排序方式: 共有164条查询结果,搜索用时 0 毫秒
51.
When intermediate mass stars reach their last stages of evolution they show pronounced oscillations. This phenomenon happens when these stars reach the so-called asymptotic giant branch (AGB), which is a region of the Hertzsprung-Russell diagram located at about the same region of effective temperatures but at larger luminosities than those of regular giant stars. The period of these oscillations depends on the mass of the star. There is growing evidence that these oscillations are highly correlated with mass loss and that, as the mass loss increases, the pulsations become more chaotic. In this paper we study a simple oscillator which accounts for the observed properties of this kind of stars. This oscillator was first proposed and studied in Icke et al. [Astron. Astrophys. 258, 341 (1992)] and we extend their study to the region of more massive and luminous stars -the region of super-AGB stars. The oscillator consists of a periodic nonlinear perturbation of a linear Hamiltonian system. The formalism of dynamical systems theory has been used to explore the associated Poincare map for the range of parameters typical of those stars. We have studied and characterized the dynamical behavior of the oscillator as the parameters of the model are varied, leading us to explore a sequence of local and global bifurcations. Among these, a tripling bifurcation is remarkable, which allows us to show that the Poincare map is a nontwist area preserving map. Meandering curves, hierarchical-islands traps and sticky orbits also show up. We discuss the implications of the stickiness phenomenon in the evolution and stability of the super-AGB stars. (c) 2002 American Institute of Physics. 相似文献
52.
Andreea Irina Barzic Dan Gheorghe Dimitriu Dana Ortansa Dorohoi 《International Journal of Polymer Analysis and Characterization》2015,20(6):565-571
The proposed method of channeled spectrum employs the recorded channeled spectrum of an optical active polymer solution placed between two crossed polarizers. The approach allows the evaluation of the optical rotatory dispersion of poly(propylene oxide) in benzene. The specific rotation, circular birefringence, and dispersion parameter depend on the wavelengths of the maxima and minima from the channeled spectrum. 相似文献
53.
Y. H. Cheng H. Kupfer F. Richter Andreea Maria Paraian 《Applied Surface Science》2002,200(1-4):117-124
MgO films were deposited by pulsed mid-frequency magnetron sputtering from metallic targets in the mixture of Ar and O2 gas. The surface morphology, crystalline structure, and optical properties were characterized by using atomic force microscopy (AFM), X-ray diffraction (XRD), and spectroscopic ellipsometry, respectively. The secondary electron emission coefficients of MgO films were measured by using a self-made apparatus in He gas. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O2 flow rate was observed. The structure of films deposited at a metallic mode changes from Mg phase to the mixed Mg and MgO phase, and the films have a very rough surface. All the films deposited at oxide mode have high transparency and smooth surface, and show (2 2 0) preferred orientation growth. The refractive index and extinction coefficient at a wavelength of 670 nm for MgO films deposited at oxide mode with a O2 flow rate of 3 sccm are 1.698 and 1.16×10−4, respectively. The secondary emission coefficient at a E/p of 57.8 V/(cm Torr) for MgO films deposited at a O2 flow rate of 3 sccm is 0.16, which is higher than that of MgO films deposited by e-beam evaporation. 相似文献
54.
Leclercq L Noujeim N H Sanon S Schmitzer AR 《The journal of physical chemistry. B》2008,112(45):14176-14184
N, N'-Disubstituted methylenediimidazolium bromide salts substituted with two aromatic groups present two different binding sites. In the binary complexes with cyclodextrins (CDs) or cucurbit[7]uril (CB[7]), the macrocycle is always positioned on the external aromatic residues. In the ternary complexes, CB[7] is positioned around the diimidazolium cation, where the external aromatic residue is included in the CD's cavity. The unfavored position of the CB[7] on the cationic site in the ternary complex is the result of its cooperative supramolecular interaction with the cyclodextrin. The obtained ternary complexes possess different interfacial properties, compared to those of the binary complexes. We demonstrate these hypotheses by NMR spectroscopy, ESI-HRMS spectrometry, molecular modeling simulation, and surface tension measurements. 相似文献
55.
Andreea Irina Barzic Dumitru Popovici Camelia Hulubei Iuliana Stoica Magdalena Aflori Simona Dunca 《International Journal of Polymer Analysis and Characterization》2016,21(1):77-84
This article has been retracted. 相似文献
56.
Lingli Cui Hamza Armghan Noushahi Yipeng Zhang Jinxin Liu Andreea Cosoveanu Ying Liu Ling Yan Jing Zhang Shaohua Shu 《Molecules (Basel, Switzerland)》2021,26(4)
As the population ages globally, there seem to be more people with Alzheimer’s disease. Unfortunately, there is currently no specific treatment for the disease. At present, Huperzine A (HupA) is one of the best drugs used for the treatment of Alzheimer’s disease and has been used in clinical trials for several years in China. HupA was first separated from Huperzia serrata, a traditional medicinal herb that is used to cure fever, contusions, strains, hematuria, schizophrenia, and snakebite for several hundreds of years in China, and has been confirmed to have acetylcholinesterase inhibitory activity. With the very slow growth of H. serrata, resources are becoming too scarce to meet the need for clinical treatment. Some endophytic fungal strains that produce HupA were isolated from H. serrate in previous studies. In this article, the diversity of the endophytic fungal community within H. serrata was observed and the relevance to the production of HupA by the host plant was further analyzed. A total of 1167 strains were obtained from the leaves of H. serrata followed by the stems (1045) and roots (824). The richness as well as diversity of endophytic fungi within the leaf and stem were higher than in the root. The endophytic fungal community was similar within stems as well as in leaves at all taxonomic levels. The 11 genera (Derxomyces, Lophiostoma, Cyphellophora, Devriesia, Serendipita, Kurtzmanomyces, Mycosphaerella, Conoideocrella, Brevicellicium, Piskurozyma, and Trichomerium) were positively correlated with HupA content. The correlation index of Derxomyces with HupA contents displayed the highest value (CI = 0.92), whereas Trichomerium showed the lowest value (CI = 0.02). Through electrospray ionization mass spectrometry (ESI-MS), it was confirmed that the HS7-1 strain could produce HupA and the total alkaloid concentration was 3.7 ug/g. This study will enable us to screen and isolate the strain that can produce HupA and to figure out the correlation between endophytic fungal diversity with HupA content in different plant organs. This can provide new insights into the screening of strains that can produce HupA more effectively. 相似文献
57.
Ana Cezarina Morosanu Andreea Celia Benchea Daniela Babusca Dana Ortansa Dorohoi 《Analytical letters》2017,50(17):2725-2739
ABSTRACTQuercetin was subjected to a quantum-mechanical analysis and energetic and electro-optic parameters were evaluated. A solvatochromic study was used to estimate the dipole moment of quercetin in the excited state and to determine the angle between the dipole moments of this molecule in the electronic states responsible for the visible absorption band. The contribution of each type of intermolecular interactions to the total spectral shift experimentally recorded in quercetin solution was also established in this study. 相似文献
58.
59.
Elinor Gardner Escamilla Andreea Cristina Nicolae Paul Russell Salerno Shahriar Shahriari Jordan Olliver Tirrell 《Order》2011,28(2):357-373
In 1975, J. Griggs conjectured that a normalized matching rank-unimodal poset possesses a nested chain decomposition. This
elegant conjecture remains open even for posets of rank 3. Recently, Hsu, Logan, and Shahriari have made progress by developing
techniques that produce nested chain decompositions for posets with certain rank numbers. As a demonstration of their methods,
they prove that the conjecture is true for all rank 3 posets of width at most 7. In this paper, we present new general techniques
for creating nested chain decompositions, and, as a corollary, we demonstrate the validity of the conjecture for all rank
3 posets of width at most 11. 相似文献
60.
Alexandru C. Razus Eugenia Andreea DraguSimona Nica Alina Nicolescu 《Tetrahedron letters》2011,52(16):1858-1862
The azulen-1-yl group strongly influences the course of the TiCl4-Zn promoted McMurry coupling of 1-acetylazulene which afforded, along with the normal coupling products, pinacol 2dl and alkene 3, the pinacolone 4. The latter was likely formed via rearrangement of the pinacolate intermediate. An improved, microwave-assisted protocol was developed for this reaction that provided high yields of products in a short reaction time. The steric reaction route is consistent with the proposed mechanism. 相似文献