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Apeldoorn, Netherlands

Kamp I.,European Space Agency | Kamp I.,Kapteyn Institute
EAS Publications Series | Year: 2012

In NLTE computations of trace elements in stellar atmospheres, background opacities are generally treated in LTE. It is thus important to assess the impact of different methods of including this background opacity on the statistical equilibrium of the trace element and its resulting NLTE abundance. This article illustrates these effects using two examples, nitrogen in Vega and carbon in the Sun. © EAS, EDP Sciences 2010. Source

Kamp I.,European Space Agency | Kamp I.,Kapteyn Institute
EAS Publications Series | Year: 2012

A-type stars with their shallow convection zones serve as ideal physics laboratories for stellar atmosphere research. In the absence of large scale mixing, processes such as diffusion, mass loss and accretion leave their characteristic imprint on the chemical composition of the photosphere. This characteristic surface pattern can be studied by means of stellar abundance analysis. However, such patterns can be hidden in the large uncertainties of LTE abundances. Thus, detailed NLTE studies that can push stellar abundance analysis beyond the 0.1 dex uncertainty limit are a pre-requisite for using A star atmospheres as physics laboratories. © EAS, EDP Sciences 2010. Source

Fillion J.H.,University Pierre and Marie Curie | Fillion J.H.,French National Center for Scientific Research | Dulieu F.,Cergy-Pontoise University | Romanzin C.,University Pierre and Marie Curie | And 2 more authors.
EPJ Web of Conferences | Year: 2012

Detailed laboratory studies and progress in surface science technique, have allowed in recent years the first experimental confirmation of surface reaction schemes, as introduced by Tielens, Hagen and Charnley [1,2]. In this paper, we review few heterogeneous processes which give routes to form elementary molecules considered as precursors for explaining the variety and richness of molecular species in the interstellar medium. Adsorption, diffusion and reaction processes are discussed. With emphasis on the experimental approaches, but also supported by theoretical developments, progresses in the understanding of the "catalytic role" of a dust grain surface in various physical conditions are described. Recent advances made on few important species (H2, H2O, CH3OH) are used to illustrate basic properties and raise open questions. © 2011 Owned by the authors, published by EDP Sciences. Source

Swaters R.A.,University of Maryland University College | Swaters R.A.,Johns Hopkins University | Swaters R.A.,US Space Telescope Science Institute | Sanders R.H.,Kapteyn Institute | McGaugh S.S.,University of Maryland University College
Astrophysical Journal | Year: 2010

Dwarf and low surface brightness (LSB) galaxies are ideal objects to test modified Newtonian dynamics (MOND), because in most of these galaxies the accelerations fall below the threshold where MOND supposedly applies. We have selected from the literature a sample of 27 dwarf and LSB galaxies. MOND is successful in explaining the general shape of the observed rotation curves for roughly three quarters of the galaxies in the sample presented here. However, for the remaining quarter, MOND does not adequately explain the observed rotation curves. Considering the uncertainties in distances and inclinations for the galaxies in our sample, a small fraction of poor MOND predictions is expected and is not necessarily a problem for MOND. We have also made fits taking the MOND acceleration constant, a0, as a free parameter in order to identify any systematic trends. We find that there appears to be a correlation between central surface brightness and the best-fit value of a0, in the sense that lower surface brightness galaxies tend to have lower a0. However, this correlation depends strongly on a small number of galaxies whose rotation curves might be uncertain due to either bars or warps. Without these galaxies, there is less evidence of a trend, but the average value we find for a0 ≈ 0.7 × 10-8 cm s-2 is somewhat lower than derived from previous studies. Such lower fitted values of a0 could occur if external gravitational fields are important. © 2010. The American Astronomical Society. All rights reserved. Source

Brocksopp C.,University College London | Kaiser C.R.,University of Southampton | Schoenmakers A.P.,Stichting ASTRON | De Bruyn A.G.,Stichting ASTRON | De Bruyn A.G.,Kapteyn Institute
Monthly Notices of the Royal Astronomical Society | Year: 2011

Double-double radio galaxies (DDRGs) offer a unique opportunity for us to study multiple episodes of jet activity in large-scale radio sources. We use radio data from the Very Large Array and the literature to model two DDRGs, B1450+333 and B1834+620, in terms of their dynamical evolution. We find that the standard Fanaroff-Riley II model is able to explain the properties of the two outer lobes of each source, whereby the lobes are formed by ram-pressure balance of a shock at the end of the jet with the surrounding medium. The inner pairs of lobes, however, are not well described by the standard model. Instead we interpret the inner lobes as arising from the emission of relativistic electrons within the outer lobes, which are compressed and re-accelerated by the bow shock in front of the restarted jets and within the outer lobes. The predicted rapid progression of the inner lobes through the outer lobes requires the eventual development of a hotspot at the edge of the outer lobe, causing the DDRG ultimately to resemble a standard Fanaroff-Riley II radio galaxy. This may suggest that DDRGs are a brief, yet normal, phase of the evolution of large-scale radio galaxies. © 2010 The Authors. Journal compilation © 2010 RAS. Source

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