On the basis for ELF - An Extensible Language Facility

Computer language for data processing and information retrieva

Interferometric Observations of the Hierarchical Triple System Algol

Algol is a triple stellar system consisting of a close semidetached binary orbited by a third object. Due to the disputed spatial orientation of the close pair, the third body perturbation of this pair is a subject of much research. In this study, we determine the spatial orientation of the close pair orbital plane using the CHARA Array, a six-element optical/IR interferometer located on Mount Wilson, and state-of-the-art e-EVN interferometric techniques. We find that the longitude of the line of nodes for the close pair is \Omega_1=48\degr\pm2\degr and the mutual inclination of the orbital planes of the close and the wide pairs is 95\degr\pm3\degr. This latter value differs by 5\degr from the formerly known 100\degr which would imply a very fast inclination variation of the system, not borne out by the photometric observations. We also investigated the dynamics of the system with numerical integration of the equations of motions using our result as an initial condition. We found large variations in the inclination of the close pair (its amplitude \sim 170\degr) with a period of about 20 millennia. This result is in good agreement with the photometrically observed change of amplitude in Algol's primary minimum.Comment: ApJ, in press. This is the accepted version; will be changed with the final version later (minor language corrections

The pragmatic formalization of computing systems relative to a given high-level language

Imperial Users onl

A unified solution for the orbit and light-time effect in the V505 Sgr system

The multiple system V505 Sagittarii is composed of at least three stars: a compact eclipsing pair and a distant component, which orbit is measured directly using speckle interferometry. In order to explain the observed orbit of the third body in V505 Sagittarii and also other observable quantities, namely the minima timings of the eclipsing binary and two different radial velocities in the spectrum, we thoroughly test a fourth-body hypothesis - a perturbation by a dim, yet-unobserved object. We use an N-body numerical integrator to simulate future and past orbital evolution of 3 or 4 components in this system. We construct a suitable chi^2 metric from all available speckle-interferometry, minima-timings and radial-velocity data and we scan a part of a parameter space to get at least some of allowed solutions. In principle, we are able to explain all observable quantities by a presence of a fourth body, but the resulting likelihood of this hypothesis is very low. We also discuss other theoretical explanations of the minima timings variations. Further observations of the minima timings during the next decade or high-resolution spectroscopic data can significantly constrain the model

Constraints on stellar convection from multi-colour photometry of Delta Scuti stars

In Delta Scuti star models, the calculated amplitude ratios and phase differences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for determination of the spherical harmonic degree of the excited modes. The dependence on convection enters through the complex parameter f, which describes bolometric flux perturbation. We present a method of simultaneous determination of f and spherical harmonic degree from multi-colour data and apply it to three Delta Scuti stars. The method indeed works. Determination of the degree appears unique and the inferred f's are sufficiently accurate to yield a useful constraint on models of stellar convection. Furthermore, the method helps to refine stellar parameters, especially if the identified mode is radial.Comment: 9 pages, 12 figures, to appear in Astronomy and Astrophysic

The Importance of Being Eelco

Programming language designers and implementers are taught that: semantics are more worthwhile than syntax, that programs exist to embody proofs, rather than to get work done, and to value Dijkstra more than Van Wijngaarden. Eelco Visser believed that, while there is value in the items on the left, there is at least as much value in the items on the right. This short paper explores how Eelco Visser embodied these values, and how he encouraged our work on the Grace programming language, supported that work withio Spoofax, and provided a venue for discussion within the WG2.16 Programming Language Design working group

Model computations of blue stragglers and W UMa-type stars in globular clusters

It was recently demonstrated that contact binaries occur in globular clusters (GCs) only immediately below turn-off point and in the region of blue straggler stars (BSs). In addition, observations indicate that at least a significant fraction of BSs in these clusters was formed by the binary mass-transfer mechanism. The aim of our present investigation is to obtain and analyze a set of evolutionary models of cool, close detached binaries with a low metal abundance, which are characteristic of GC. We computed the evolution of 975 models of initially detached, cool close binaries with different initial parameters. The models include mass exchange between components as well as mass and angular momentum loss due to the magnetized winds for very low-metallicity binaries with Z = 0.001. The models are interpreted in the context of existing data on contact binary and blue straggler members of GCs. The model parameters agree well with the observed positions of the GC contact binaries in the Hertzsprung-Russell diagram. Contact binaries in the lower part of the cluster main sequence are absent because there are no binaries with initial orbital periods shorter than 1.5 d. Contact binaries end their evolution as mergers that appear in the BS region. Binary-formed BSs populate the whole observed BS region in a GC, but a gap is visible between low-mass mergers that are concentrated along the zero-age main sequence and binary BSs occupying the red part of the BS region. Very few binary mergers are expected to rotate rapidly and/or possess chemical peculiarities resulting from the exposure of the layers processed by CNO nuclear reactions. All other binary mergers are indistinguishable from the collisionally formed mergers. The results show that binary-formed BSs may constitute at least a substantial fraction of all BSs in a GC.Comment: Accepted for publication in A&

On the Origin of Recursive Procedures

We investigate the origin of recursive procedures in imperative programming languages. We attempt to set the record straight, and to identify the trend that led to recursive procedures, by means of an analysis of the related concepts and of the most reliable available documents, as far as known to us. We show that not all of those who were involved in defining these concepts in these documents were fully aware of the implications of their proposals. Our aim is not primarily historical, but to contribute to a clarification of some of the concepts related to recursion. In particular, we demonstrate that recursive procedure declarations and recursive procedure activations are logically disjoint concepts