or temperature distributions are generally reduced in the simulated intensity maps (even without beam convolution), especially at the longer wavelengths.
16070993
Post-T Tauri Stars: A False Problem
We consider the problem of the apparent lack of old T Tauri stars in nearby star-forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetimes and the ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. Thus, models based on the assumption of a constant star formation rate overestimate the predicted number of stars of ages greater than 2-5 Myr. We argue that the probability of finding a large population of old stars in a star-forming region is intrinsically very small. It is therefore unlikely that the dispersed X-ray sources found by ROSAT at large distances from molecular cloud complexes can be identified with such a population. We conclude that the post-T Tauri problem is by and large nonexistent.
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.
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.
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.
118241507
Searching for micro black holes at LHC
An exciting feature of brane-world models with large extra dimensions [1] is that the fundamental scale of gravity M G could be as low as the electroweak scale (M G ≃ 1TeV). Since the mass of a black hole must be larger than M G, otherwise the classical concept of event horizon would be lost, micro black holes of a few TeV’s may therefore be produced at the LHC in this scenario (for some recent reviews, see [2]).
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.
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.
14310220
Probing quantum gravity effects in black holes at LHC
We study modifications of the Hawking emission in the evaporation of miniature black holes possibly produced in accelerators when their mass approaches the fundamental scale of gravity, set to 1TeV according to some extra dimension models. Back-reaction and quantum gravity corrections are modelled by employing modified relations between the black hole mass and temperature. We release the assumption that black holes explode at 1TeV or leave a remnant, and let them evaporate to much smaller masses. We have implemented such modified decay processes into an existing micro-black hole event generator, performing a study of the decay products in order to search for phenomenological evidence of quantum gravity effects.