1,972 research outputs found
Survival probabilities in the double trapping reaction A +B -> B, B + C -> C
We consider the double trapping reaction A + B -> B, B + C -> C in one
dimension. The survival probability of a given A particle is calculated under
various conditions on the diffusion constants of the reactants, and on the
ratio of initial B and C particle densities. The results are of more general
form than those obtained in previous work on the problem.Comment: 5 page
Looking for pulsations in HgMn stars through CoRoT lightcurves
HgMn Chemically Peculiar stars are among the quietest stars of the
main-sequence. However, according to theoretical predictions, these stars could
have pulsations related to the very strong overabundances of iron peak
elements, which are produced by atomic diffusion in upper layers. Such
pulsations have never been detected from ground based observations.
Our aim is to search for signatures of pulsations in HgMn stars using the
high quality lightcurves provided by the CoRoT satellite.
We identified three faint stars (V>12), from VLT-GIRAFFE multiobject
spectrograph survey in a field which was planned for observation by CoRoT. They
present the typical characteristics of HgMn stars. They were observed by the
CoRoT satellite during the long run (131 days) which started from the 24th of
October 2007, with the exoplanets CCD's (Additional Programme). In the present
work, we present the analysis of the ground based spectra of these three stars
and the analysis of the corresponding CoRoT lightcurves.
Two of these three HgMn candidates show low amplitude (less than 1.6 mmag)
periodic variations (4.3 and 2.53 days respectively, with harmonics) which are
compatible with periods predicted by theoretical models.Comment: Accepted paper in A&A (7 May 2009
Synthesis and photophysical properties of a poly(methyl methacrylate) polymer with carbazolyl side groups
The photophysical properties of solutions and films of poly(methyl methacrylate) (PMMA) containing 1.6 mol % of randomly distributed pendant ethyl carbazolyl groups have been studied under steady-state and time-resolved conditions. The polymer was prepared by esterification of poly(methyl methacrylate-co-methacrylic acid) by 9H-carbazole-9-ethanol. At room temperature, the steady-state fluorescence spectrum is attributed to isolated lumophores in solution, but is much more complex in films where emission is detected from isolated groups, ground-state dimers and excimers. At 77 K, emission from both solutions and films contains a component of phosphorescence whose wavelengths overlap fluorescence from excited ground-state dimers and excimers. From the temperature dependence of the steady-state emission spectra of the films, recorded from 30 to 410 K, several types of relaxation processes are identified at temperatures similar to those of unmodified PMMA. Data by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and fluorescence spectroscopy are compared. They include the γ-relaxations from motions of small segments of the chains at 120-130 K (by fluorescence spectroscopy), the β-relaxation from motions of lateral ester groups (by DMTA and fluorescence spectroscopy) at 310 K and α-relaxations (glass transition onset) at 380 K (by DSC and DMTA)
Synthesis and photophysical properties of a poly(methyl methacrylate) polymer with carbazolyl side groups
The photophysical properties of solutions and films of poly(methyl methacrylate) (PMMA) containing 1.6 mol % of randomly distributed pendant ethyl carbazolyl groups have been studied under steady-state and time-resolved conditions. The polymer was prepared by esterification of poly(methyl methacrylate-co-methacrylic acid) by 9H-carbazole-9-ethanol. At room temperature, the steady-state fluorescence spectrum is attributed to isolated lumophores in solution, but is much more complex in films where emission is detected from isolated groups, ground-state dimers and excimers. At 77 K, emission from both solutions and films contains a component of phosphorescence whose wavelengths overlap fluorescence from excited ground-state dimers and excimers. From the temperature dependence of the steady-state emission spectra of the films, recorded from 30 to 410 K, several types of relaxation processes are identified at temperatures similar to those of unmodified PMMA. Data by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and fluorescence spectroscopy are compared. They include the γ-relaxations from motions of small segments of the chains at 120-130 K (by fluorescence spectroscopy), the β-relaxation from motions of lateral ester groups (by DMTA and fluorescence spectroscopy) at 310 K and α-relaxations (glass transition onset) at 380 K (by DSC and DMTA).As propriedades fotofísicas de um poli(metacrilato de metila) (PMMA) contendo 1,6 mol % de grupos etil-carbazolila aleatoriamente distribuídos pela cadeia, foram estudadas pelas técnicas de espectroscopia de fluorescência em condições fotoestacionárias e com resolução temporal. O material foi preparado a partir da esterificação do poli(metacrilato de metila- co-ácido metacrílico) com 9H-carbazolila-9-etanol. A temperatura ambiente, o espectro de fluorescência que se obtém para a amostra em solução é devido ao lumóforo isolado. Em filme, suas características espectroscópicas se tornam mais complexas, com contribuições do lumóforo isolado, dímeros formados no estado eletrônico fundamental e excímeros. A 77 K, a emissão obtida das amostras, tanto em filme quanto em solução, apresenta também a contribuição da fosforescência, que ocorre na mesma região espectral que a emissão de fluorescência dos agregados emissores. Os diversos tipos de relaxações mecânicas do polímero puderam ser observados através dos espectros de fluorescência do filme, obtidos na faixa de temperatura de 30 a 410 K e são similares àqueles relativos ao PMMA. Os dados obtidos por calorimetria diferencial de varredura (DSC) e análise térmica dinâmico-mecânica (DMTA) são comparados aos dados fornecidos pela espectroscopia de fluorescência e as temperaturas onde ocorrem estas relaxações puderam ser determinadas. Assim os processos da γ-relaxação, que é resultante de movimentos de pequenos segmentos de cadeia, ocorrem a 120-130 K (observados por espectroscopia de fluorescência), a β-relaxação, resultante dos movimentos de grupos éster laterais (observada por DMTA e espectroscopia de fluorescência), a 310 K e α-relaxação (temperatura de transição vítrea) ocorre a 380 K (observada por DSC e DMTA).14501461Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES
Our Sun. IV. The Standard Model and Helioseismology: Consequences of Uncertainties in Input Physics and in Observed Solar Parameters
Helioseismology provides a powerful tool to explore the deep interior of the
Sun: for example, the adiabatic sound speed can be inferred with an accuracy of
a few parts in 10,000. This has become a serious challenge to theoretical
models of the Sun. Therefore, we have undertaken a self-consistent, systematic
study of sources of uncertainties in the standard solar model, which must be
understood before the helioseismic observations can be used as constraints on
theory. We find that the largest uncertainty in the sound speed in the solar
interior, namely, 3 parts in 1000, arises from uncertainties in the observed
photospheric abundances of the elements; uncertainties of 1 part in 1000 arise
from (1) the 4% uncertainty in the OPAL opacities, (2) the 5% uncertainty in
the basic pp nuclear reaction rate, (3) the 15% uncertainty in the diffusion
constants for the gravitational settling of helium, and (4) the 50%
uncertainties in diffusion constants for the heavier elements. (Other
investigators have shown that similar uncertainties arise from uncertainties in
the interior equation of state and in rotation-induced turbulent mixing.) The
predicted pre-main-sequence solar lithium depletion is a factor of order 20 (an
order of magnitude larger than that predicted by earlier models that neglected
gravitational settling and used older opacities), and is uncertain by a factor
of 2. The predicted neutrino capture rate is uncertain by 30% for the Cl-37
experiment and by 3% for the Ga-71 experiments (not including uncertainties in
the capture cross sections), while the B-8 neutrino flux is uncertain by 30%.Comment: LaTeX, 38 pages (including 8 figures); ApJ, in press. Added
figures/color figurea available at
http://www.cita.utoronto.ca/~boothroy/sun4.htm
Structure and expression of the human globin genes and murine histocompatibility antigen genes
Dissecting kinetic pathways to formation of the fibrillar objects in molecular gels using synchrotron FT-IR †
The processes associated with sol-to-molecular gel transitions are often difficult to follow in real time due to the characteristically short time-frames for pseudo-crystallization leading to fibrillar objects and establishment of their 3-dimensional networks. We use fast infrared spectroscopic methods to overcome these difficulties in a 'demonstration of principle' study: the nucleation and growth of crystallites of two amide derivatives of IJR)-12-hydroxystearic acid, IJR)-12-hydroxy-N-propyloctadecanamide (1) and IJR)-12-hydroxyoctadecanamide (2), in mineral oil over a range of temperatures. First, the gel properties were established by optical microscopic, differential scanning calorimetric, and rheological measurements. Thereafter, the environments experienced by individual functional groups within 1 and 2 have been followed as a function of gelation time, and the data were treated by Avrami and fractal models in order to extract the kinetics, activation energies, and natures of the growth processes. The data both provide insights and raise new questions concerning the manner by which the amide and hydroxyl groups interact over the critical time periods during which the fibrillar networks of the gels are being established. The results demonstrate the utility of fast infrared spectroscopy as a valuable new tool to probe intimate details of the processes associated with self-assembly. The techniques and methodologies described are amenable to use with a wide range of gelating (or other aggregating) systems
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