Better Use Case Diagrams by Using Work System Snapshots

Research to date shows significant variability in the success of applying the common technique of use case diagramming for identifying information system scope in terms of use cases performed by actors interacting with an information system or performed automatically by the information system. The current research tests a) the benefits of using a work system snapshot, a basic analytical tool from the work system method, before producing use case diagrams, and b) the additional benefits of enhancing use case diagramming constructs to distinguish between automated activities, activities supported by the information system, and relevant manual activities. Teams of student subjects in an experiment produced substantially better use case diagrams - containing far more use cases and qualitatively better use cases than did the teams in control group - when provided with a work system snapshot that summarized a test scenario in terms of work system concept

Constraint Diagrams: Visualizing Assertions in OO Modelling

Describes a notation, constraint diagrams, which allows pre/post conditions and invariants to be expressed visually, rather than in the notation of mathematical logic. The notation is explored through a small case study (a library system). Some conclusions are drawn about the use of the notation in modelling, and its possible impact on tools and semantics. This report has been split into two and considerable revised and updated: Kent (1997b), Kent (1997c)

Semantics Through Pictures: towards a diagrammatic semantics for object-oriented modelling notations

An object-oriented (OO) model has a static component, the set of allowable snapshots or system states, and a dynamic component, the set of filmstrips or sequences of snapshots. Diagrammatic notations, such as those in UML, each places constraints on the static and/or dynamic models. A formal semantics of OO modeling notations can be constructed by providing a formal description of (i) sets of snapshots and filmstrips, (ii) constraints on those sets, and (iii) the derivation of those constraints from diagrammatic notations. In addition, since constraints are contributed by many diagrams for the same model, a way of doing this compositionally is desirable. One approach to the semantics is to use first-order logic for (i) and (ii), and theory inclusion with renaming, as in Larch, to characterize composition. A common approach to (iii) is to bootstrap: provide a semantics for a kernel of the notation and then use the kernel to give a semantics to the other notations. This only works if a kernel which is sufficiently expressive can be identified, and this is not the case for UML. However, we have developed a diagrammatic notation, dubbed constraint diagrams, which seems capable of expressing most if not all static and dynamic constraints, and it is proposed that this be used to give a diagrammatic semantics to OO models

GMC Collisions as Triggers of Star Formation. V. Observational Signatures

We present calculations of molecular, atomic and ionic line emission from simulations of giant molecular cloud (GMC) collisions. We post-process snapshots of the magneto-hydrodynamical simulations presented in an earlier paper in this series by Wu et al. (2017) of colliding and non-colliding GMCs. Using photodissociation region (PDR) chemistry and radiative transfer we calculate the level populations and emission properties of $^{12}$CO $J=1-0$, [CI] $^3{\rm P}_1\rightarrow{^3{\rm P}}_0$ at $609\,\mu$m, [CII] $158\,\mu$m and [OI] $^3{\rm P}_1\rightarrow{^3{\rm P}}_0$ transition at $63\,\mu$m. From integrated intensity emission maps and position-velocity diagrams, we find that fine-structure lines, particularly the [CII] $158\,\mu$m, can be used as a diagnostic tracer for cloud-cloud collision activity. These results hold even in more evolved systems in which the collision signature in molecular lines has been diminished.Comment: 10 pages, 7 figures, accepted for publication in ApJ, comments welcom

Characterizing the velocity field in hydrodynamical simulations of low-mass star formation using spectral line profiles

When low-mass stars form, the collapsing cloud of gas and dust goes through several stages which are usually characterized by the shape of their spectral energy distributions. Such classification is based on the cloud morphology only and does not address the dynamical state of the object. In this paper we investigate the initial cloud collapse and subsequent disk formation through the dynamical behavior as reflected in the sub-millimeter spectral emission line profiles. If a young stellar object is to be characterized by its dynamical structure it is important to know how accurately information about the velocity field can be extracted and which observables provide the best description of the kinematics. Of particular interest is the transition from infalling envelope to rotating disk, because this provides the initial conditions for the protoplanetary disk, such as mass and size. We use a hydrodynamical model, describing the collapse of a core and formation of a disk, to produce synthetic observables which we compare to calculated line profiles of a simple parameterized model. Because we know the velocity field from the hydrodynamical simulation we can determine in a quantitative way how well our best-fit parameterized velocity field reproduces the original. We use a molecular line excitation and radiation transfer code to produce spectra of both our hydro dynamical simulation as well as our parameterized model. We find that information about the velocity field can reasonably well be derived by fitting a simple model to either single-dish lines or interferometric data, but preferentially by using a combination of the two. Our result shows that it is possible to establish relative ages of a sample of young stellar objects using this method, independently of the details of the hydrodynamical model.Comment: 12 pages, 11 figures, accepted for publication in A&A on June 1

A Simple Computer Model for Liquid Lipid Bilayers

We present a simple coarse-grained bead-and-spring model for lipid bilayers. The system has been developed to reproduce the main (gel-liquid) transition of biological membranes on intermediate length scales of a couple of nanometres and is very efficient from a computational point of view. For the solvent environment, two different models are proposed. The first model forces the lipids to form bilayers by confining their heads in two parallel planes. In the second model, the bilayer is stabilised by a surrounding gas of "phantom" solvent beads, which do not interact with each other. This model takes only slightly more computing time than the first one, while retaining the full membrane flexibility. We calculate the liquid-gel phase boundaries for both models and find that they are very similar.Comment: 11 pages, 6 figure