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Magnetic fields, non-thermal radiation and particle acceleration in colliding winds of WR-O stars
(2015)
Non-thermal emission has been detected in WR-stars for many years at long wavelengths spectral range, in general attributed to synchrotron emission. Two key ingredients are needed to explain such emissions, namely magnetic fields and relativistic particles. Particles can be accelerated to relativistic speeds by Fermi processes at strong shocks. Therefore, strong synchrotron emission is usually attributed to WR binarity. The magnetic field may also be amplified at shocks, however the actual picture of the magnetic field geometry, intensity, and its role on the acceleration of particles at WR binary systems is still unclear. In this work we discuss the recent developments in MHD modelling of wind-wind collision regions by means of numerical simulations, and the coupled particle acceleration processes related.
Using a code that employs a self-consistent method for computing the effects of photoionization on circumstellar gas dynamics, we model the formation of wind-driven nebulae around massive Wolf-Rayet (W-R) stars. Our algorithm incorporates a simplified model of the photo-ionization source, computes the fractional ionization of hydrogen due to the photoionizing flux and recombination, and determines self-consistently the energy balance due to ionization, photo-heating and radiative cooling. We take into account changes in stellar properties and mass-loss over the star's evolution. Our multi-dimensional simulations clearly reveal the presence of strong ionization front instabilities. Using various X-ray emission models, and abundances consistent with those derived for W-R nebulae, we compute the X-ray flux and spectra from our wind bubble models. We show the evolution of the X-ray spectral features with time over the evolution of the star, taking the absorption of the X-rays by the ionized bubble into account. Our simulated X-ray spectra compare reasonably well with observed spectra of Wolf-Rayet bubbles. They suggest that X-ray nebulae around massive stars may not be easily detectable, consistent with observations.∗
Die Beziehungen zwischen den Vereinten Nationen und nichtstaatlichen Akteuren
haben seit den 1990er Jahren einen radikalen Wandel erlebt. Nach der
Rio-Konferenz über Umwelt und Entwicklung 1992 stand in den Vereinten Nationen
zunächst die Frage im Vordergrund, wie der gewachsenen Bedeutung
der Nichtregierungsorganisationen (NGOs) in der Arbeit und den Strukturen der
Weltorganisation Rechnung getragen werden könnte. Seit Ende der 1990er Jahre
dominierten innerhalb der Vereinten Nationen und einiger ihrer Spezialorgane
und Sonderorganisationen zunehmend die Bemühungen, Privatunternehmen
und ihre Interessenvertreter aktiver in die Arbeit der Vereinten Nationen zu integrieren.
Dies geschah zum einen in Form unterschiedlichster bilateraler Kontakte
und Kooperationsvorhaben zwischen Unternehmen und UN-Akteuren, zum anderen
im Rahmen von Dialogveranstaltungen und gemeinsamen Initiativen von
Regierungen, zwischenstaatlichen Gremien, Wirtschaftsvertretern und NGOs, für
die im Folgeprozess der Rio-Konferenz der Begriff der Multistakeholder-Partnerschaften
geprägt wurde.
Der folgende Beitrag nimmt diese Entwicklung kritisch unter die Lupe. Er
zeichnet im Zeitraffer nach, wie sich die Beziehungen zwischen den Vereinten
Nationen und nichtstaatlichen Akteuren gewandelt haben, beschreibt das Ausmaß
und die Bandbreite der neuen Partnerschaftsansätze, erörtert Risiken und
Nebenwirkungen dieses Paradigmenwechsels in der internationalen Politik und
skizziert zum Schluss, welche Konsequenzen sich daraus für die Vereinten Nationen
abzeichnen.
I. Gründung, Aufgaben und Struktur des Hochkommissariats
II. Zur Rolle des OHCHR im und gegenüber dem Menschenrechtsrat
III. Zu Erfolgschancen von Feldmissionen in eher schwierigen Ländern: China, Russische Föderation, Afghanistan, Sudan und Kolumbien
IV. Zur Frage der politischen Unabhängigkeit des OHCHR
V. Schlussfolgerungen und Ausblick
The evolution of massive stars in very low metallicity galaxies is less well observationally
constrained than in environments more similar to the Milky Way, M33, or the LMC. We discuss
in this contribution the current state of our program to search for and characterize Wolf-Rayet stars (and other massive emission line stars) in low metallicity galaxies in the Local Volume.
PopIII-star siblings in IZw18 and metal-poor WR galaxies unveiled from integral field spectroscopy
(2015)
Here, we highlight our recent results from the IFS study of Mrk178, the closest metal-poor WR galaxy, and of IZw18, the most metal-poor star-forming galaxy known in the local Universe. The IFS data of Mrk178 show the importance of aperture effects on the search for WR features, and the extent to which physical variations in the ISM properties can be detected. Our IFS data of IZw18 reveal its entire nebular HeIIλ4686-emitting region, and indicate for the very first time that peculiar, hot (nearly) metal-free ionizing stars (called here PopIII-star siblings) might hold the key to the HeII-ionization in IZw18.
The main objective of this work is to investigate the evolution of massive stars, and the interplay between them and the ionized gas for a sample of local metal-poor Wolf-Rayet galaxies.
Optical integral field spectrocopy was used in combination with multi-wavelength radio data.
Combining optical and radio data, we locate Wolf-Rayet stars and supernova remnants across the Wolf-Rayet galaxies to study the spatial correlation between them. This study will shed light on the massive star formation and its feedback, and will help us to better understand
distant star-forming galaxies.
We highlight the basic physics that allows fundamental parameters, such as the effective
temperature, luminosity, abundances, and mass-loss rate, of Wolf-Rayet (W-R) stars to be
determined. Since the temperature deduced from the spectrum of a W-R star is an ionization
temperature, a detailed discussion of the ionization structure of W-R winds, and how it is set, is given. We also provide an overview of line and continuum formation in W-R stars. Mechanisms that contribute to the strength of different emission lines, such as collisional excitation, radiative recombination, dielectronic recombination, and continuum uorescence, are discussed.