Bayesian Asteroseismology of 23 Solar-Like Kepler Targets

We study 23 previously published Kepler targets to perform a consistent grid-based Bayesian asteroseismic analysis and compare our results to those obtained via the Asteroseismic Modelling Portal (AMP). We find differences in the derived stellar parameters of many targets and their uncertainties. While some of these differences can be attributed to systematic effects between stellar evolutionary models, we show that the different methodologies deliver incompatible uncertainties for some parameters. Using non-adiabatic models and our capability to measure surface effects, we also investigate the dependency of these surface effects on the stellar parameters. Our results suggest a dependence of the magnitude of the surface effect on the mixing length parameter which also, but only minimally, affects the determination of stellar parameters. While some stars in our sample show no surface effect at all, the most significant surface effects are found for stars that are close to the Sun's position in the HR diagram.Comment: 14 pages, 9 figures, accepted for publication in MNRA

A Study of the Dynamics of Cardiac Ischemia using Experimental and Modeling Approaches

The dynamics of cardiac ischemia was investigated using experimental studies and computer simulations. An experimental model consisting of an isolated and perfused canine heart with full control over blood flow rate to a targeted coronary artery was used in the experimental study and a realistically shaped computer model of a canine heart, incorporating anisotropic conductivity and realistic fiber orientation, was used in the simulation study. The phenomena investigated were: (1) the influence of fiber rotation on the epicardial potentials during ischemia and (2) the effect of conductivity changes during a period of sustained ischemia. Comparison of preliminary experimental and computer simulation results suggest that as the ischemic region grows from the endocardium towards the epicardium, the epicardial potential patterns follow the rotating fiber orientation in the myocardium. Secondly, in the experimental studies it was observed that prolonged ischemia caused a subsequent reduction in the magnitude of epicardial potentials. Similar results were obtained from the computer model when the conductivity of the tissue in the ischemic region was reduce

Time Variability of Quasars: the Structure Function Variance

Significant progress in the description of quasar variability has been recently made by employing SDSS and POSS data. Common to most studies is a fundamental assumption that photometric observations at two epochs for a large number of quasars will reveal the same statistical properties as well-sampled light curves for individual objects. We critically test this assumption using light curves for a sample of $\sim$2,600 spectroscopically confirmed quasars observed about 50 times on average over 8 years by the SDSS stripe 82 survey. We find that the dependence of the mean structure function computed for individual quasars on luminosity, rest-frame wavelength and time is qualitatively and quantitatively similar to the behavior of the structure function derived from two-epoch observations of a much larger sample. We also reproduce the result that the variability properties of radio and X-ray selected subsamples are different. However, the scatter of the variability structure function for fixed values of luminosity, rest-frame wavelength and time is similar to the scatter induced by the variance of these quantities in the analyzed sample. Hence, our results suggest that, although the statistical properties of quasar variability inferred using two-epoch data capture some underlying physics, there is significant additional information that can be extracted from well-sampled light curves for individual objects.Comment: Presented at the "Classification and Discovery in Large Astronomical Surveys" meeting, Ringberg Castle, 14-17 October, 200

The chaotic solar cycle II. Analysis of cosmogenic 10Be data

Context. The variations of solar activity over long time intervals using a solar activity reconstruction based on the cosmogenic radionuclide 10Be measured in polar ice cores are studied. Methods. By applying methods of nonlinear dynamics, the solar activity cycle is studied using solar activity proxies that have been reaching into the past for over 9300 years. The complexity of the system is expressed by several parameters of nonlinear dynamics, such as embedding dimension or false nearest neighbors, and the method of delay coordinates is applied to the time series. We also fit a damped random walk model, which accurately describes the variability of quasars, to the solar 10Be data and investigate the corresponding power spectral distribution. The periods in the data series were searched by the Fourier and wavelet analyses. The solar activity on the long-term scale is found to be on the edge of chaotic behavior. This can explain the observed intermittent period of longer lasting solar activity minima. Filtering the data by eliminating variations below a certain period (the periods of 380 yr and 57 yr were used) yields a far more regular behavior of solar activity. A comparison between the results for the 10Be data with the 14C data shows many similarities. Both cosmogenic isotopes are strongly correlated mutually and with solar activity. Finally, we find that a series of damped random walk models provides a good fit to the 10Be data with a fixed characteristic time scale of 1000 years, which is roughly consistent with the quasi-periods found by the Fourier and wavelet analyses.Comment: 8 pages, 11 figure

<i>‘What retention’ means to me</i>: the position of the adult learner in student retention

Studies of student retention and progression overwhelmingly appear adopt definitions that place the institution, rather than the student, at the centre. Retention is most often conceived in terms of linear and continuous progress between institutionally identified start and end points. This paper reports on research that considered data from 38 in-depth interviews conducted with individuals who had characteristics often associated with non-traditional engagement in higher education who between 2006 and 2010 had studied an ‘Introduction to HE’ module at one distance higher education institution, some of whom had progressed to further study at that institution, some of whom had not. The research deployed a life histories approach to seek a finer grained understanding of how individuals conceptualise their own learning journey and experience, in order to reflect on institutional conceptions of student retention. The findings highlight potential anomalies hidden within institutional retention rates – large proportions of the interview participants who were not ‘retained’ by the institution reported successful progression to and in other learning institutions and environments, both formal and informal. Nearly all described positive perspectives on lifelong learning which were either engendered or improved by the learning undertaken. This attests to the complexity of individuals’ lives and provides clear evidence that institution-centric definitions of retention and progression are insufficient to create truly meaningful understanding of successful individual learning journeys and experiences. It is argued that only through careful consideration of the lived experience of students and a re-conception of measures of retention, will we be able to offer real insight into improving student retention