480 research outputs found
Galactic Centre stellar winds and Sgr A* accretion
(ABRIDGED) We present in detail our new 3D numerical models for the accretion
of stellar winds on to Sgr A*. In our most sophisticated models, we put stars
on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account
for radiative cooling. We first model only one phase `fast' stellar winds (1000
km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5
Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models.
In contrast, Mdot decreases by an order of magnitude for stars following
circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to
produce slow winds. Much of these winds cool radiatively, forming cold clumps
immersed into the X-ray emitting gas. We test two orbital configurations for
the stars in this scenario, an isotropic distribution and two rotating discs
with perpendicular orientation. The morphology of cold gas is quite sensitive
to the orbits. In both cases, however, most of the accreted gas is hot, with an
almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The
cold gas accretes in intermittent, short but powerful episodes which may give
rise to large amplitude variability in the luminosity of Sgr A* on time scales
of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and
1e4 Rsch, for the one and two-phase wind simulations, respectively, never
forming the quasi-spherical accretion flows suggested in some previous work.
Our work suggests that, averaged over time scales of 100s to 1000s of years,
the radiative and mechanical luminosity of Sgr A* may be substantially higher
than it is in its current state. Further improvements of the wind accretion
modelling of Sgr A* will rely on improved observational constraints for the
wind properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper
and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2:
minor changes
OCL2Trigger: Deriving active mechanisms for relational databases using Model-Driven Architecture
16 pages, 10 figures.-- Issue title: "Best papers from the 2007 Australian Software Engineering Conference (ASWEC 2007), Melbourne, Australia, April 10-13, 2007, Australian Software Engineering Conference 2007".Transforming integrity constraints into active rules or triggers for verifying database consistency produces a serious and complex problem related to real time behaviour that must be considered for any implementation. Our main contribution to this work is to provide a complete approach for deriving the active mechanisms for Relational Databases from the specification of the integrity constraints by using OCL. This approach is designed in accordance with the MDA approach which consists of transforming the specified OCL clauses into a class diagram into SQL:2003 standard triggers, then transforming the standard triggers into target DBMS triggers. We believe that developing triggers and plugging them into a given model is insufficient because the behaviour of such triggers is invisible to the developers, and therefore not controllable. For this reason, a DBMS trigger verification model is used in our approach, in order to ensure the termination of trigger execution. Our approach is implemented as an add-in tool in Rational Rose called OCL2Trigger.This work is part of the "Software Process Management Platform: Modelling, reuse and measurement" TIN2004/07083 project.Publicad
Applying a Fuzzy Approach to Relaxing Cardinality Constraints
9 pages, 3 figures.-- Contributed to: 15th International Conference on Database and Expert Systems Applications (DEXA 2004, Zaragoza, Spain, Aug 30 - Sep 3, 2004).In database applications the verification of cardinality constraints is a serious and complex problem that appears when the modifications operations are performed in a large cascade. Many efforts have been devoted to solve this problem, but some solutions lead to other problems such as the complex execution model or an impact on the database performance. In this paper a method to reducing and simplifying the complex verification of cardinality constraints by relaxing these constraints using fuzzy concepts is proposed.Publicad
Topological Quantum Critical Points in the Extended Bose-Hubbard Model
The combination of topology and quantum criticality can give rise to an exotic mix of counterintuitive effects. Here, we show that unexpected topological properties take place in a paradigmatic strongly correlated Hamiltonian: the 1D extended Bose-Hubbard model. In particular, we reveal the presence of two distinct topological quantum critical points with localized edge states and gapless bulk excitations. Our results show that the topological critical points separate two phases, one topologically protected and the other topologically trivial, both characterized by a long-range ordered string correlation function. The long-range order persists also at the topological critical points and explains the presence of localized edge states protected by a finite charge gap. Finally, we introduce a superresolution quantum gas microscopy scheme for dipolar dysprosium atoms, which provides a reliable route towards the experimental study of topological quantum critical points
Plugging active mechanisms to control dynamic aspects derived from the Multiplicity Constraint in UML
8 pages, 5 figures.-- Contributed to: 7th International Conference on the Unified Modeling Language (UML'2004, Lisbon, Portugal, Oct 11-15, 2004).Multiple efforts have been devoted to face the problem of database modelling. One of them is the automatization of database design process using CASE tools. Frequently, these tools do not completely support all phases of database analysis and the design methodology that they propose. Therefore, we propose to incorporate new features to these tools enhancing them and solving some of the modelling problems. In this paper we present an add-in module that aims to generate triggers for preserving the multiplicity constraints of a conceptual scheme in the transformation to a relational scheme. The module is integrated into the RATIONAL ROSE case tool.Publicad
Optical and Infrared Photometry of the Unusual Type Ia Supernova 2000cx
We present optical and infrared photometry of the unusual Type Ia supernova
2000cx. With the data of Li et al. (2001) and Jha (2002), this comprises the
largest dataset ever assembled for a Type Ia SN, more than 600 points in
UBVRIJHK. We confirm the finding of Li et al. regarding the unusually blue B-V
colors as SN 2000cx entered the nebular phase. Its I-band secondary hump was
extremely weak given its B-band decline rate. The V minus near infrared colors
likewise do not match loci based on other slowly declining Type Ia SNe, though
V-K is the least ``abnormal''. In several ways SN 2000cx resembles other slow
decliners, given its B-band decline rate (Delta m_15(B) = 0.93), the appearance
of Fe III lines and weakness of Si II in its pre-maximum spectrum, the V-K
colors and post-maximum V-H colors. If the distance modulus derived from
Surface Brightness Fluctuations of the host galaxy is correct, we find that the
rate of light increase prior to maximum, the characteristics of the bolometric
light curve, and the implied absolute magnitude at maximum are all consistent
with a sub-luminous object with Delta m_15(B) ~ 1.6-1.7 having a higher than
normal kinetic energy.Comment: 46 pages, 17 figures, to be published in Publications of the
Astronomical Society of the Pacifi
Guidelines for representing complex cardinality constraints in binary and ternary relationships
Ternary relationships represent the association among three entities whose constraints database designers do not always know how to manage. In other words, it is very difficult for the designer to detect, represent and add constraints in a ternary relationship according to the domain requirements. To remedy the shortcomings in capturing the semantics required for the representation of this kind of relationship, the present paper discusses a practical method to motivate the designer's use of ternary relationships in a methodological framework. The method shows how to calculate cardinality constraints in binary and ternary relationships and to preserve the associated semantics until the implementation phase of the database development method.This work forms part of the ‘Thuban: Natural
Interaction Platform for Virtual Attending in Real Environments’ project
(TIN2008-02711), the Spanish Ministry of Industry, Tourism and
Trade through the project Semants (TSI-020100-2009-419) and also
by the Spanish research projects: MA2VICMR: Improving the access,
analysis and visibility of the multilingual and multimedia information
in web for the Region of Madrid (S2009/TIC-1542).Publicad
Using UML’s Sequence Diagrams for Representing Execution Models Associated to Triggers
11 pages, 3 figures.-- Contributed to: 23rd British National Conference on Databases (BNCOD 23, Belfast, Northern Ireland, UK, July 18-20, 2006).Using active rules or triggers to verify integrity constraints is a serious and complex problem because these mechanisms have behaviour that could be difficult to predict in a complex database. The situation is even worse as there are few tools available for developing and verifying them. We believe that automatic support for trigger development and verification would help database developers to adopt triggers in the database design process. Therefore, in this work we suggest a visualization add-in tool that represents and verifies triggers execution by using UML’s sequence diagrams. This tool is added in RATIONAL ROSE and it simulates the execution sequence of a set of triggers when a DML operation is produced. This tool uses the SQL standard to express the triggers semantics and execution.This work is part of the project "Software Process Management Platform: modelling, reuse and measurement". TIN2004/07083.Publicad
Infalling clouds onto super-massive black hole binaries - I. Formation of discs, accretion and gas dynamics
There is compelling evidence that most -if not all- galaxies harbour a
super-massive black hole (SMBH) at their nucleus, hence binaries of these
massive objects are an inevitable product of the hierarchical evolution of
structures in the universe, and represent an important but thus-far elusive
phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to
be important for the dynamical evolution of SMBH binaries, as well as in
producing luminous emission that can be used to infer their properties. One
plausible source of the gaseous fuel is clumps of gas formed due to turbulence
and gravitational instabilities in the interstellar medium, that later fall
toward and interact with the binary. In this context, we model numerically the
evolution of turbulent clouds in near-radial infall onto equal-mass SMBH
binaries, using a modified version of the SPH code GADGET-3. We present a total
of 12 simulations that explore different possible pericentre distances and
relative inclinations, and show that the formation of circumbinary discs and
discs around each SMBH ('mini-discs') depend on those parameters. We also study
the dynamics of the formed discs, and the variability of the feeding rate onto
the SMBHs in the different configurations.Comment: Accepted for publication in MNRAS, 17 pages, 13 figures. Animations
available at http://www.astro.puc.cl/~fgarrido/animation
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