22 research outputs found
The 228 valence electron rule for onion-like inorganic fullerenes X1@Y12@Z20 of Ih and Th symmetry
No full textBased on an analysis of the experimentally known 228e- valence electron systems AsNi12As203-, SbNi12Sb203-, SbPd12Sb203-, and SnCu12Sn2012- of Ih symmetry, the new species GeZn12Ge20, KrNi12As20, BrNi12As201-, SeNi12As202-, GeMn12Br20, SeCo12Se206+, and SeFe12Se206- have been designed according to the 228 electron rule introduced in this study for onion-like inorganic fullerenes composed of late main group and transition metal elements. Of these GeZn12Ge20, KrNi12As20, BrNi12As201-, SeNi12As202-, and GeMn12Br20 together with the known AsNi12As203- and its neutral counterpart were investigated at the DFT B3LYP/6-31G(d) level of theory. With one exception they show, in fact, two stable energetic minima of Ih and Th symmetry. Relaxing the symmetry constraints further, e.g. Y12 = {Q6,R6}, additional new neutral species of the general composition Z@Q6R6@Z20 of C5v and C2v symmetry can be formulated, i.e. Z = Se, Q = Fe, R = Mn, following this new building principle, therefore, supporting and inspiring the development of new materials of icosahedral and tetrahedral symmetry.
Dedicated to Professor Helmut Schwarz on the occasion of his 80th birthda
Class Attendance and Students’ Evaluations of Teaching: Do No-Shows Bias Course Ratings and Rankings?
No full textBackground: Many university departments use students’ evaluations of teaching (SET) to compare and rank courses. However, absenteeism from class is often nonrandom and, therefore, SET for different courses might not be comparable. Objective: The present study aims to answer two questions. Are SET positively biased due to absenteeism? Do procedures, which adjust for absenteeism, change course rankings? Research Design: The author discusses the problem from a missing data perspective and present empirical results from regression models to determine which factors are simultaneously associated with students’ class attendance and course ratings. In order to determine the extent of these biases, the author then corrects average ratings for students’ absenteeism and inspect changes in course rankings resulting from this adjustment. Subjects: The author analyzes SET data on the individual level. One or more course ratings are available for each student. Measures: Individual course ratings and absenteeism served as the key outcomes. Results: Absenteeism decreases with rising teaching quality. Furthermore, both factors are systematically related to student and course attributes. Weighting students’ ratings by actual absenteeism leads to mostly small changes in ranks, which follow a power law. Only a few, average courses are disproportionally influenced by the adjustment. Weighting by predicted absenteeism leads to very small changes in ranks. Again, average courses are more strongly affected than courses of very high or low in quality. Conclusions: No-shows bias course ratings and rankings. SET are more appropriate to identify high- and low-quality courses than to determine the exact ranks of average courses
A detailed pathway analysis of the chemical reaction system generating the Martian vertical ozone profile
No full textAtmospheric chemical composition is crucial in determining a planet's atmospheric structure, stability, and evolution. Attaining a quantitative understanding of the essential chemical mechanisms governing atmospheric composition is nontrivial due to complex interactions between chemical species. Trace species, for example, can participate in catalytic cycles – affecting the abundance of major and other trace gas species. Specifically, for Mars, such cycles dictate the abundance of its primary atmospheric constituent, carbon dioxide (CO), but also for one of its trace gases, ozone (O). The identification of chemical pathways/cycles by hand is extremely demanding; hence, the application of numerical methods, such as the Pathway Analysis Program (PAP), is crucial to analyze and quantitatively exemplify chemical reaction networks. Here, we carry out the first automated quantitative chemical pathway analysis of Mars’ atmosphere with respect to O. PAP was applied to JPL/Caltech's 1-D updated photochemical Mars model's output data. We determine all significant chemical pathways and their contribution to O production and consumption (up to 80 km) in order to investigate the mechanisms causing the characteristic shape of the O volume mixing ratio profile, i.e. a ground layer maximum and an ozone layer at ∼50 km. These pathways explain why an O layer is present, why it is located at that particular altitude and what the different processes forming the near-surface and middle atmosphere O maxima are. Furthermore, we show that the Martian atmosphere can be divided into two chemically distinct regions according to the O(P):O ratio. In the lower region (below approximately 24 km altitude) O is the most abundant O (= O + O(P)) species. In the upper region (above approximately 24 km altitude), where the O layer is located, O(P) is the most abundant O species. Earlier results concerning the formation of O on Mars can now be explained with the help of chemical pathways leading to a better understanding of the vertical O profile.
© 2016 Elsevier Inc. All rights reservedThis research has been partly supported by the Helmholtz As-
sociation through the research alliance “Planetary Evolution and
Life”. This work was partially funded by grant AyA 2012–32237
awarded by the Spanish Ministerio de Economia y Competitividad.
The authors would like to thank Run-Lie Shia and Dickens Saint-
Hilaire for their contributions to the discussion
A reassessment of the role of Hippias of Elis as a proto-doxographical source
No full textIn 1944, Bruno Snell inaugurated a new field of research we may call Protodoxographical Studies by arguing that there should be a common source for parallel passages with doxographic features in Plato and Aristotle, which he identified as the sophist Hippias of Elis. Later scholars – in particular, Joachim Classen, Jaap Mansfeld, and Andreas Patzer – replicated his method for other parallel passages, and thereby expanded the range of possible content from Hippias. This generalization, however, has not come without methodological losses, in particular a gradual dismissal of parallel confirmatory anchors for the suggested passages: for Patzer, for example, the mere occurrence of parallel texts with a structure similar to the one originally identified by Snell would be taken as an unequivocal sign of the presence of Hippias. One of the effects of this precedent was the propagation of a hypertrophied image of Hippias as philosopher, historian of Philosophy, and inaugurator of the doxographic genre, anticipating the Peripatetic doxographical tradition, and setting him up as a kind of grey eminence behind Plato and Aristotle’s appreciation of the so-called “Presocratic Philosophy”. Although Mansfeld elaborated his contributions on Snell’s theory more carefully, even producing a major advance with identification of a Gorgian protodoxographical matrix parallel to the Hippian one, the lack of continuity in the protodoxographical studies (that remain practically frozen since 1986), resulted in a biased reception in which Mansfeld even started to function as a confirmer of Patzer’s excesses. Against this backdrop, this thesis intends to return to Snell and his successors in order to retrace the evolution of the protodoxographical theory, critically re-evaluate the evidence they produced, and add evidence that has been neglected, including recent studies on Hippias and other implicated characters. From this analysis, a more adequate profile for Hippias as an author, and a more adequate characterization of the structure and purpose of his Collection will emerge, as well as a more accurate characterization and consequent appreciation of the proto-doxographic contribution of Gorgias, and the identification of a possible new proto-doxographic source in the naturalistic physician Hippo.Em 1944, Bruno Snell inaugurou um novo campo de pesquisas – que podemos chamar de Estudos Protodoxográficos – ao defender que haveria uma fonte comum para passagens paralelas com características doxográficas em Platão e Aristóteles, que ele identificou como sendo o sofista Hípias de Élis. Estudiosos posteriores – em particular Joachim Classen, Jaap Mansfeld e Andreas Patzer – replicaram seu método para outras passagens paralelas e, com isso, acabaram expandindo o rol de possíveis conteúdos oriundos de Hípias. Esta generalização, contudo, não virá sem perdas de caráter metodológico, em particular uma gradual dispensa de âncoras paralelas de confirmação para as passagens aventadas: para Patzer, por exemplo, a simples ocorrência de textos paralelos com estrutura similar à que fora identificada originalmente por Snell será tomada como signo inequívoco da presença de Hípias. Um dos efeitos desse procedimento, será a propagação de uma imagem hipertrofiada de Hípias como filósofo, historiador da filosofia e inaugurador do discurso doxográfico, antecipando a tradição doxográfica peripatética e configurando-se como uma espécie de eminência parda por trás da apreciação platônico-aristotélica da chamada “Filosofia Pré-Socrática”. Embora Mansfeld tenha elaborado suas contribuições sobre a teoria de Snell de forma mais cuidadosa, produzindo inclusive um importante avanço, ao identificar uma matriz protodoxográfica gorgiana, paralela à hipiana, a falta de continuidade nos estudos protodoxográficos (que praticamente ficaram congelados no ano de 1986), resultou em uma recepção enviesada, na qual Mansfeld passou a funcionar até mesmo como confirmador dos excessos de Patzer. Diante desse cenário, esta tese pretende retornar a Snell e seus sucessores, a fim de retraçar a evolução da teoria protodoxográfica, reavaliar criticamente as evidências apresentadas e acrescentar evidências que foram negligenciadas, incluindo estudos recentes sobre Hípias e outras personagens implicadas. Desta análise, emergirão um perfil mais adequado para Hípias enquanto autor, e uma caracterização mais adequada da estrutura e da finalidade de sua Coletânea, além de uma caracterização mais precisa e consequente valorização da contribuição protodoxográfica de Górgias, e a identificação de uma possível nova fonte protodoxográfica no médico naturalista Hípon.CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superio
Habitable Zone Lifetime of Exoplanets around Main Sequence Stars
No full textFunding: Dean's Scholarship at the University of East Anglia.The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star’s main sequence lifetime. We describe the time that a planet spends within the HZ as its ‘‘habitable zone lifetime.’’ The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the ‘‘classic’’ HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a ‘‘hybrid’’ HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79 · 109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns.Peer reviewe
Sensitivity of Biomarkers to Chemical Emissions in Proterozoic-type Atmospheres
No full text27 pages, 5 Figures, 4 TablesInternational audienceThe search for life beyond the Solar System is a major activity in exoplanet science. However, even if an Earth-like planet were to be found, it is unlikely to be at a similar stage of evolution as the modern Earth. It is therefore of interest to investigate the sensitivity of biomarker signals for life as we know it for an Earth-like planet but at earlier stages of evolution. Here, we assess biomarkers i.e. species almost exclusively associated with life, in present-day and in 10% present atmospheric level oxygen atmospheres corresponding to the Earth's Proterozoic period. We investigate the impact of proposed enhanced microbial emissions of the biomarker nitrous oxide, which photolyses to form nitrogen oxides which can destroy the biomarker ozone. A major result of our work is regardless of the microbial activity producing nitrous oxide in the early anoxic ocean, a certain minimum ozone column can be expected to persist in Proterozoic-type atmospheres due to a stabilising feedback loop between ozone, nitrous oxide and the ultraviolet radiation field. Atmospheric nitrous oxide columns were enhanced by a factor of 51 for the Proterozoic "Canfield ocean" scenario with 100 times increased nitrous oxide surface emissions. In such a scenario nitrous oxide displays prominent spectral features, so may be more important as a biomarker than previously considered in such cases. The run with "Canfield ocean" nitrous oxide emissions enhanced by a factor of 100 also featured additional surface warming of 3.5K. Our results suggest that the Proterozoic ozone layer mostly survives the changes in composition which implies that it is indeed a good atmospheric biomarker
3D climate modeling of Earth-like extrasolar planets orbiting different types of host stars
No full textInternational audienceThe potential habitability of a terrestrial planet is usually defined by the possible existence of liquid water on its surface. The potential presence of liquid water depends on many factors such as, most importantly, surface temperatures. The properties of the planetary atmosphere and its interaction with the radiative energy provided by the planet's host star are thereby of decisive importance. In this study we investigate the influence of different main-sequence stars upon the climate of Earth-like extrasolar planets and their potential habitability by applying a 3D Earth climate model accounting for local and dynamical processes. The calculations have been performed for planets with Earth-like atmospheres at orbital distances where the total amount of energy received from the various host stars equals the solar constant. In contrast to previous 3D modeling studies, we include the effect of ozone radiative heating upon the vertical temperature structure of the atmospheres. The global orbital mean results obtained have been compared to those of a 1D radiative convective climate model. The different stellar spectral energy distributions lead to different surface temperatures and due to ozone heating to very different vertical temperature structures. As previous 1D studies we find higher surface temperatures for the Earth-like planet around the K-type star, and lower temperatures for the planet around the F-type star compared to an Earth-like planet around the Sun. However, this effect is more pronounced in the 3D model results than in the 1D model because the 3D model accounts for feedback processes such as the ice-albedo and the water vapor feedback. Whether the 1D model may approximate the global mean of the 3D model results strongly depends on the choice of the relative humidity profile in the 1D model, which is used to determine the water vapor profile
Electron-Beam Irradiated Corneal versus Gamma-irradiated Scleral Patch Graft Erosion Rates in Glaucoma Drainage Device Surgery
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