Spatial distribution and galactic model parameters of cataclysmic variables

The spatial distribution, galactic model parameters and luminosity function of cataclysmic variables (CVs) in the solar neighbourhood have been determined from a carefully established sample of 459 CVs. The sample contains all of the CVs with distances computed from the Period-Luminosity-Colours (PLCs) relation of CVs which has been recently derived and calibrated with {\em 2MASS} photometric data. It has been found that an exponential function fits best to the observational z-distributions of all of the CVs in the sample, non-magnetic CVs and dwarf novae, while the sech^{2} function is more appropriate for nova-like stars and polars. The vertical scaleheight of CVs is 158$\pm$14 pc for the {\em 2MASS} J-band limiting apparent magnitude of 15.8. On the other hand, the vertical scaleheights are 128$\pm$20 and 160$\pm$5 pc for dwarf novae and nova-like stars, respectively. The local space density of CVs is found to be $\sim3\times10^{-5}$ pc^{-3} which is in agreement with the lower limit of the theoretical predictions. The luminosity function of CVs shows an increasing trend toward higher space densities at low luminosities, implying that the number of short-period systems should be high. The discrepancies between the theoretical and observational population studies of CVs will almost disappear if for the z-dependence of the space density the sech^{2} density function is used.Comment: 29 pages, 9 figures and 5 tables, accepted for publication in New Astronom

Validity and usefulness of COVID-19 models

Mathematical models have become central to the public and policy debate about the recent COVID-19 pandemic. On the one hand, they provide guidance to policy-makers about the development of the epidemic and healthcare demand overtime; on the other hand, they are heavily criticized for their lack of credibility. This commentary reflects on three such models from a validity and usefulness perspective. Specifically, it discusses the complexity, validation, and communication of models informing the government decisions in the UK, US and Austria, and concludes that, although these models are useful in many ways, they currently lack a thorough validation and a clear communication of their uncertainties. Therefore, prediction claims of these models should be taken cautiously, and their merits on scenario analysis should be the basis for decision-making. The lessons that can be learned from the COVID models in terms of the communication of uncertainties and assumptions can guide the use of quantitative models in other policy-making areas

Kinematics of W UMa-type binaries and evidences on the two types of formation

The kinematics of 129 W UMa binaries is studied and its implications on the contact binary evolution is discussed. The sample is found to be heterogeneous in the velocity space that kinematically younger and older contact binaries exist in the sample. Kinematically young (0.5 Gyr) sub-sample (MG) is formed by selecting the systems which are satisfying the kinematical criteria of moving groups. After removing the possible MG members and the systems which are known to be members of open clusters, the rest of the sample is called Field Contact Binaries (FCB). The FCB has further divided into four groups according to The orbital period ranges. Then a correlation has been found in the sense that shorter period less massive systems have larger velocity dispersions than the longer period more massive systems. Dispersions in the velocity space indicates 5.47 Gyr kinematical age for the FCB group. Comparing with the field chromospherically active binaries (CAB), presumably detached binary progenitors of the contact systems, the FCB appears to be 1.61 Gyr older. Assuming an equilibrium in the formation and destruction of CAB and W UMa systems in the Galaxy, this age difference is treated as empirically deduced lifetime of the contact stage. Since the kinematical ages of the four sub groups of FCB are much longer than the 1.61 Gyr lifetime of the contact stage, the pre-contact stages of FCB must dominantly be producing the large dispersions. The kinematically young (0.5 Gyr) MG group covers the same total mass, period and spectral ranges as the FCB. But, the very young age of this group does not leave enough room for pre-contact stages, thus it is most likely that those systems were formed in the beginning of the main-sequence or during the pre-main-sequence contraction phase.Comment: 19 pages, including 11 figures and 5 tables, accepted for publication in MNRA

Lazy AC-Pattern Matching for Rewriting

We define a lazy pattern-matching mechanism modulo associativity and commutativity. The solutions of a pattern-matching problem are stored in a lazy list composed of a first substitution at the head and a non-evaluated object that encodes the remaining computations. We integrate the lazy AC-matching in a strategy language: rewriting rule and strategy application produce a lazy list of terms.Comment: In Proceedings WRS 2011, arXiv:1204.531

Understanding the Mechanisms behind Fragmentation in the Housing Construction and Retrofit

Housing and energy-efficiency policies often cause unintended consequences in the built-environment and beyond it, and encounter resistance by the potential users. Systems thinking and an integrated approach are suggested both for research and decision-making in order to avoid such unintended consequences and policy resistance. However, previous studies revealed that such an integrated decision making takes place neither at the policy nor at the industry level, and the resulting fragmented processes lead to performance gaps in the housing sector. This study investigates the mechanisms behind fragmentation based on a large set of stakeholder interviews conducted for the housing sector in the UK. For this purpose, the information obtained from these interviews is transformed into a system dynamics model. The model focuses on the improvement of housing performance by immediate actions such as resource allocation or by longer-term actions such as competence development both at the policy and industry level. Future research will focus on extending the model to the user level and elaborating it in participatory sessions

Using Textual Data in System Dynamics Model Conceptualization

Qualitative data is an important source of information for system dynamics modeling. It can potentially support any stage of the modeling process, yet it is mainly used in the early steps such as problem identification and model conceptualization. Existing approaches that outline a systematic use of qualitative data in model conceptualization are often not adopted for reasons of time constraints resulting from an abundance of data. In this paper, we introduce an approach that synthesizes the strengths of existing methods. This alternative approach (i) is focused on causal relationships starting from the initial steps of coding; (ii) generates a generalized and simplified causal map without recording individual relationships so that time consumption can be reduced; and (iii) maintains the links from the final causal map to the data sources by using software. We demonstrate an application of this approach in a study about integrated decision making in the housing sector of the UK

Systems Thinking to Understand National Well-Being from a Human Capital Perspective

Well-being has become an important policy goal to replace gross domestic product (GDP) as an indicator of national progress. Several multidimensional metrics and indicators of well-being have been developed mostly based on the four-capital model that includes natural, economic, human and social capital. These multidimensional measures of well-being, however, are highly categorical and lack a systems perspective that focuses on underlying mechanisms of the metrics and the interconnections between them. This study aims at bringing a systems thinking approach to understanding and measuring national well-being, particularly from a human capital perspective. For this purpose, we employ a qualitative systems mapping approach and identify the direct or indirect relationships between the well-being indicators related to human capital. The results show that the human capital system is governed by several reinforcing feedback loops through economic progress, health and life expectancy, which gives a central role to human capital to enhance well-being. There are balancing loops, however, that may have adverse effects on human capital formation and well-being, for instance through migration and ageing. Future studies can focus on the other three subsystems in the four-capital model, and on quantifying the relationships between different dimensions of well-being

Participatory system dynamics modelling for housing, energy and wellbeing interactions

The built environment is a key target of decarbonization policies. However, such policies often have a narrow objective and narrow focus, resulting in ‘policy-resistance’ and unintended consequences. The literature attributes these unintended consequences to a narrow financial focus, adverse incentives, and inadequate handling of knowledge, skills, communication and feedback gaps, but it provides little advice on how these complex interactions can be captured. This paper illustrates the development and application of an integrated approach to address these complex interactions with regard to housing performance, energy, communal spaces and wellbeing. In particular, it explores the dynamics created by these relationships with simulation modelling in participatory settings, and with a diverse group of stakeholders. The simulation results suggest that monitoring is key to improve the performance of the housing stock besides energy efficiency; and investments in communal spaces positively affect the adoption of energy-efficiency measures and the wellbeing of residents. The evaluation results for participatory workshops show this approach was found useful by the stakeholders for supporting more integrated decision-making about housing. In future research, this approach can be implemented for policy problems in specific contexts