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-$z$ galaxies in the metal enrichment simulations presented in Pallottini et al. 2014. At $z=4$, 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 (${\rm EW}_{\rm HI}$) of CGM absorbers as a function of the line of sight impact parameter ($b$). We test our predictions against mock spectra extracted from the simulations, and show that the model reproduces the ${\rm EW}_{\rm HI}(b)$ profile extracted from the synthetic spectra analysis. When compared with available data, our CGM model nicely predicts the observed ${\rm EW}_{\rm HI}(b)$ in $z\lesssim2$ 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. 75