Surface features, rotation and atmospheric variability of ultra cool dwarfs

Photometric I band light curves of 21 ultra cool M and L dwarfs are presented. Variability with amplitudes of 0.01 to 0.055 magnitudes (RMS) with typical timescales of an hour to several hours are discovered in half of these objects. Periodic variability is discovered in a few cases, but interestingly several variable objects show no significant periods, even though the observations were almost certainly sensitive to the expected rotation periods. It is argued that in these cases the variability is due to the evolution of the surface features on timescales of a few hours. This is supported in the case of 2M1145 for which no common period is found in two separate light curves. It is speculated that these features are photospheric dust clouds, with their evolution possibly driven by rotation and turbulence. An alternative possibility is magnetically-induced surface features. However, chromospheric activity undergoes a sharp decrease between M7 and L1, whereas a greater occurrence of variability is observed in objects later than M9, lending support to the dust interpretation.Comment: To appear in "Ultracool Dwarf Stars" (Lecture Notes in Physics), H.R.A. Jones, I. Steele (eds), Springer-Verlag, 2001. Also available from http://www.mpia-hd.mpg.de/homes/calj/ultra.htm

The glial growth factors deficiency and synaptic destabilization hypothesis of schizophrenia

BACKGROUND: A systems approach to understanding the etiology of schizophrenia requires a theory which is able to integrate genetic as well as neurodevelopmental factors. PRESENTATION OF THE HYPOTHESIS: Based on a co-localization of loci approach and a large amount of circumstantial evidence, we here propose that a functional deficiency of glial growth factors and of growth factors produced by glial cells are among the distal causes in the genotype-to-phenotype chain leading to the development of schizophrenia. These factors include neuregulin, insulin-like growth factor I, insulin, epidermal growth factor, neurotrophic growth factors, erbB receptors, phosphatidylinositol-3 kinase, growth arrest specific genes, neuritin, tumor necrosis factor alpha, glutamate, NMDA and cholinergic receptors. A genetically and epigenetically determined low baseline of glial growth factor signaling and synaptic strength is expected to increase the vulnerability for additional reductions (e.g., by viruses such as HHV-6 and JC virus infecting glial cells). This should lead to a weakening of the positive feedback loop between the presynaptic neuron and its targets, and below a certain threshold to synaptic destabilization and schizophrenia. TESTING THE HYPOTHESIS: Supported by informed conjectures and empirical facts, the hypothesis makes an attractive case for a large number of further investigations. IMPLICATIONS OF THE HYPOTHESIS: The hypothesis suggests glial cells as the locus of the genes-environment interactions in schizophrenia, with glial asthenia as an important factor for the genetic liability to the disorder, and an increase of prolactin and/or insulin as possible working mechanisms of traditional and atypical neuroleptic treatments

Metaphor in Chemistry: An Examination of Chemical Metaphor

The function of metaphor in science has been labeled as decorative, persuasive, heuristic, instrumental, facilitating or obstructing. It has sometimes been regarded as inspiring, provoking, perverting or destroying rational thought. Metaphor’s positive role has been noted by philosophers, historians of chemistry, and science education researchers. It has been hailed as a descriptive and explanatory device that stimulates and shapes concept development. I discuss how metaphor functions in science generally, then refine this idea through an examination metaphor’s role in chemical thinking in three contexts: the history and philosophy of chemistry, laboratory research practice, and chemical education. I aim to show how metaphor is already operative in the chemist’s use of the concept of chemical element and that this understanding characterizes chemical thinking in general. The chapter concludes with a discussion of a specifically chemical understanding of metaphor

How do models give us knowledge? The case of Carnot's ideal heat engine.

Our concern is to explain how and why models give us useful knowledge. We argue that if we are to understand how models function in the actual scientific practice the representational approach to models proves either misleading or too minimal—depending on how representation is defined. By ‘representational approach’ we mean one that attributes the epistemic value of models to the representational relationship between a model and some real target system. In contrast we propose turning from the representational approach to the artefactual, which also implies a new unit of analysis: the activity of modelling. Modelling, we suggest, could fruitfully be approached as a scientific practice in which concrete artefacts, i.e., models, are constructed with specific representational means and used in various ways, for example, for the purposes of scientific reasoning, theory construction and design of experiments and other artefacts. Furthermore, we propose that in the activity of modelling the construction of models is intertwined with the construction of new phenomena, concepts, and theoretical principles. We will illustrate these claims by studying the construction of the ideal heat engine by Sadi Carnot

What is Validation of Computer Simulations? Toward a Clarification of the Concept of Validation and of Related Notions

This chapter clarifies the concept of validation of computer simulations by comparing various definitions that have been proposed for the notion. While the definitions agree in taking validation to be an evaluation, they differ on the following questions: 1. What exactly is evaluated – results from a computer simulation, a model, a computer code? 2. What are the standards of evaluation – truth, accuracy and credibility or also something else? 3. What type of verdict does validation lead to – that the simulation is such and such good, or that it passes a test defined by a pre-defined threshold? 4. How strong needs the case to be for the verdict? 5. Does validation necessarily proceed by comparing simulation outputs with measured data? Along with the questions, the chapter explains notions that figure prominently in them, e.g. the concepts of accuracy and credibility. It further discusses natural answers to the questions as well as arguments that speak in favor and against these answers. The aim is to obtain a better understanding of the options we have for defining validation and how they are related to each other

Model templates within and between disciplines: from magnets to gases – and socio-economic systems

One striking feature of the contemporary modelling practice is its interdisciplinary nature. The same equation forms, and mathematical and computational methods, are used across different disciplines, as well as within the same discipline. Are there, then, differences between intra- and interdisciplinary transfer, and can the comparison between the two provide more insight on the challenges of interdisciplinary theoretical work? We will study the development and various uses of the Ising model within physics, contrasting them to its applications to socio-economic systems. While the renormalization group (RG) methods justify the transfer of the Ising model within physics – by ascribing them to the same universality class – its application to socio-economic phenomena has no such theoretical grounding. As a result, the insights gained by modelling socio-economic phenomena by the Ising model may remain limited