Quantitative models of AGB populations with subsolar metallicities and their mass loss

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Synthetic photometry for carbon-rich giants. IV. An extensive grid of dynamic atmosphere and wind models

The evolution and spectral properties of stars on the AGB are significantly affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind models are an essential tool for studying these evolved stars, both individually and as members of stellar populations, to understand their contribution to the integrated light and chemical evolution of galaxies. This paper is part of a series testing state-of-the-art atmosphere and wind models of carbon stars against observations, and making them available for use in various theoretical and observational studies. We have computed low-resolution spectra and photometry (in the wavelength range 0.35-25 mu) for a grid of 540 dynamic models with stellar parameters typical of solar-metallicity C-rich AGB stars and with a range of pulsation amplitudes. The models cover the dynamic atmosphere and dusty outflow (if present), assuming spherical symmetry, and taking opacities of gas-phase species and dust grains consistently into account. To characterize the time-dependent dynamic and photometric behaviour of the models in a concise way we defined a number of classes for models with and without winds. Comparisons with observed data in general show a quite good agreement for example regarding mass-loss rates vs. (J-K) colours or K magnitudes vs. (J-K) colours. Some exceptions from the good overall agreement, however, are found and attributed to the range of input parameters (e.g. relatively high carbon excesses) or intrinsic model assumptions (e.g. small particle limit for grain opacities). While current results indicate that some changes in model assumptions and parameter ranges should be made in the future to bring certain synthetic observables into better agreement with observations, it seems unlikely that these pending improvements will significantly affect the mass-loss rates of the models.Comment: 28 pages, 15 figures. Table B.1, an 11-page table, is only available at CD

Profiling Microglia From Alzheimer's Disease Donors and Non-demented Elderly in Acute Human Postmortem Cortical Tissue

Microglia are the tissue-resident macrophages of the central nervous system (CNS). Recent studies based on bulk and single-cell RNA sequencing in mice indicate high relevance of microglia with respect to risk genes and neuro-inflammation in Alzheimer's disease (AD). Here, we investigated microglia transcriptomes at bulk and single-cell levels in non-demented elderly and AD donors using acute human postmortem cortical brain samples. We identified seven human microglial subpopulations with heterogeneity in gene expression. Notably, gene expression profiles and subcluster composition of microglia did not differ between AD donors and non-demented elderly in bulk RNA sequencing nor in single-cell sequencing

Quantitative Modelle von AGB Populationen mit subsolaren Metallizitäten und ihr Massenverlust

In dieser Arbeit wird der Massenverlust von Sternen untersucht, mit besonderem Schwerpunkt auf denen, die sich im Hertzsprung-Russell-Diagramm auf dem Asymptotischen Riesenast (AGB, engl. Abk.) befinden und durch hohe Leuchtkräfte und niedrige Effektivtemperaturen gekennzeichnet sind. Der AGB ist die späte Entwicklungsphase von Sternen mit niedriger und mittlerer Anfangsmasse. Charakteristisch für sie sind Instabilitäten, die zu Pulsationen der Sterne führen, wie sie z. B. bei Langperiodischen Veränderlichen oder Mira Sternen beobachtet werden. Außerdem ist diese Phase mit starkem Massenverlust von bis zu ~10^{-4} M_{sun} yr^{-1} verbunden. Durch ihren Massenverlust tragen diese Sterne erheblich zur Anreicherung der interstellaren Materie mit durch Kernfusion prozessierter Materie und Staub bei. AGB Sterne sind daher nicht nur als Objekte der Sternentwicklung interessant, sondern auch im Hinblick auf die galaktische chemische Entwicklung. Die Existenz von Staub in der Atmosphäre eines AGB Sterns hat einen immensen Einfluß auf deren Physik und Chemie. Wurde erst einmal Staub gebildet, spielt er eine wichtige Rolle als Antriebsmechanismus des Massenverlustes, da sein Absorptionsquerschnitt um vieles größer ist als der von (molekularem) Gas. Durch die Pulsation wird die Atmosphäre angehoben, was zu günstigen Bedingungen für die Staubbildung führt. Der Strahlungsdruck auf die gebildeten Staubkörner und Reibungskopplung zwischen Staub und umgebendem Gas beschleunigt die Materie nach außen und führt zu beträchtlichem Massenverlust des Sterns. In dieser Arbeit kombinieren wir Sternentwicklungssimulationen mit einer Massenverlust-Beschreibung, die auf detaillierten Windmodellen basiert, und untersuchen von theoretischer Seite den sich ergebenden Massenverlust. Während in früheren Untersuchungen Modelle mit solarer Metallizität ausgewertet wurden, konzentrieren wir uns hier auf staubgetriebene Windmodelle mit Elementhäufigkeiten, die in den Magellanschen Wolken beobachtet werden. Die Metallizität der Großen Magellanschen Wolke (LMC) ist Z=0.008, d. h. ungefähr ein gutes Drittel des solaren Wertes, während die Kleine Magellansche Wolke (SMC) eine im Schnitt sogar noch niedrigere Metallizität von Z=0.004 hat, was der Hälfte des LMC bzw. ein Fünftel des solaren Wertes entspricht. Basierend auf Sätzen dieser Windmodelle leiten wir parametrisierte Massenverlustbeschreibungen ab, die nur von den Sterngrößen selbst abhängen und dann in Sternentwicklungsrechnungen angewandt werden. Diese Rechnungen führen zu einer quantitativen Darstellung des Massenverlusts von Sternen mit subsolaren Metallizitäten. Unter Verwendung der neuen Raster von Entwicklungswegen generieren wir synthetische Sternsätze, die ausgehend von einer gegebenen Anfangsmassenverteilung und Sternbildungsrate den heutigen Zustand widerspiegeln. Diese werden erfolgreich mit einem beobachteten Satz von Riesensternen aus der Datenbank des "DENIS Catalogue toward Magellanic Clouds" verglichen.In this work the mass loss of stars is investigated with special focus on those populating the asymptotic giant branch (AGB) in the Hertzsprung-Russell diagram characterised by high luminosities and low effective temperatures. On the AGB stars with low- and intermediate initial mass are in their late stage of evolution. This phase is characterised by instabilities leading to stellar pulsations as observed in long-period variables or Mira stars. Furthermore, this phase is connected to strong mass-loss of up to ~10^{-4} M_{sun} yr^{-1}. By means of their mass loss, these stars contribute enormously to the enrichment of the interstellar matter with material processed through nuclear burning reactions and dust. Therefore, AGB stars are not only interesting as objects of stellar evolution but also in terms of galactic chemical evolution. In the atmosphere of an AGB star, the existence of dust has an immense influence on its physics and chemistry. Once dust is formed, it plays an important role as driving-mechanism of the mass loss, because its absorption cross section is by far larger than that of the (molecular) gas. Due to the pulsation the stellar atmosphere is levitated leading to favourable conditions for dust formation. The radiation pressure on the formed dust grains and the frictional coupling between the dust and surrounding gas accelerates the material outwards and results in a substantial stellar mass loss. In this work we theoretically investigate mass loss resulting from stellar evolution simulation in combination with a mass-loss description based on detailed dust-driven wind models. While in earlier investigations models with solar metallicity have been evaluated, we focus here on dust-driven wind models with element abundances as observed in the Magellanic Clouds. The metallicity of the Large Magellanic Cloud (LMC) is Z=0.008, i.e. roughly a good third of the solar value, while the Small Magellanic Cloud (SMC) has on average an even lower metallicity of Z=0.004, being half of the LMC or a fifth of the solar metallicity. Based on grids of these wind models we derive parameterised mass-loss descriptions dependent on stellar quantities only, which then are applied to stellar evolution calculations. Those lead to a quantitative account of stellar mass loss at subsolar metallicities. On the basis of the new grids of evolutionary tracks, we are generating synthetic stellar samples which represent the present day state for a given initial mass function and star formation rate in dependence of the considered metallicity. These are successfully compared to an observed sample of giants from the DENIS Catalogue toward Magellanic Clouds database

Preparative, structural, and electrochemical investigations on peralkylated niobocene dichlorides and difluorides

CP(dagger)2NbCl2 (CP(dagger) = 175-C5Me4Et) (1b) was synthesized from NbCl5 and a slight excess of Cp(dagger)Li and NaBH4. Its electrochemical (2 e-, E1/2 = -1.6 V) and chemical reduction (2 equiv of Na/Hg) was studied showing that peralkylation of the Cp ligand facilitates two-electron reduction compared to less substituted CP2NbHal2 derivatives. The reduced solutions of CP*2NbCl2 (CP* = C5Me5) (1a) and of lb were shown by means of EPR spectroscopy to contain two new paramagnetic species 1A,B for which bent niobocene (A(Nb) = 100 G) and fulvene-like structures (A(Nb) = 55 G) are proposed. The high reduction potential of these species led in their reaction with HPF6 to the first stable difluoroniobocene derivative, [CP(dagger)2NbF2]PF6 (2), in good yield. The molecular structures of 2 and a cocrystallizate of 2 and [CP(dagger)2NbCl2]PF6 (3), analyzing as [CP(dagger)2NbCl(Cl,F)]PF6 (4), were studied by X-ray diffraction techniques. Comparison with the structure of 1b shows a marked decrease of the angle Hal-Nb-Hal from 102.3(4)-degrees (2) to 85.2(1)-degrees (1b). Electrochemical 1 e- reduction of 2 gave rise to the formation of CP(dagger)2NbF2, which exhibits a much lower potential (E1/2 = -2.38 V) than other niobocene dihalides. Reaction of 2 with Li2S2 gave CP(dagger)2Nb(eta-S2)F, whereas 3 was reduced by Li2S2 to give 1b

Professors and lecturers at Friedrich-Alexander-Universität, 1743-1960 : Pt. 3: Humanities, Science

Das Verzeichnis enthält die Kurzbiographien aller ordentlichen und außerordentlichen Professoren und Privatdozenten, die zwischen 1743 und 1960 mindestens ein Semester lang in der Philosophischen oder Naturwissenschaftlichen Fakultät der Universität Erlangen gelehrt haben. Die Einträge sind alphabetisch nach dem Familiennamen geordnet, jedoch nach den Fakultäten getrennt.The biographical dictionary contains short biographies of all professors and lecturers who taught humanities or science at Erlangen University from 1743 through 1960 for at least one semester. According to faculties, the names are arranged alphabetically