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Search Results: 1 - 10 of 228517 matches for " R. Chan "
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N-Body Simulations of Gas-Free Disc Galaxies with SMBH Seed in Binary Systems  [PDF]
R. Chan
International Journal of Astronomy and Astrophysics (IJAA) , 2019, DOI: 10.4236/ijaa.2019.93013
Abstract: We have shown the outcome of N-body simulations of the interactions of two disc galaxies without gas with the same mass. Both disc galaxies have halos of dark matter, central bulges and initial supermassive black hole (SMBH) seeds at their centers. The purpose of this work is to study the mass and dynamical evolution of the initial SMBH seed during a Hubble cosmological time. It is a complementation of our previous paper with different initial orbit conditions and by introducing the SMBH seed in the initial galaxy. The disc of the secondary galaxy has a coplanar or polar orientation in relation to the disc of the primary galaxy and their initial orbit are eccentric and prograde. The primary and secondary galaxies have mass and size of Milky Way with an initial SMBH seed. We have found that the merger of the primary and secondary discs can result in a final normal disc or a final warped disc. After the fusion of discs, the final one is thicker and larger than the initial disc. The tidal effects are very important, modifying the evolution of the SMBH in the primary and secondary galaxy differently. The mass of the SMBH of the primary galaxy has increased by a factor ranging from 52 to 64 times the initial seed mass, depending on the experiment. However, the mass of the SMBH of the secondary galaxy has increased by a factor ranging from 6 to 33 times the initial SMBH seed mass, depending also on the experiment. Most of the accreted particles have come from the bulge and from the halo, depleting their particles. This could explain why the observations show that the SMBH with masses of approximately\"\"is found in many bulgeless galaxies. Only a small number of the accreted particles has come from the disc. In some cases of final merging stage of the two galaxies, the final SMBH of the secondary galaxy was ejected out of the galaxy.
The use of sunflower transcription factors as biotechnological tools to improve yield and stress tolerance in crops
Phyton (Buenos Aires) , 2009,
Abstract: transcription factors (tfs) are proteins able to specifically recognize dna sequences in the regulatory regions of their target genes. they bind these specific sequences, an event that leads to the activation or repression of whole signal transduction pathways. in plants about 1500 tfs were informatically identified; identification was mainly based in the presence of dna-binding domains in the translated sequences. they were classified in families and subfamilies according to several features, including the conservation of the dna binding domain, the genes structures and the functions they exert. among transcription factors, several seem to be potential powerful biotechnological tools to improve crops via obtaining transgenic plants. assigned purposes include: yield improvements, abiotic and biotic stress tolerances, and a combination of them. none of them is up to date a product market, since from the gene discovery to the regulation process (which differs in each country) there is a long pipeline to run. since a few years ago, our research group is devoted to the structural and functional characterization of sunflower transcription factors, especially those belonging to the hd-zip family. members of this family exhibit in their structure a homeodomain (hd) associated to a leucine zipper (lz). this association is unique to plants, being these two domains common to transcription factors from several kingdoms. it was proposed by other authors, and also by us, that this unique association is probably due to specific plant responses as plant development, which depends on environmental conditions. in this work we show an overview and new insights of the sunflower hd-zip proteins, demonstrating to be useful biotechnological tools to confer drought, salt, herbicide, and herbivory (e.g. insect) tolerance, and other abiotic and biotic tolerance to stressgenerating factors. like other tfs, sunflower hd-zip transcription factors confer a complex phenotype to transgenic plants
Gauche-Trans Energy Differences in Dimethoxymethane and Dimethoxyethane
E. V. R. Chan
Physics , 2005,
Abstract: Ab-initio self consistent field calculations using double zeta Gaussian basis set expansions were performed on conformers of Dimethoxymethane and Dimethoxyethane. The gauche-trans energy differences using the rigid rotor approximation were calculated.
Radiating Shear-Free Gravitational Collapse with Charge
G. Pinheiro,R. Chan
Physics , 2011, DOI: 10.1007/s10714-012-1468-7
Abstract: We present a new shear free model for the gravitational collapse of a spherically symmetric charged body. We propose a dissipative contraction with radiation emitted outwards. The Einstein field equations, using the junction conditions and an ansatz, are integrated numerically. A check of the energy conditions is also performed. We obtain that the charge delays the black hole formation and it can even halt the collapse.
Long-Time Evolution of Gas-Free Disc Galaxies in Binary Systems
R. Chan,S. Junqueira
Physics , 2013,
Abstract: We present the results of several detailed numerical N-body simulations of the dynamical interactions of two equal mass disc galaxies. Both galaxies are embedded in spherical halos of dark matter and contain central bulges. Our analysis of the dynamical evolution of the binary system focuses on the morphological evolution of the stellar distribution of the discs. The satellite galaxy has coplanar or polar disc orientation in relation to the disc of the primary galaxy and their initial orbits are prograde eccentric ($e=0.1$, $e=0.4$ or $e=0.7$). Both galaxies have mass and size comparable to the Milky Way. We show that the merger of the two disc galaxies, depending on the relative orientation of the discs, can yield either a disc or lenticular remnant, instead of an elliptical one. These are the first simulations in the literature to show the formation of S0-like galaxies from protracted binary galaxy interactions. Additionally, we demonstrate that the time to merger increases linearly with the initial apocentric distance between the galaxies, and decreases with the initial orbital eccentricity. We also show that the tidal forces of the discs excite transient $m=1$ and $m=2$ wave modes, i.e., lopsidedness, spiral arms, and bars. However, after the merging of the discs, such instabilities fade completely, and the remnant is thicker and bigger than the original discs. The maximum relative amplitude of these waves is at most about 15 times greater compared to the control case. of these two wave modes. Finally, the disc settles down quickly, after the merger, in less than one outer disc rotation period.
Line Shape Analysis of Linear X Ray Magnetic Scattering CoPt Thin Films
E. V. R. Chan
Physics , 2004,
Abstract: Data analysis of the CCD files from x ray magnetic resonance scattering linearly polarized in transmission geometry produces information about the radial and azimuthal intensities. In a series of measurements of increasing photon energies trends in data are analyzed.
Monte Carlo Simulation for Spheres with Two Length Scales
E. V. R. Chan
Physics , 2004,
Abstract: Canonical ensemble Monte Carlo calculations have been carried out on spheres with two different length scales. Radial distribution functions at various temperatures and densities were computed and compared. Preliminary results indicate that there are differences but because these may be subtle, more calculations including self-diffusion coefficient, heat capacity, etc. would be necessary in order to determine a phase diagram.
Radiating Gravitational Collapse with Shearing Motion and Bulk Viscosity Revisited
G. Pinheiro,R. Chan
Physics , 2013, DOI: 10.1142/S0218271810018050
Abstract: A new model is proposed to a collapsing star consisting of an anisotropic fluid with bulk viscosity, radial heat flow and outgoing radiation. In a previous paper one of us has introduced a function time dependent into the $g_{rr}$, besides the time dependent metric functions $g_{\theta\theta}$ and $g_{\phi\phi}$. The aim of this work is to generalize this previous model by introducing bulk viscosity and compare it to the non-viscous collapse. The behavior of the density, pressure, mass, luminosity and the effective adiabatic index is analyzed. Our work is also compared to the case of a collapsing fluid with bulk viscosity of another previous model, for a star with 6 $M_{\odot}$. The pressure of the star, at the beginning of the collapse, is isotropic but due to the presence of the bulk viscosity the pressure becomes more and more anisotropic. The black hole is never formed because the apparent horizon formation condition is never satisfied, in contrast of the previous model where a black hole is formed. An observer at infinity sees a radial point source radiating exponentially until reaches the time of maximum luminosity and suddenly the star turns off. In contrast of the former model where the luminosity also increases exponentially, reaching a maximum and after it decreases until the formation of the black hole. The effective adiabatic index diminishes due to the bulk viscosity, thus increasing the instability of the system, in both models, in the former paper and in this work.
Code for RXMS Data Analysis
E. V. R. Chan
Physics , 2004,
Abstract: The source code for resonant x-ray magnetic scattering (RXMS) data analysis is available. It includes measures of the radial variation of intensity and ability to extract azimuthal angular variation of intensity within the transmission geometry. If you need assistance to modify my code please email.
PET/CT Is Complementary to Fine-Needle Aspiration Cytology in Assessment of Irradiated Neck in Head and Neck Cancers
R. C. L. Chan,Y. W. Chan
Surgery Research and Practice , 2014, DOI: 10.1155/2014/191267
Abstract: Background. Accurate assessment of irradiated neck in squamous cell carcinoma of the head and neck (HNSCC) is essential. Fine-needle aspiration cytology is often performed for suspicious lesions but it is limited by its low negative predictive value (NPV). We postulated that F-18 fluorodeoxyglucose (FDG) positron emission tomography combined with computed tomography (PET/CT) can overcome this limitation by its high NPV value and allow for a more accurate assessment of irradiated neck in HNSCC. Methods. Fifty-four HNSCC patients were included for the study. They all received previous irradiation to the neck. Clinical characteristics, details of radiotherapy, PET/CT results, follow-up findings, and final histological diagnosis were analyzed. Results. The sensitivity, specificity, positive predictive value (PPV), and NPV were 95.8%, 96.7%, 95.8%, and 96.7%, respectively. Age, sex, radiation dose, interval between PET/CT and radiotherapy completion, nature of radiotherapy, and use of second course of radiotherapy were not found to affect diagnostic accuracy of PET/CT. A new algorithm for investigation of masses in irradiated neck is proposed. Conclusions. PET/CT is an effective diagnostic tool and has a complementary role to FNAC in the management of irradiated neck in head and neck cancers, particularly in cases where suspicious lesions were identified but FNAC showed negative results. 1. Background Radiotherapy is frequently employed in the management of head and neck squamous cell carcinoma (HNSCC). Locoregionally advanced tumours are frequently given adjuvant radiotherapy with or without chemotherapy to improve locoregional control. Definitive radiation is also given for organ preservation and to avoid operative morbidity. Accurate assessment of irradiated neck is essential in management of HNSCC. Unfortunately, radiation-associated fibrosis of skin and subcutaneous tissue can easily mask underlying lesions on clinical examination. Various modalities of imaging were used to detect residual or recurrent neck diseases. However, anatomical distortion caused by radiotherapy (and often together with surgery) renders that the assessment of disease status by imaging alone is difficult. Ideally, when a suspicious lesion is identified clinically or radiologically, tissue biopsy should be sampled from suspicious lesions. In reality, however, they are often limited by inaccessibility and/or potential morbidity associated, especially in irradiated necks. Thus, fine-needle aspiration cytology (FNAC) is often performed instead. However, we have recently shown that
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