Building Blue Stragglers with Stellar Collisions

The evolution of stellar collision products in cluster simulations has usually been modelled using simplified prescriptions. Such prescriptions either replace the collision product with an (evolved) main sequence star, or assume that the collision product was completely mixed during the collision. It is known from hydrodynamical simulations of stellar collisions that collision products are not completely mixed, however. We have calculated the evolution of stellar collision products and find that they are brighter than normal main sequence stars of the same mass, but not as blue as models that assume that the collision product was fully mixed during the collision.Comment: 2 pages, 1 figure. To appear in the proceedings of Dynamical Evolution of Dense Stellar Systems, IAU Symposium 24

When Stars Collide

When two stars collide and merge they form a new star that can stand out against the background population in a starcluster as a blue straggler. In so called collision runaways many stars can merge and may form a very massive star that eventually forms an intermediate mass blackhole. We have performed detailed evolution calculations of merger remnants from collisions between main sequence stars, both for lower mass stars and higher mass stars. These stars can be significantly brighter than ordinary stars of the same mass due to their increased helium abundance. Simplified treatments ignoring this effect give incorrect predictions for the collision product lifetime and evolution in the Hertzsprung-Russell diagram.Comment: 8 pages, 5 figures to appear in the proceedings for "Unsolved Problems in Stellar Physics", Cambridge, 2-6 July 200

Eerherstel van de eer:We hebben de inhaligheid van publieke dienstverleners over onszelf afgeroepen

Niet geld maar waardering is de krachtigste motivatie die er bestaat. Wetenschappers zoeken naar middelen om hiervan maatschappelijk gebruik te maken. Minister Ronald Plasterk van Onderwijs, Cultuur en Wetenschap heeft een merkwaardig probleem. Om ervoor te zorgen dat er ook straks nog mensen voor de klas staan, wil hij de lerarensalarissen verhogen. Hoewel hij hiervoor vrijwel unanieme politieke steun krijgt, had het parlementaire debat over dit voornemen een zure ondertoon. Kamerleden vroegen zich bezorgd af of het geld de leraren wel zal bereiken en niet in de regionen van het management zal blijven hangen. Zouden we straks een parlementaire enquête nodig hebben om uit te vinden waar het extra miljard gebleven is? Er is namelijk iets aan de hand met het geld van scholen. Toen die in de jaren negentig de beschikking kregen over eigen budgetten was er een breed vertrouwen dat zij de vergrote beleidsruimte op de best mogelijke manier voor het onderwijs zouden benutten. Gaandeweg is echter gebleken dat een aantal scholen (van hoog tot laag) grote sommen gelds zijn gaan reserveren. En er zelfs mee zijn gaan speculeren. De publieke opinie is daarom al enige tijd vervuld van wantrouwen jegens schoolbesturen en -directies. Wie had dat durven voorspellen? Deze omslag in de publieke opinie werd nog eens gevoed door het feit dat met de komst van de budgettaire decentralisatie schoolleiders zijn begonnen om eerst zichzelf een salarisverhoging te bezorgen. Schooldirecties zijn zich ‘College van bestuur’ gaan noemen en wisten de aldus verworven gewichtigheid met een ‘marktconform beloningsniveau’ te onderstrepen. Bovendien hebben zij zich omringd met een hofhouding van managers en beleidsontwikkelaars waarvan de toegevoegde waarde voor het onderwijs omstreden is. En schoolbestuurders zijn niet de enige publieke dienstverleners die onder verdenking staan te goed voor zichzelf te zorgen. [...

Labour turnover and its effects on performance:an empirical test using firm data

In this article we test the hypothesis that the relationship between labour turnover and the economic performance of the firm is bell-shaped: a turnover level too low has a negative effect and likewise does a level too high. Our analysis is based on economic performance data of 110 offices of a temp agency. Since these offices vary highly in labour turnover but are similar in product and operational management, the data enabled us to control for a number of important intervening variables. From a regression analysis it could be shown that labour turnover indeed is related to office performance in a curvilinear way, indicating that it is especially excessive turnover that matters. This result proved robust for both performance level and change of performance as the dependent variables

The Origins of Blue Stragglers and Binarity in Globular Clusters

(abridged) We use newly available empirical binary fractions for globular clusters to carry out a direct test of the binary evolution hypothesis, and of collisional channels that involve binary stars. More specifically, using the previously reported correlation between blue straggler numbers and core mass as a benchmark, we test for correlations with the number of binary stars, as well as with the rates of single-single, single-binary, and binary-binary encounters. Surprisingly, we find that the simple correlation with core mass remains by far the strongest predictor of blue straggler population size, even in our joint models. This is despite the fact that the binary fractions themselves strongly anti-correlate with core mass, just as expected in the binary evolution model. At first sight, these results do not fit neatly with either binary evolution or collisional models in their simplest forms. Arguably the simplest and most intriguing possibility to explain this unexpected result is that observational errors on the core binary fractions are larger than the true intrinsic dispersion associated with their dependence on core mass. In the context of the binary evolution model, this would explain why the combination of binary fraction and core mass is a poorer predictor of blue straggler numbers than core mass alone. It would also imply that core mass is a remarkably clean predictor of core binary fractions. This would be of considerable importance for the dynamical evolution of globular clusters, and provides an important benchmark for models attempting to understand their present-day properties.Comment: 10 pages, 9 figures, accepted for publication in MNRA