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Für Margot Faak (1926–2015)
Am 14. Juni 2015 starb unsere Kollegin Margot Faak im 89. Lebensjahr. Wir erinnern an sie mit Worten des Abschieds von Eberhard Knobloch und dem Wiederabdruck eines Aufsatzes aus ihrer Feder von 1999.
Die Neuveröffentlichung erfolgt mit freundlicher Genehmigung der Alexander von Humboldt-Stiftung.
Der Text wurde in den Fußnoten durch einige aktuelle bibliographische und sachliche Angaben ergänzt.
HiN VII, 13 (2006) war Margot Faak zu ihrem 80. Geburtstag gewidmet. Das Heft enthält einen Lebenslauf und ein Verzeichnis ihrer Schriften.
Berlin, Oktober 2015
Regina Mikosch und Ingo Schwarz
Bewegte Systematik
(2015)
Die Amerikanischen Reisetagebücher stellen eine Herausforderung für die Erforschung historischer Epistemologie dar. Der vorliegende Aufsatz will ausgehend vom Scheitern einer eindeutigen gattungsgeschichtlichen Zuschreibung die Sonderstellung der Bände als Instrumente der Erfahrung herausarbeiten, welche an der Schwelle zum 19. Jh. den ontologischen Optimismus sowie das methodische Tasten eines Wissenschaftlers, Schriftstellers und Forschungsreisenden in subjektbezogener Form zugänglich machen. Gerade der multifunktionale Charakter der ‚Journale‘ als Log- bzw. Reisetagebücher und Skizzenhefte, die noch weit entfernt von der Ordnung wissenschaftlichen Schreibens scheinen, lassen jene Voraussetzungen wissenschaftlicher Erkenntnisprozesse sichtbar werden, mithilfe derer Humboldt innerhalb der kantschen Koordinaten Raum und Zeit einen ‚Rhythmus‘ situativ verarbeiteter Natur- und Welterfahrung in ein dynamisches Speichermedium einfließen lässt.
Tras la huella de la vida
(2015)
El proyecto de larga duración “Centro Alejandro de Humboldt – Ciencia en movimiento” (iniciado en 2015) de la Academia de las Ciencias y Humanidades Berlín-Brandemburgo tiene como objetivo la edición de los manuscritos de viaje del famoso autor, naturalista, cosmopólito y viajero. La edición incluirá los manuscritos de sus dos viajes hemisféricos (América, Asia) como también los de su legado literario. La pregunta esencial que acompaña las investigaciones en torno al proyecto de la Academia es la importancia que Humboldt le da a la vida y a la convivencia. Los tres pilares en los que descansa la reflexión son: la relacionalidad, la concomitancia entre ciencia y estética y el movimiento.
Schulen der Zukunft
(2015)
Inklusion für alle
(2015)
While the majority of very massive stars is clearly found in clusters, there are also very massive objects not associated with any cluster, suggesting they may have been born in isolation. In order to gain more insights, we studied the regions around two WR stars in the Galactic Center region. To understand the nature of the potential cluster around massive stars, photometry alone is not sufficient. We therefore used the ESO VLT/SINFONI integral field spectrograph to obtain photometry and spectra for the whole region around our two candidate stars. In total, more than 60 stars have been found and assigned a spectral type.
Under the assumption of spherical symmetry, the run of intensity with impact parameter for a spatially resolved and optically thin bubble can be inverted for an "effective emissivity" as a function of radius. The effective emissivity takes into account instrumental sensitivity and even interstellar absorption. This work was supported by a grant from NASA (G03-14008X).
As WR 6 is a putatively single WN4 star, and is relatively bright (V = 6.9), it is an ideal case for studying the wind mechanisms in these extremely luminous stars. To obtain higher resolution spectra at higher energy (above 1 keV) than previously obtained with the XMM/Newton RGS, we have observed WR 6 with the Chandra High Energy Transmission Grating Spectrometer for 450 ks. We have resolved emission lines of S, Si, Mg, Ne, and Fe, which all show a “fin"-shaped prole, characteristic of a self-absorbed uniformly expanding shell. Steep blue edges gives robust maximal expansion velocities of about 2000 km/s, somewhat larger than the 1700km/s derived from UV lines. The He-like lines all indicate that X-ray emitting plasmas are far from the photosphere – even at the higher energies where opacity is lowest { as was also the case for the longer wavelength lines observed with XMM-Newton/RGS. Abundances determined from X-ray spectral modeling indicate enhancements consistent with nucleosynthesis. The star was also variable in X-rays and in simultaneous optical photometry obtained with Chandra aspect camera, but not coherently with the optically known period of 3.765 days.
In this review I briefly summarize our knowledge of the X-ray emission from single WN, WC, and WO stars. These stars have relatively modest X-ray luminosities, typically not exceeding 1L⊙. The analysis of X-ray spectra usually reveals thermal plasma with temperatures reaching a few x10 MK. X-ray variability is detected in some WN stars. At present we don't fully understand how X-ray radiation in produced in WR stars, albeit there are some promising research avenues, such as the presence of CIRs in the winds of some stars. To fully understand WR stars we need to unravel mechanisms of X-ray production in their winds.
A significant number of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient, showing a chemical composition of helium, carbon, and oxygen. Most of them exhibit Wolf-Rayet-like emission line spectra, similar to those of the massive WC Pop I stars, and are therefore classified as of spectral type [WC]. In the last years, CSPNe of other Wolf-Rayet spectral subtypes have been identified, namely PB 8, which is of spectral type [WN/C], and IC 4663 and Abell 48, which are of spectral type [WN]. We review spectral analyses of Wolf-Rayet type central stars of different evolutionary stages and discuss the results in the context of stellar evolution. Especially we consider the question of a common evolutionary channel for [WC] stars. The constraints on the formation of [WN] or [WC/N] subtype stars will also be addressed.
The distribution of angular momentum in massive stars is a critical component of their evolution, yet not much is known on the rotation velocities of Wolf-Rayet stars. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars by implementing a 3D integration scheme in the PoWR code. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may imply rapid rotation, infer the corresponding rotation parameters, and discuss the implications of our results. We find that rotation helps to reproduce the unique spectra analyzed here. However, if rotation is indeed involved, the inferred rotational velocities at the stellar surface are large (∼ 200 km/s), and the implied co-rotation radii (∼ 10R∗) suggest the existence of very strong photospheric magnetic fields (∼ 20 kG).
In the last decades, stellar atmosphere codes have become a key tool in understanding massive stars, including precise calculations of stellar and wind parameters, such as temperature, massloss rate, and terminal wind velocity. Nevertheless, for these models the hydrodynamic equation is not solved in the wind. Motivated by the results of the CAK theory, the models typically use a beta velocity law, which however turns out not to be adequate for stars with very strong winds, and treat the mass-loss rate as a free parameter. In a new branch of the Potsdam Wolf-Rayet model atmosphere (PoWR) code, we solve the hydrodynamic equation consistently throughout the stellar atmosphere. The PoWR code performs the calculation of the radiative force without approximations (e.g. Sobolev). We show the impact of hydrodynamically consistent modelling on OB and WR stars in comparison to conventional models and discuss the obtained velocity fields and their impact on the observed spectral lines.
Macroclumping in WR 136
(2015)
Macroclumping proved to resolve the discordance between different mass-loss rate diagnostics for O-type stars, in particular between Hα and the P v resonance lines. In this paper, we report first results from a corresponding investigation for WR stars. We apply our detailed 3-D Monte Carlo (MC) line formation code to the P v resonance doublet and show, for the Galactic WNL star WR136, that macroclumping is require to bring this line in accordance with the mass-loss rate derived from the emission-line spectrum.
A detailed and comprehensive study of the Wolf-Rayet stars of the nitrogen sequence (WN
stars) in the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC) is presented.
We derived the fundamental stellar and wind parameters for more than 100 massive stars, encompassing almost the whole WN population in the Magellanic Clouds (MCs). The observations are fitted with synthetic spectra, using the PotsdamWolf-Rayet model atmosphere
code (PoWR). For this purpose, large grids of line-blanket models for different metallicities have been calculated, covering a wide range of stellar temperatures, mass-loss rates, and hydrogen abundances. Our comprehensive sample facilitates statistical studies of the WN properties in the MCs without selection bias. To investigate the impact of the low LMC metallicity and the even lower SMC metallicity, we compare our new results to previous analyses of the Galactic WN population and the late type WN stars from M31. Based on these studies we derived an empirical relation between the WN mass-loss rates and the metallicity. Current stellar evolution tracks, even when accounting for rotationally induced mixing, partly fail to reproduce the observed ranges of luminosities and initial masses.
The emission-line dominated spectra of Wolf-Rayet stars are formed in expanding layers of their atmosphere, i.e. in their strong stellar wind. Adequate modeling of such spectra has to face a couple of difficulties. Because of the supersonic motion, the radiative transfer is preferably formulated in the co-moving frame. The strong deviations from local thermodynamical equilibrium (LTE) require to solve the equations of statistical equilibrium for the population numbers, accounting for many hundred atomic energy levels and thousands of line transitions. Moreover, millions of lines from iron-group elements must be taken into account for their blanketing effect. Model atmospheres of the described kind can reproduce the observed WR spectra satisfyingly, and have been widely applied for corresponding spectral analyses.
En las pinturas que retratan al erúdito Alexander von Humboldt encontramos densos signos visuales las cuales simbolizan su vida científica. La imagen del Chimborazo, cuya cima Humboldt no había alcanzado, recuerda la función epistémica de lo inacabado en su obra, mientras que la carta, el globo y su postura típica remiten al central significado del movimiento y al “escribir de cara a las cosas”. La presencia de sus objetos de escritura en las imagenes llama la atención, ya que se puede entender como referencia al significado de los diarios del viaje americano escritos durante toda la vida. En aquellos se ponen de manifiesto los momentos centrales del Humboldtian Science. El carácter del work in progress y la relacionalidad consecuente de la representación muestran las constantes epistemológicas de esta ciencia así como la dimensión de lo corporal en los manuscritos.