612 research outputs found
Abundances and possible diffusion of elements in M67 stars
We present a spectroscopic study at high resolution, R~50,000, of 14 stars
located on the main sequence, at the turn-off point and on the early subgiant
branch in the cluster M67 in order to investigate its detailed chemical
composition, for comparison with the Sun and solar twins in the solar
neighbourhood, and to explore selective atomic diffusion of chemical elements
as predicted by stellar-structure theory. We have obtained VLT/FLAMES-UVES
spectra and analysed these strictly differentially in order to explore
chemical-abundance similarities and differences between the M67 stars and the
Sun, and among the M67 stars themselves. Individual abundances of 19 different
chemical elements are obtained for the stars. They are found to agree very well
with solar abundances, with abundance ratios closer to solar than those of most
solar twins in the solar neighbourhood. An exception is Li which shows a
considerable scatter among the cluster stars. There is a tendency for the
cluster-star abundances to be depleted relative to the abundances in the field
stars in correlation with the condensation temperature of the elements, a
tendency earlier found also for the Sun. The results support the hypothesis
that the gas of the proto-cluster was depleted by formation and cleansing of
dust before the stars formed. They also add support to the proposal that the
Sun was once formed in a dense stellar environment. Moreover, the observed
minor reductions of heavy elements in the atmospheres of the dwarfs and
turn-off point stars relative to our standard star M67-1194 and the subgiants
seem to suggest that diffusion processes are at work in these stars, although
the evidence is not compelling. Based on theoretical models the
diffusion-corrected initial metallicity of M67 is estimated to be [Fe/H]=+0.06.Comment: 14 pages, 9 figures, 5 tables. Accepted for publication in section 8.
Stellar atmospheres of Astronomy and Astrophysics. Minor language corrections
and an update of section 4.1 as compared to previous publicatio
Habitual Criminal Statutes: Shield or Sword
An essential part of future collision avoidance systems is to be able to predict road curvature. This can be based on vision data, but the lateral movement of leading vehicles can also be used to support road geometry estimation. This paper presents a method for detecting lane departures, including lane changes, of leading vehicles. This information is used to adapt the dynamic models used in the estimation algorithm in order to accommodate for the fact that a lane departure is in progress. The goal is to improve the accuracy of the road geometry estimates, which is affected by the motion of leading vehicles. The significantly improved performance is demonstrated using sensor data from authentic traffic environments
Reduced models for dynamic analysis of high-speed railway bridges
Today a dynamic analysis is often performed with the use of advanced finite element software. An advanced model normally produces precise results. Analyzing these large models of bridges is however time consuming and a detailed description of the geometry, supports and material properties must be known. In an early stage of a project a reduced model could significantly reduce the time needed to perform the final dynamic analysis. The aim of this thesis was to investigate the dynamic response of railway bridges for high-speed trains and to develop a reduced model that can be used early in a project, to provide a quick dynamic analysis of bridges with several spans. The reduced model was based on the fact that the fundamental dynamic behavior of certain type of bridges may be described by the dynamic behavior of 2-dimensional Bernoulli beam elements. The bridge model was developed as a MATLAB function and consists of a selectable number of Bernoulli beam elements. Three different approaches of modeling the train load were compared. The first method of modelling the train was made as moving point-loads, the second as a distributed load and the third as a separate mass and spring model. In order to make the developed MATLAB function easy to use, to perform quick analysis and with good accuracy, the numbers of input parameters were minimized. This was made by setting some of the properties of the reduced model as fixed standard values. The values were based on result of extensive modelling studies. Finally, two bridges with realistic dimensions and materials were analyzed using both the reduced model and a more advanced model. The reduced model was proved to provide basically the same results as the more advanced model. The time needed to model the bridge and to perform the analysis was approximately 10 minutes when using a standard home pc, which was a significant reduction of the time needed to make the advanced model and to perform the analysis
Constraint damping for the Z4c formulation of general relativity
One possibility for avoiding constraint violation in numerical relativity
simulations adopting free-evolution schemes is to modify the continuum
evolution equations so that constraint violations are damped away. Gundlach et.
al. demonstrated that such a scheme damps low amplitude, high frequency
constraint violating modes exponentially for the Z4 formulation of General
Relativity. Here we analyze the effect of the damping scheme in numerical
applications on a conformal decomposition of Z4. After reproducing the
theoretically predicted damping rates of constraint violations in the linear
regime, we explore numerical solutions not covered by the theoretical analysis.
In particular we examine the effect of the damping scheme on low-frequency and
on high-amplitude perturbations of flat spacetime as well and on the long-term
dynamics of puncture and compact star initial data in the context of spherical
symmetry. We find that the damping scheme is effective provided that the
constraint violation is resolved on the numerical grid. On grid noise the
combination of artificial dissipation and damping helps to suppress constraint
violations. We find that care must be taken in choosing the damping parameter
in simulations of puncture black holes. Otherwise the damping scheme can cause
undesirable growth of the constraints, and even qualitatively incorrect
evolutions. In the numerical evolution of a compact static star we find that
the choice of the damping parameter is even more delicate, but may lead to a
small decrease of constraint violation. For a large range of values it results
in unphysical behavior.Comment: 13 pages, 24 figure
Merging Techniques for Faster Derivation of WCET Flow Information using Abstract Execution
Static Worst-Case Execution Time (WCET) analysis derives upper bounds for the execution times of programs. Such bounds are crucial when designing and verifying real-time systems. A key component in static WCET analysis is to derive flow information, such as loop bounds and infeasible paths. We have previously introduced abstract execution (AE), a method capable of deriving very precise flow information. This paper present different merging techniques that can be used by AE for trading analysis time for flow information precision. It also presents a new technique, ordered merging, which may radically shorten AE analysis times, especially when analyzing large programs with many possible input variable values
Toward Static Timing Analysis of Parallel Software
The current trend within computer, and even real-time, systems is to incorporate parallel hardware, e.g., multicore processors, and parallel software. Thus, the ability to safely analyse such parallel systems, e.g., regarding the timing behaviour, becomes necessary. Static timing analysis is an approach to mathematically derive safe bounds on the execution time of a program, when executed on a given hardware platform. This paper presents an algorithm that statically analyses the timing of parallel software, with threads communicating through shared memory, using abstract interpretation. It also gives an extensive example to clarify how the algorithm works
Editorial: EATAW2019: Selected papers from the 10th Conference of the European Association for the Teaching of Academic Writing, Chalmers University of Technology, Gothenburg, Sweden, July, 2019
https://publications.coventry.ac.uk/index.php/joaw/article/view/71
ALF - A Language for WCET Flow Analysis
Static Worst-Case Execution Time (WCET) analysis derives upper bounds for the execution times of programs. Such bounds are crucial when designing and verifying real-time systems. A key component in static WCET analysis is the flow analysis, which derives bounds on the number of times different code entities can be executed. Examples of flow information derived by a flow analysis are loop bounds and infeasible paths.
Flow analysis can be performed on source code, intermediate code, or binary code: for the latter, there is a proliferation of instruction sets. Thus, flow analysis must deal with many code formats. However, the basic flow analysis techniques are more or less the same regardless of the code format. Thus, an interesting option is to define a common code format for flow analysis, which also allows for
easy translation from the other formats. Flow analyses for this common format will then be portable, in principle supporting all types of code formats which can be translated to this format. Further, a common format simplifies the development of flow analyses, since only one specific code format needs to be targeted.
This paper presents such a common code format, the ALF language (ARTIST2 Language for WCET Flow Analysis)
- …