The Experts below are selected from a list of 90 Experts worldwide ranked by ideXlab platform

Srikanth Sastry - One of the best experts on this subject based on the ideXlab platform.

  • Force networks and jamming in shear-deformed sphere packings.
    Physical Review E, 2019
    Co-Authors: H. A. Vinutha, Srikanth Sastry
    Abstract:

    The emergence of rigidity upon changes of temperature, density, or applied stresses in disordered assemblies of particles is of interest in a wide range of soft matter, from glass formers, gels, foams, and granular matter. Shear jamming of frictional grains presents an interesting special case wherein the application of shear stress leads to rigidity rather than its loss. The formation of self-organized structures that resist shear deformation offers an appealing geometric picture of shear jamming, which nevertheless is incompletely developed, and not well integrated with ideas concerning rigidity in frictionless systems. Exploiting the observation that athermally sheared sphere assemblies develop structural features necessary for shear jamming even in the absence of friction [H. A. Vinutha and S. Sastry, Nature Physics 12, 578 (2016)1745-247310.1038/nphys3658], we analyze Conditions for jamming in such assemblies computationally. Solving force and torque balance Conditions for their contact geometry, we show, and validate with frictional simulations, that the mean contact number Z equals D+1 (for spatial dimension D=2,3) at jamming for both finite and infinite friction, above the "random loose packing" limit density, at variance with previous analyses of frictional jamming. We show that the shear jamming threshold satisfies the Marginal Stability Condition recently proposed for jamming in frictionless systems. Along lines explored in studying covalent glasses, we perform rigidity percolation analysis for D=2 and find that rigidity percolation precedes shear jamming, which, however, coincides with the percolation of over-constrained regions, leading to the identification of a regime analogous to the intermediate phase observed in covalent glasses. Together, these results provide a geometric description of shear jamming that relate closely with analyses of jamming, rigidity, and the glass transition in frictionless systems, and thus help develop a unified description of jamming phenomenology in diverse disordered matter.

Daniel Verscharen - One of the best experts on this subject based on the ideXlab platform.

  • PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction
    The Astrophysical Journal, 2016
    Co-Authors: Mario A. Riquelme, Eliot Quataert, Daniel Verscharen
    Abstract:

    In low-collisionality plasmas, velocity-space instabilities are a key mechanism providing an effective collisionality for the plasma. We use particle-in-cell (PIC) simulations to study the interplay between electron and ion-scale velocity-space instabilities and their effect on electron pressure anisotropy, viscous heating, and thermal conduction. The adiabatic invariance of the magnetic moment in low-collisionality plasmas leads to pressure anisotropy, $p_{\perp,j} > p_{||,j}$, if the magnetic field $\vec{B}$ is amplified ($p_{\perp,j}$ and $p_{||,j}$ denote the pressure of species $j$ [electron, ion] perpendicular and parallel to $\vec{B}$). If the resulting anisotropy is large enough, it can in turn trigger small-scale plasma instabilities. Our PIC simulations explore the nonlinear regime of the mirror, ion-cyclotron, and electron whistler instabilities, through continuous amplification of the magnetic field $|\vec{B}|$ by an imposed shear in the plasma. In the regime $1 \lesssim \beta_j \lesssim 20$ ($\beta_j \equiv 8\pi p_j/|\vec{B}|^2$), the saturated electron pressure anisotropy, $\Delta p_e/p_{||,e}$, is determined mainly by the (electron-lengthscale) whistler Marginal Stability Condition, with a modest factor of $\sim 1.5-2$ decrease due to the trapping of electrons by the mirrors. We explicitly calculate the mean free path of the electrons and ions along the mean magnetic field and provide a simple physical prescription for the mean free path and thermal conductivity in low-collisionality $\beta_j \gtrsim 1$ plasmas. Our results imply that velocity-space instabilities likely decrease the thermal conductivity of plasma in the outer parts of massive, hot, galaxy clusters. We also discuss the implications of our results for electron heating and thermal conduction in low-collisionality accretion flows onto black holes, including Sgr A* in the Galactic Center.

Anna S. Saburova - One of the best experts on this subject based on the ideXlab platform.

  • Gravitational Stability and mass estimation of stellar disks
    Astronomische Nachrichten, 2013
    Co-Authors: Anna S. Saburova, A. V. Zasov
    Abstract:

    We estimate the masses of disks of galaxies using the Marginal gravitational Stability criterion and compare them with the photometrical disk mass evaluations. The comparison reveals that the stellar disks of most of spiral galaxies we considered cannot be substantially overheated (at least within several radial scalelengths) and are therefore unlikely to have experienced a significant merging event in their history. However, for substantial part of S0- type galaxies a stellar velocity dispersion is well in excess of the gravitational Stability threshold suggesting a major merger event in the past. For four low surface brightness galaxies we found that the disk masses corresponding to the Marginal Stability Condition are significantly higher than it may be expected from their brightness. Either their disks are dynamically overheated, or they contain a large amount of non-luminous matter.

  • Is dynamic heating of stellar disk inevitable
    2013
    Co-Authors: A. V. Zasov, Anna S. Saburova, Ivan Yu. Katkov
    Abstract:

    Major mergers or/and the repeated minor mergers lead to dynamical heating of disks of galaxies. We analyze the available data on the velocity dispersion of stellar disks of S-S0 galaxies, including the new observational data obtained at 6m telescope of SAO RAS. As a measure of dynamical (over)heating, we use the ratio of the observed velocity dispersion to the minimal dispersion which provides the local Stability of the stellar disks with respect to gravitational perturbations. We came to conclusion that stellar disks in a significant part of galaxies (including LSB and some S0 galaxies) are close to the Marginal Stability Condition (or are slightly overheated) – at least at radial distances r ∼ 2-3 radial scalelenghts. It enables to constrain the role of merging in the heating of stellar disks: in many cases it seems to be non-efficient. Marginal Stability Condition may also be successfully used to estimate the mass of a disk and the midplane volume gas (stars) densities on the basis of kinematic measurements.

  • Gravitational Stability and dynamical overheating of stellar disks of galaxies
    Astronomy Letters, 2011
    Co-Authors: A. V. Zasov, Alexander Khoperskov, Anna S. Saburova
    Abstract:

    We use the Marginal Stability Condition for galactic disks and the stellar velocity dispersion data published by different authors to place upper limits on the disk local surface density at two radial scalelengths $R=2h$. Extrapolating these estimates, we constrain the total mass of the disks and compare these estimates to those based on the photometry and color of stellar populations. The comparison reveals that the stellar disks of most of spiral galaxies in our sample cannot be substantially overheated and are therefore unlikely to have experienced a significant merging event in their history. The same conclusion applies to some, but not all of the S0 galaxies we consider. However, a substantial part of the early type galaxies do show the stellar velocity dispersion well in excess of the gravitational Stability threshold suggesting a major merger event in the past. We find dynamically overheated disks among both seemingly isolated galaxies and those forming pairs. The ratio of the Marginal Stability disk mass estimate to the total galaxy mass within four radial scalelengths remains within a range of 0.4---0.8. We see no evidence for a noticeable running of this ratio with either the morphological type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter

H. A. Vinutha - One of the best experts on this subject based on the ideXlab platform.

  • Force networks and jamming in shear-deformed sphere packings.
    Physical Review E, 2019
    Co-Authors: H. A. Vinutha, Srikanth Sastry
    Abstract:

    The emergence of rigidity upon changes of temperature, density, or applied stresses in disordered assemblies of particles is of interest in a wide range of soft matter, from glass formers, gels, foams, and granular matter. Shear jamming of frictional grains presents an interesting special case wherein the application of shear stress leads to rigidity rather than its loss. The formation of self-organized structures that resist shear deformation offers an appealing geometric picture of shear jamming, which nevertheless is incompletely developed, and not well integrated with ideas concerning rigidity in frictionless systems. Exploiting the observation that athermally sheared sphere assemblies develop structural features necessary for shear jamming even in the absence of friction [H. A. Vinutha and S. Sastry, Nature Physics 12, 578 (2016)1745-247310.1038/nphys3658], we analyze Conditions for jamming in such assemblies computationally. Solving force and torque balance Conditions for their contact geometry, we show, and validate with frictional simulations, that the mean contact number Z equals D+1 (for spatial dimension D=2,3) at jamming for both finite and infinite friction, above the "random loose packing" limit density, at variance with previous analyses of frictional jamming. We show that the shear jamming threshold satisfies the Marginal Stability Condition recently proposed for jamming in frictionless systems. Along lines explored in studying covalent glasses, we perform rigidity percolation analysis for D=2 and find that rigidity percolation precedes shear jamming, which, however, coincides with the percolation of over-constrained regions, leading to the identification of a regime analogous to the intermediate phase observed in covalent glasses. Together, these results provide a geometric description of shear jamming that relate closely with analyses of jamming, rigidity, and the glass transition in frictionless systems, and thus help develop a unified description of jamming phenomenology in diverse disordered matter.

A. V. Zasov - One of the best experts on this subject based on the ideXlab platform.

  • Gravitational Stability and mass estimation of stellar disks
    Astronomische Nachrichten, 2013
    Co-Authors: Anna S. Saburova, A. V. Zasov
    Abstract:

    We estimate the masses of disks of galaxies using the Marginal gravitational Stability criterion and compare them with the photometrical disk mass evaluations. The comparison reveals that the stellar disks of most of spiral galaxies we considered cannot be substantially overheated (at least within several radial scalelengths) and are therefore unlikely to have experienced a significant merging event in their history. However, for substantial part of S0- type galaxies a stellar velocity dispersion is well in excess of the gravitational Stability threshold suggesting a major merger event in the past. For four low surface brightness galaxies we found that the disk masses corresponding to the Marginal Stability Condition are significantly higher than it may be expected from their brightness. Either their disks are dynamically overheated, or they contain a large amount of non-luminous matter.

  • Is dynamic heating of stellar disk inevitable
    2013
    Co-Authors: A. V. Zasov, Anna S. Saburova, Ivan Yu. Katkov
    Abstract:

    Major mergers or/and the repeated minor mergers lead to dynamical heating of disks of galaxies. We analyze the available data on the velocity dispersion of stellar disks of S-S0 galaxies, including the new observational data obtained at 6m telescope of SAO RAS. As a measure of dynamical (over)heating, we use the ratio of the observed velocity dispersion to the minimal dispersion which provides the local Stability of the stellar disks with respect to gravitational perturbations. We came to conclusion that stellar disks in a significant part of galaxies (including LSB and some S0 galaxies) are close to the Marginal Stability Condition (or are slightly overheated) – at least at radial distances r ∼ 2-3 radial scalelenghts. It enables to constrain the role of merging in the heating of stellar disks: in many cases it seems to be non-efficient. Marginal Stability Condition may also be successfully used to estimate the mass of a disk and the midplane volume gas (stars) densities on the basis of kinematic measurements.

  • Gravitational Stability and dynamical overheating of stellar disks of galaxies
    Astronomy Letters, 2011
    Co-Authors: A. V. Zasov, Alexander Khoperskov, Anna S. Saburova
    Abstract:

    We use the Marginal Stability Condition for galactic disks and the stellar velocity dispersion data published by different authors to place upper limits on the disk local surface density at two radial scalelengths $R=2h$. Extrapolating these estimates, we constrain the total mass of the disks and compare these estimates to those based on the photometry and color of stellar populations. The comparison reveals that the stellar disks of most of spiral galaxies in our sample cannot be substantially overheated and are therefore unlikely to have experienced a significant merging event in their history. The same conclusion applies to some, but not all of the S0 galaxies we consider. However, a substantial part of the early type galaxies do show the stellar velocity dispersion well in excess of the gravitational Stability threshold suggesting a major merger event in the past. We find dynamically overheated disks among both seemingly isolated galaxies and those forming pairs. The ratio of the Marginal Stability disk mass estimate to the total galaxy mass within four radial scalelengths remains within a range of 0.4---0.8. We see no evidence for a noticeable running of this ratio with either the morphological type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter