170,579 research outputs found
Abundance Tomography of Type Ia Supernovae
No full textMany uncertainties about the physics of Type Ia Supernovae have been revealed in the recent past, and numerous pieces are puzzled together to achieve a complete description of the phenomenon of thermonuclear explosions in the sky. However, very important parts are still missing. In particular, the concept lacks a proper connection between the various evolutionary steps, namely the progenitor scenario, explosion theory, nucleosynthesis from the burning, and the observations. Early time spectra of Type Ia Supernovae naturally contain information about all of these processes and are at the centre of the entire scenario. Appropriate models of that phase can provide the missing link and improve our understanding of this field enormously.
The goal of this thesis is to advance new methods to calculate synthetic spectra in order to extract the information contained in the observations more efficiently. Based on a well established radiation transfer code, a new technique called "Abundance Tomography" is developed to derive the abundance distribution of Type Ia Supernovae ejecta. While previous approaches were limited to the determination of the abundances of specific species in restricted regions of the supernova envelope, here a complete stratified distribution of all major elements is obtained. This method is applied to the very well observed normal SN 2002bo. Combining the early spectra with those of the nebular phase leads to a coverage of the entire ejecta from the centre out to the highest velocities. The abundances derived are used to compute a synthetic bolometric light curve to test the radial distribution of Fe group and intermediate-mass elements.
The sampling procedure of the incident radiation field at the lower boundary is modified to obtain a better description of the real situation in Type Ia Supernovae. This improves the overall flux distribution significantly, especially in the red part of the spectrum, where almost no real line opacity is found. Synthetic spectra with this new procedure reproduce the observations much more accurately, as is shown by models of SN 2002er.
Hydrogen lines have never been detected convincingly in Type Ia Supernovae spectra. However, using spectra that were observed more than 10 days before maximum light, it is shown that small amounts of hydrogen in the outer parts of the ejecta can explain high velocity line absorptions, seen rather frequently in various objects, e.g. SN 2002dj, SN 2003du, and SN 1999ee. The hydrogen is not claimed to be primordial to the white dwarf but it is rather the effect of the supernova ejecta interacting with circumstellar material, namely the white dwarf's accretion disk build up prior to the explosion.
Finally, UV spectra of Type Ia Supernovae are discussed. The ability of the Monte Carlo technique to deal naturally with this wavelength region is proven. Applications are presented by modelling spectra of SN 2001ep and SN 2001eh obtained with the Hubble Space Telescope.
The results are discussed in the broader context of Type Ia Supernovae physics: What causes the diversity in the nearby sample? What are the progenitors and how does the explosion work? What is the influence on cosmological models?
A detailed knowledge of the abundances, their distribution in the Supernova ejecta, and their ultimate causes delivers the key to these fundamental issues.Die Entdeckung der Dunklen Energie basiert nicht zuletzt auf der Interpretation von Beobachtungen weit entfernter bzw. hoch rotverschobener Supernovae vom Typ Ia (z>0,1). Gleichwohl steht eine vollständige und konsistente Beschreibung des physikalischen Ablaufs thermonuklearer Explosionen, die dem Phänomen der Supernovae Ia zugrunde liegen, immer noch aus.
So konnte bislang der Zusammenhang zwischen den einzelnen Entwicklungsstufen - vom Vorläuferstern über die explosive
Nukleosynthese bis hin zur Phase der homologen Expansion - und den Beobachtungen noch nicht hinreichend geklärt werden. Die Spektren aus der frühen Phase, d.h. einige Tage nach der Explosion, enthalten wichtige Informationen über diesen Ablauf; deren Interpretation steht damit im Zentrum der derzeitigen Supernova Ia Forschung. Geeignete Modelle zur Erzeugung synthetischer Spektren sind das notwendige Werkzeug, um wesentliche Details der thermonuklearen Explosion zu überprüfen und so eine Möglichkeit zur Quantifizierung der Dunklen Energie bereitzustellen.
Ziel dieser Arbeit ist es, mit Hilfe neuer Methoden zur Berechnung synthetischer Spektren die in den Beobachtungen enthaltenen Informationen zu extrahieren und zu analysieren. Den entscheidenden methodischen Schritt stellt dabei die Entwicklung der Tomographie der Elementhäufigkeiten dar. Als wesentlicher Fortschritt kann die Verteilung der Elemente in der Hülle der Supernovae detailliert untersucht werden. Bisherige Verfahren beschränkten sich hierbei lediglich auf die Bestimmung globaler Häufigkeiten spezieller Elemente, während die neue Methode eine vollständige Analyse der radialabhängigen Verteilung aller wichtigen Elemente zulässt. Die Ergebnisse einer ersten Anwendung werden für die Supernova 2002bo präsentiert. Dabei wurde die Spektralanalyse der frühen Phase mit den Ergebnissen der Nebelphase kombiniert, um so eine vollständige Bestimmung der Häufigkeiten - vom innersten Punkt der Hülle bis hin zu den höchsten Radialgeschwindigkeitswerten - zu erhalten. Im
nächsten Schritt wurden die Ergebnisse der Häufigkeitsverteilung dazu verwendet, die bolometrische Lichtkurve zu berechnen. Durch den Vergleich mit der beobachteten Lichtkurve konnte so die geschichtete Verteilung der Eisengruppen- und mittelschweren Elemente präzise bestätigt werden.
Desweiteren wurde das Samplingverfahren zur Berechnung des
Strahlungsfeldes im innersten Bereich erheblich verbessert und so modifiziert, dass es den bei Supernovae Ia vorherrschenden physikalischen Bedingungen besser entspricht. Damit konnte die Strahlungsflussverteilung signifikant verbessert werden, was insbesondere im opazitätsarmen roten Bereich des Spektrums von entscheidender Bedeutung ist. Anhand von Modellen zur Supernova 2002er wird ferner gezeigt, dass die auf der neuen Methode basierenden synthetischen Spektren die Beobachtungen in wesentlichen Punkten, wie beispielsweise die dominanten Spektrallinien von S, Si, Ca und Fe, erheblich besser repräsentieren.
Ein weiterer für das Gesamtverständnis wichtiger Punkt bezieht sich auf die Frage nach der Wasserstoffhäufigkeit.
Bislang konnten in Typ Ia Supernova Spektren keine Wasserstofflinien nachgewiesen werden. Anhand von Spektren, die mehr als 10 Tage vor dem Helligkeitsmaximum aufgenommen wurden, konnten wir jedoch zeigen, dass bislang ungeklärte
Linienabsorptionen, hauptsächlich von Ca und Si bei sehr hohen Geschwindigkeiten durch kleine Mengen Wasserstoff aus der Akkretionsscheibe erklärt werden können. Dieses Ergebnis
birgt erhebliche Implikationen für das Verständnis der
Supernova Ia Vorläufersterne.
Für die Diskussion der Ergebnisse in einem größeren astrophysikalischen Rahmen sind desweiteren folgende Fragestellungen relevant: Was verursacht bei den nahen Objekten das unterschiedliche Verhalten ihrer Lichtkurven? Woraus entstehen sie und wie läuft die Explosion im Detail ab? Welche Konsequenzen hat die präzise Parameterstudie von Supernovae Ia für die Kosmologie?
Wie in dieser Arbeit gezeigt wird, liefert die exakte Kenntnis der Häufigkeitsverteilung der Elemente in den
Supernovae Ia Hüllen den Schlüssel zur Beantwortung dieser
elementaren Fragen
The cardiac troponin C mutation Leu29Gln found in a patient with hypertrophic cardiomyopathy does not alter contractile parameters in skinned murine myocardium
No full textThe present study investigates the effects of the first mutation of troponin C (hcTnC(L29Q)) found in a patient with hypertrophic cardiomyopathy (HCM) on force-pCa relations and the interplay with phosphorylation of sarcomeric PKA substrates. In triton-skinned murine cardiac fibers, the endogenous mcTnC was extracted and the fibers were subsequently reconstituted with recombinant wild-type and mutant hcTnC. Force-pCa relations of preparations containing hcTnC(L29Q) or hcTnC(WT) were similar. Incubation of fibers reconstituted with the recombinant proteins with phosphatase to dephosphorylate sarcomeric PKA substrates induced an increase in Ca(2+) sensitivity, slightly more pronounced (0.04 pCa units) in hcTnC(L29Q)-containing fibers. Incubation of the dephosphorylated fibers with PKA induced significant rightward shifts of force-pCa relations of similar magnitude with both, hcTnC(L29Q) and hcTnC(WT). No significant effects of hcTnC(L29Q) on the velocity of unloaded shortening were observed. In conclusion, no major differences in contractile parameters of preparations containing hcTnC(L29Q) compared to hcTnC(WT) were observed. Therefore, it appears unlikely that hcTnC(L29Q) induces the development of HCM by affecting the regulation of Ca(2+)-activated force and interference with PKA-mediated modulation of the Ca(2+) sensitivity of contraction
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Interdisziplinäre Entwicklung eines Instruments zur Evaluation von Lehrkräftefortbildungen – zu Zeiten der Corona-Pandemie
No full textStehle S, Baier-Mosch F, Burgwald C, Corvacho I, McLean P, Jehle M. Interdisziplinäre Entwicklung eines Instruments zur Evaluation von Lehrkräftefortbildungen – zu Zeiten der Corona-Pandemie. In: Maurer C, Rincke K, Holzäpfel L, Lipowsky F, eds. Wie viel Wissenschaft braucht die Lehrerfortbildung - Arbeitsbündnisse im analogen und virtuellen Raum. Online-QLB-Tagung an der Universität Regensburg 2021. Regensburg: Universität; 2021
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
A 0.12mm<sup>2</sup> Wien-Bridge Temperature Sensor with 0.1°C (3σ) Inaccuracy from -40°C to 180°C
No full text Resistor-based temperature sensors can achieve much higher resolution and energy efficiency than conventional BJT-based sensors [1], but they typically occupy more area (> 0.25 mm 2 ) and have lower operating temperatures (le 125 {circ} {C}) [2]-[4]. This work describes a 0.12mm 2 resistor-based sensor that uses a Wien-bridge (WB) filter to achieve 0.1 {circ} {C} (3 sigma) inaccuracy from - 40 {circ} {C} to 180 {circ} {C}. Compared to a state-of-the-art WB sensor [4], it occupies 6 × less area and achieves comparable relative accuracy over a 76% wider operating range. Session 10.3 Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic InstrumentationMicroelectronic
Accretion shocks in young stars: the role of local absorption on the X-ray emission
No full text. Weinvestigate the X-ray emission from accretion shocks in classical T Tauri
stars, due to the infalling material impacting the stellar surface. Several aspects in both
observations and models of the accretion process are still unclear: the observed X-ray
luminosity of the post-shock plasma is below the predicted value, the density vs temper
ature structure of the shocked plasma, with increasing densities at higher temperature,
is opposite of what expected from simple accretion shock models. To address these issues
we performed numerical magnetohydrodynamic simulations describing the impact of an
accretion stream onto the stellar surface and considered the local absorption due to the
surrounding medium. We explored the effects of absorption for different viewing angles
and for the He-like line triplets commonly used for density diagnostic. From the model
results we synthesize the X-ray emission from the accretion shock, producing maps and
spectra. We perform density and temperature diagnostics on the synthetic spectra, and
we directly compare our results with the observations. Our model shows that the X-ray
f
luxes detected are lower than expected because of the local absorption. The emerging
spectra suggest a complex density vs temperature distribution proving that a detailed
model accounting for a realistic treatment of the local absorption is needed to interpret
the observations of X-ray emitting accretion shock
Improving the solid electrolyte interphase of silicon-based anodes for lithium-ion batteries
No full textSilicon (Si) is the most promising anode material for next-generation lithium-ion batteries due to its high availability and outstanding specific capacity. However, Si-based anodes are suffering from high volume changes during cycling, causing unstable solid electrolyte interphase (SEI). The accumulated SEI growth leads to increased cell resistance, capacity fading and results in a low cycling stability. Therefore, the present work aims to modify the SEI of Si-based anode materials to increase their capacity retention.
A reliable reference electrode (RE) was required for the electrochemical testing. To this end, an in-house designed perforated RE (PRE) was adapted to a small cell format and validated with graphite anodes as reference. The work demonstrated that the PRE can be reliably applied in this format for potential monitoring and electrochemical impedance spectroscopy, while showing a low blockage of Li ions.
Furthermore, in this work lithium difluorophosphate (LiDFP) was applied as an electrolyte additive in different concentrations to investigate its effect on the SEI of Si anodes. It was found that the addition of LiDFP enhanced coulombic efficiencies, cycling stability, and resistance of the Si anodes in full cells. A concentration of 1% provided the best benefits when applied at medium to high C-rates. It enhanced the capacity retention by 13% compared to the reference electrolyte that did not contain LiDFP. Post-mortem analyses via scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) revealed that the microstructure of the Si anodes in cells containing 1% LiDFP remained more intact after cycling, and the decomposition reactions of certain electrolyte components were reduced.
New strategies towards the stabilization of SEI, and thus enhancing cycling stability were designed and successfully employed. To achieve this, polyurea coatings applied on the Si particles via molecular layer deposition were investigated. These coatings significantly reduced the coulombic efficiencies and cycling stability of the anodes, which was attributed to their electrically insulating character, thus increasing the cell resistance. As an alternative, Si/C composite particles with intrinsically higher conductivity were coated with metal oxides via atomic layer deposition. Depending on coating material and thickness, significant improvements in capacity retention of up to 12% could be achieved. XPS showed that the coatings partially suppressed the decomposition reactions of the electrolyte.
Coating the Si anodes with the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prior to cell assembly via an electropolymerization process was another approach used for the improvement of cycling performance of Si-based anodes. The problem of conductivity regarding polymer coatings on Si anodes should be tackled by using a conductive polymer. Different PEDOT coating thicknesses could be achieved by simply variating the process parameters. Electrochemical testing of the Si anodes in full pouch cell confirmed that the thinner PEDOT coatings exhibited lower cell resistance and higher cycling stability by up to 7%. Post-mortem analysis via cross-sectional SEM and XPS analysis, revealed that the microstructure of the PEODT-coated anodes was better-maintained, and the chemical SEI composition was positively influenced
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