1,418 research outputs found
Web Services: A Process Algebra Approach
It is now well-admitted that formal methods are helpful for many issues
raised in the Web service area. In this paper we present a framework for the
design and verification of WSs using process algebras and their tools. We
define a two-way mapping between abstract specifications written using these
calculi and executable Web services written in BPEL4WS. Several choices are
available: design and correct errors in BPEL4WS, using process algebra
verification tools, or design and correct in process algebra and automatically
obtaining the corresponding BPEL4WS code. The approaches can be combined.
Process algebra are not useful only for temporal logic verification: we remark
the use of simulation/bisimulation both for verification and for the
hierarchical refinement design method. It is worth noting that our approach
allows the use of any process algebra depending on the needs of the user at
different levels (expressiveness, existence of reasoning tools, user
expertise)
The circumgalactic medium of high redshift galaxies
We study the properties of the circumgalactic medium (CGM) of high-
galaxies in the metal enrichment simulations presented in Pallottini et al.
2014. At , we find that the simulated CGM gas density profiles are
self-similar, once scaled with the virial radius of the parent dark matter
halo. We also find a simple analytical expression relating the neutral hydrogen
equivalent width () of CGM absorbers as a function of the
line of sight impact parameter (). We test our predictions against mock
spectra extracted from the simulations, and show that the model reproduces the
profile extracted from the synthetic spectra analysis.
When compared with available data, our CGM model nicely predicts the observed
in galaxies, and supports the idea that the
CGM profile does not evolve with redshift.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
On the Formation of Molecular Clumps in QSO Outflows
We study the origin of the cold molecular clumps in quasar outflows, recently
detected in CO and HCN emission. We first describe the physical properties of
such radiation-driven outflows and show that a transition from a momentum- to
an energy-driven flow must occur at a radial distance of R ~ 0.25 kpc. During
this transition, the shell of swept up material fragments due to
Rayleigh-Taylor instabilities, but these clumps contain little mass and are
likely to be rapidly ablated by the hot gas in which they are immersed. We then
explore an alternative scenario in which clumps form from thermal instabilities
at R >~ 1 kpc, possibly containing enough dust to catalyze molecule formation.
We investigate this processes with 3D two-fluid (gas+dust) numerical
simulations of a kpc^3 patch of the outflow, including atomic and dust cooling,
thermal conduction, dust sputtering, and photoionization from the QSO radiation
field. In all cases, dust grains are rapidly destroyed in ~10,000 years; and
while some cold clumps form at later times, they are present only as transient
features, which disappear as cooling becomes more widespread. In fact, we only
find a stable two-phase medium with dense clumps if we artificially enhance the
QSO radiation field by a factor 100. This result, together with the complete
destruction of dust grains, renders the interpretation of molecular outflows a
very challenging problem.Comment: 17 pages, 12 figures, ApJ, In pres
The infant Milky Way
We investigate the physical properties of the progenitors of today living
Milky Way-like galaxies that are visible as Damped Lya Absorption systems and
Lya Emitters at higher redshifts (z ~ 2.3,5.7). To this aim we use a
statistical merger-tree approach that follows the formation of the Galaxy and
its dwarf satellites in a cosmological context, tracing the chemical evolution
and stellar population history of the progenitor halos. The model accounts for
the properties of the most metal-poor stars and local dwarf galaxies, providing
insights on the early cosmic star-formation. Fruitful links between Galactic
Archaeology and more distant galaxies are presented.Comment: 6 pages, 6 figures; to appear in the proceedings of the Subaru
conference on Galactic Archaeology, Shuzenji, Japan (Nov. 1-4 2011);
Astronomical Society of the Pacific Conference Series 201
Dynamics of Conductive/Cooling Fronts: Cloud Implosion and Thermal Solitons
We investigate the evolution of interfaces among phases of the interstellar
medium with different temperature. It is found that, for some initial
conditions, the dynamical effects related to conductive fronts are very
important even if radiation losses, which tend to decelerate the front
propagation, are taken into account. We have also explored the consequences of
the inclusion of shear and bulk viscosity, and we have allowed for saturation
of the kinetic effects. Numerical simulations of a cloud immersed in a hot
medium have been performed; depending on the ratio of conductive to dynamical
time, the density is increased by a huge factor and the cloud may become
optically thick. Clouds that are highly compressed are able to stop the
evaporation process even if their initial size is smaller than the Field
length. In addition to the numerical approach, the time dependent evolution has
been studied also analytically. Simple techniques have been applied to the
problem in order to study the transition stages to a stationary state. The
global properties of the solution for static and steady fronts and useful
relations among the various physical variables are derived; a mechanical
analogy is often used to clarify the physics of the results. It is demonstrated
that a class of soliton-like solutions are admitted by the hydrodynamical
equations appropriate to describe the conduction/cooling fronts (in the
inviscid case) that do not require a heat flux at the boundaries. Some
astrophysical consequences are indicated along with some possible applications
to the structure of the Galactic ISM and to extragalactic objects.Comment: 29 pages, Plain TeX, 14 figures, Space Telescope Preprint Series-No.
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