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Search Results: 1 - 10 of 672826 matches for " S. A. Rodionov "
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Extensions and applications of the iterative method
S. A. Rodionov,E. Athanassoula
Physics , 2011, DOI: 10.1051/0004-6361/201116540
Abstract: Aims: We aim to develop an algorithm for constructing equilibrium initial conditions for simulations of disk galaxies with a triaxial halo and/or a gaseous component. This will pave the way for N-body simulations of realistic disk galaxies. Methods: We use the iterative method, which we presented in a previous article. The idea of this method is very simple. It relies on constrained evolution. Results: We develop an algorithm for constructing equilibrium models of disk galaxies including a gaseous disk and a triaxial or axisymmetric halo. We discuss two test models. The first model consists of a spherical halo, a stellar disk, and an isothermal gaseous disk. The second model consists of a triaxial halo, a stellar disk, and a star-forming gaseous disk. We demonstrate that both test models are very close to equilibrium, as we had intended.
Dynamical models of the elliptical galaxy NGC 4494
S. A. Rodionov,E. Athanassoula
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17429.x
Abstract: We present dynamical models of NGC 4494, which we built using our iterative method presented in a previous paper. These models are live N-body models consisting of equal mass particles, and they are steady state as confirmed by a fully self-consistent evolution. Our goals were twofold. The first one -- namely to test whether our iterative method could indeed be used to construct galactic models following given observational constraints, both photometric and kinematic -- was fully achieved. Our method allowed us to go beyond a simple spherical model and to make full sets of rotating, axisymmetric models without any limitations to the velocity distribution. Our second goal was to understand better the structure of NGC 4494, and more specifically to set constraints on its halo mass. For this we tried three families of models: without halo, with a light halo and with a heavy halo, respectively. Our models reproduce well the photometry and the kinematics, the latter except specific regions where some non-equilibrium or non-axisymmetric structure could be present in the galaxy (e.g. the kinematically decoupled core). However, the lower order moments of the velocity distribution (up to and including the second order) do not allow us to discriminate between the three halos. On the other hand, when we extend the comparison to the higher order moments of the velocity distribution obtained from the long-slit data, we find that our light halo model fits the data better than the no halo, or the heavy halo models. They also reproduce the shape of the angular dependence of the PNe velocity dispersion in the outermost parts of the galaxy, but not the amplitude of its azimuthal variation. This may imply that a yet more general class of models, such as triaxial, may be necessary for a yet better fit.
Mechanisms of the Vertical Secular Heating of a Stellar Disk
N. Ya. Sotnikova,S. A. Rodionov
Physics , 2003, DOI: 10.1134/1.1573281
Abstract: We investigate the nonlinear growth stages of bending instability in stellar disks with exponential radial density profiles.We found that the unstable modes are global (the wavelengths are larger than the disk scale lengths) and that the instability saturation level is much higher than that following from a linear criterion. The instability saturation time scales are of the order of one billion years or more. For this reason, the bending instability can play an important role in the secular heating of a stellar disk in the $z$ direction. In an extensive series of numerical $N$-body simulations with a high spatial resolution, we were able to scan in detail the space of key parameters (the initial disk thickness $z_0$, the Toomre parameter $Q$, and the ratio of dark halo mass to disk mass $M_{\rm h} / M_{\rm d}$). We revealed three distinct mechanisms of disk heating in the $z$ direction: bending instability of the entire disk, bending instability of the bar, and heating on vertical inhomogeneities in the distribution of stellar matter.
Bending instability in galactic discs. Advocacy of the linear theory
S. A. Rodionov,N. Ya. Sotnikova
Physics , 2013, DOI: 10.1093/mnras/stt1183
Abstract: We demonstrate that in N-body simulations of isolated disc galaxies there is numerical vertical heating which slowly increases the vertical velocity dispersion and the disc thickness. Even for models with over a million particles in a disc, this heating can be significant. Such an effect is just the same as in numerical experiments by Sellwood (2013). We also show that in a stellar disc, outside a boxy/peanut bulge, if it presents, the saturation level of the bending instability is rather close to the value predicted by the linear theory. We pay attention to the fact that the bending instability develops and decays very fast, so it couldn't play any role in secular vertical heating. However the bending instability defines the minimal value of the ratio between the vertical and radial velocity dispersions $\sigma_z / \sigma_R \approx 0.3$ (so indirectly the minimal thickness) which could have stellar discs in real galaxies. We demonstrate that observations confirm last statement.
Charge relaxation resistance in the Coulomb blockade problem
Ya. I. Rodionov,I. S. Burmistrov,A. S. Ioselevich
Physics , 2009, DOI: 10.1103/PhysRevB.80.035332
Abstract: We study the dissipation in a system consisting of a small metallic island coupled to a gate electrode and to a massive reservoir via single tunneling junction. The dissipation of energy is caused by a slowly oscillating gate voltage. We compute it in the regimes of weak and strong Coulomb blockade. We focus on the regime of not very low temperatures when electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. The answers assume a particularly transparent form while expressed in terms of specially chosen physical observables. We discovered that the dissipation rate is given by a universal expression in both limiting cases.
Bar formation and evolution in disc galaxies with gas and a triaxial halo: Morphology, bar strength and halo properties
E. Athanassoula,R. E. G. Machado,S. A. Rodionov
Physics , 2012, DOI: 10.1093/mnras/sts452
Abstract: We follow the formation and evolution of bars in N-body simulations of disc galaxies with gas and/or a triaxial halo. We find that both the relative gas fraction and the halo shape play a major role in the formation and evolution of the bar. In gas-rich simulations, the disc stays near-axisymmetric much longer than in gas-poor ones, and, when the bar starts growing, it does so at a much slower rate. Due to these two effects combined, large-scale bars form much later in gas-rich than in gas-poor discs. This can explain the observation that bars are in place earlier in massive red disc galaxies than in blue spirals. We also find that the morphological characteristics in the bar region are strongly influenced by the gas fraction. In particular, the bar at the end of the simulation is much weaker in gas-rich cases. In no case did we witness bar destruction. Halo triaxiality has a dual influence on bar strength. In the very early stages of the simulation it induces bar formation to start earlier. On the other hand, during the later, secular evolution phase, triaxial haloes lead to considerably less increase of the bar strength than spherical ones. The shape of the halo evolves considerably with time. The inner halo parts may become more elongated, or more spherical, depending on the bar strength. The main body of initially triaxial haloes evolves towards sphericity, but in initially strongly triaxial cases it stops well short of becoming spherical. Part of the angular momentum absorbed by the halo generates considerable rotation of the halo particles that stay located relatively near the disc for long periods of time. Another part generates halo bulk rotation, which, contrary to that of the bar, increases with time but stays small.
Simulations of Barred Galaxies in Triaxial Dark Matter Haloes: The Effects of Gas
Rubens E. G. Machado,E. Athanassoula,S. A. Rodionov
Physics , 2012,
Abstract: The baryonic discs of galaxies are believed to alter the shapes of the dark matter haloes in which they reside. We perform a set of hydrodynamical N-body simulations of disc galaxies with triaxial dark matter haloes, using elliptical discs with a gaseous component as initial conditions. We explore models of different halo triaxiality and also of different initial gas fractions, which allows us to evaluate how each affects the formations of the bar. Due to star formation, models of all halo shapes and of all initial gas fractions reach approximately the same gas content at the end of the simulation. Nevertheless, we find that the presence of gas in the early phases has important effects on the subsequent evolution. Bars are generally weaker for larger initial gas content and for larger halo triaxiality. The presence of gas, however, is a more efficient factor in inhibiting the formation of a strong bar than halo triaxiality is.
Explicit solutions for partial differential equations of Lord-Shulman thermoelasticity
Rodionov A.
Theoretical and Applied Mechanics , 2009, DOI: 10.2298/tam0902137r
Abstract: We consider the Lord-Shulman model of thermoelasticity with one relaxation constant. The corresponding system of four linear partial differential equations is solved by means of holomorphic expansions. We prove the convergence of expansions and study the possibility to convert them in finite sums.
Nonstationary Wavelets Related to the Walsh Functions  [PDF]
Yuri A. Farkov, Evgeny A. Rodionov
American Journal of Computational Mathematics (AJCM) , 2012, DOI: 10.4236/ajcm.2012.22011
Abstract: Using the Walsh-Fourier transform, we give a construction of compactly supported nonstationary dyadic wavelets on the positive half-line. The masks of these wavelets are the Walsh polynomials defined by finite sets of parameters. Application to compression of fractal functions are also discussed.
Comparative genomic reconstruction of transcriptional networks controlling central metabolism in the Shewanella genus
Rodionov Dmitry A,Novichkov Pavel S,Stavrovskaya Elena D,Rodionova Irina A
BMC Genomics , 2011, DOI: 10.1186/1471-2164-12-s1-s3
Abstract: Background Genome-scale prediction of gene regulation and reconstruction of transcriptional regulatory networks in bacteria is one of the critical tasks of modern genomics. The Shewanella genus is comprised of metabolically versatile gamma-proteobacteria, whose lifestyles and natural environments are substantially different from Escherichia coli and other model bacterial species. The comparative genomics approaches and computational identification of regulatory sites are useful for the in silico reconstruction of transcriptional regulatory networks in bacteria. Results To explore conservation and variations in the Shewanella transcriptional networks we analyzed the repertoire of transcription factors and performed genomics-based reconstruction and comparative analysis of regulons in 16 Shewanella genomes. The inferred regulatory network includes 82 transcription factors and their DNA binding sites, 8 riboswitches and 6 translational attenuators. Forty five regulons were newly inferred from the genome context analysis, whereas others were propagated from previously characterized regulons in the Enterobacteria and Pseudomonas spp.. Multiple variations in regulatory strategies between the Shewanella spp. and E. coli include regulon contraction and expansion (as in the case of PdhR, HexR, FadR), numerous cases of recruiting non-orthologous regulators to control equivalent pathways (e.g. PsrA for fatty acid degradation) and, conversely, orthologous regulators to control distinct pathways (e.g. TyrR, ArgR, Crp). Conclusions We tentatively defined the first reference collection of ~100 transcriptional regulons in 16 Shewanella genomes. The resulting regulatory network contains ~600 regulated genes per genome that are mostly involved in metabolism of carbohydrates, amino acids, fatty acids, vitamins, metals, and stress responses. Several reconstructed regulons including NagR for N-acetylglucosamine catabolism were experimentally validated in S. oneidensis MR-1. Analysis of correlations in gene expression patterns helps to interpret the reconstructed regulatory network. The inferred regulatory interactions will provide an additional regulatory constrains for an integrated model of metabolism and regulation in S. oneidensis MR-1.
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