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Shu-ichiro Inutsuka - One of the best experts on this subject based on the ideXlab platform.
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an origin of multiple ring structure and hidden planets in hl tau a unified picture by secular Gravitational Instability
The Astronomical Journal, 2016Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:Recent ALMA observation has revealed multiple ring structures formed in a protoplanetary disk around HL Tau. Prior to the ALMA observation of HL Tau, theoretical analysis of secular Gravitational Instability (GI) described a possible formation of multiple ring structures with separations of 13 AU around a radius of 100 AU in protoplanetary disks under certain conditions. In this article, we reanalyze the viability of secular GI by adopting the physical values inferred from the observations. We derive the radial distributions of the most unstable wavelength and the growth timescale of secular GI and verify that secular GI can form the ring structures observed in HL Tau. When a turbulent viscosity coefficient $\alpha$ remains small in inner region of the disk, secular GI grows in the whole disk. Thus, the formation of planetary mass objects should occur first in the inner region as a result of Gravitational fragmentation after the non-linear growth of secular GI. In this case, resulting objects are expected to create the gaps at $r$ ~ 10 AU and ~ 30 AU. As a result, all ring structures in HL Tau can be created by secular GI. If this scenario is realized in HL Tau, the outer region corresponds to the earlier growth phase of the most unstable mode of secular GI, and the inner region corresponds to the outcome of the non-linear growth of secular GI. Therefore, this interpretation suggests that we are possibly witnessing both the beginning and end of planet formation in HL Tau.
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two component secular Gravitational Instability in a protoplanetary disk a possible mechanism for creating ring like structures
The Astrophysical Journal, 2014Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:The Instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated using linear analysis. In the case where the dust-to-gas ratio is enhanced and turbulence is weak, the Instability grows, even in Gravitationally stable disks, on a timescale of order 104-5 yr at a radius of order 100 AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the Instability reduces to the so-called "secular Gravitational Instability," which was investigated previously to be an Instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular Gravitational Instability in the simplified treatment. This may indicate that we should not neglect small terms in the equation of motion if the growth rate is small. The Instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the Instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the Instability is the accumulation of dust grains, and hence the Instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii ~100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.
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two component secular Gravitational Instability in a protoplanetary disk a possible mechanism for creating ring like structures
arXiv: Earth and Planetary Astrophysics, 2013Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:The Instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated by linear analysis. In the case where the dust to gas ratio is enhanced and turbulence is week, the Instability grows, even in Gravitationally stable disks, on a timescale of order $10^{4- 5}$yr at a radius of order 100AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the Instability reduces to the so-called "secular Gravitational Instability", which was investigated previously as an Instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular Gravitational Instability in the simplified treatment. This may indicate that we should not neglect small terms in equation of motion if the growth rate is small. The Instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the Instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the Instability is the accumulation of dust grains, and hence the Instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii $\sim$100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.
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secular Gravitational Instability of a dust layer in shear turbulence
The Astrophysical Journal, 2012Co-Authors: Shugo Michikoshi, Eiichiro Kokubo, Shu-ichiro InutsukaAbstract:We perform a linear stability analysis of a dust layer in a turbulent gas disk. Youdin investigated the secular Gravitational Instability (GI) of a dust layer using hydrodynamic equations with a turbulent diffusion term. We obtain essentially the same result independently of Youdin. In the present analysis, we restrict the area of interest to small dust particles, while investigating the secular GI in a more rigorous manner. We discuss the time evolution of the dust surface density distribution using a stochastic model and derive the advection-diffusion equation. The validity of the analysis by Youdin is confirmed in the strong drag limit. We demonstrate quantitatively that the finite thickness of a dust layer weakens the secular GI and that the density-dependent diffusion coefficient changes the growth rate. We apply the results obtained to the turbulence driven by the shear Instability and find that the secular GI is faster than the radial drift when the gas density is three times as large as that in the minimum-mass disk model. If the dust particles are larger than chondrules, the secular GI grows within the lifetime of a protoplanetary disk.
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n body simulation of planetesimal formation through Gravitational Instability of a dust layer in laminar gas disk
The Astrophysical Journal, 2010Co-Authors: Shugo Michikoshi, Eiichiro Kokubo, Shu-ichiro InutsukaAbstract:We investigate the formation process of planetesimals from the dust layer by the Gravitational Instability in the gas disk using local N-body simulations. The gas is modeled as a background laminar flow. We study the formation process of planetesimals and its dependence on the strength of the gas drag. Our simulation results show that the formation process is divided into three stages qualitatively: the formation of wake-like density structures, the creation of planetesimal seeds, and their collisional growth. The linear analysis of the dissipative Gravitational Instability shows that the dust layer is secularly unstable although Toomre's Q value is larger than unity. However, in the initial stage, the growth time of the Gravitational Instability is longer than that of the dust sedimentation and the decrease in the velocity dispersion. Thus, the velocity dispersion decreases and the disk shrinks vertically. As the velocity dispersion becomes sufficiently small, the Gravitational Instability finally becomes dominant. Then wake-like density structures are formed by the Gravitational Instability. These structures fragment into planetesimal seeds. The seeds grow rapidly owing to mutual collisions.
Shugo Michikoshi - One of the best experts on this subject based on the ideXlab platform.
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Gravitational Instability of a dust layer composed of porous silicate dust aggregates in a protoplanetary disk
The Astrophysical Journal, 2018Co-Authors: Misako Tatsuuma, Shugo Michikoshi, Eiichiro KokuboAbstract:Planetesimal formation is one of the most important unsolved problems in planet formation theory. In particular, rocky planetesimal formation is difficult because silicate dust grains are easily broken when they collide. It has recently been proposed that they can grow as porous aggregates when their monomer radius is smaller than ~10 nm, which can also avoid the radial drift toward the central star. However, the stability of a layer composed of such porous silicate dust aggregates has not been investigated. Therefore, we investigate the Gravitational Instability (GI) of this dust layer. To evaluate the disk stability, we calculate Toomre's stability parameter Q, for which we need to evaluate the equilibrium random velocity of dust aggregates. We calculate the equilibrium random velocity considering Gravitational scattering and collisions between dust aggregates, drag by mean flow of gas, stirring by gas turbulence, and Gravitational scattering by gas density fluctuation due to turbulence. We derive the condition of the GI using the disk mass, dust-to-gas ratio, turbulent strength, orbital radius, and dust monomer radius. We find that, for the minimum mass solar nebula model at 1 au, the dust layer becomes Gravitationally unstable when the turbulent strength α 10−5. If the dust-to-gas ratio is increased twice, the GI occurs for α 10−4. We also find that the dust layer is more unstable in disks with larger mass, higher dust-to-gas ratio, and weaker turbulent strength, at larger orbital radius, and with a larger monomer radius.
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secular Gravitational Instability of a dust layer in shear turbulence
The Astrophysical Journal, 2012Co-Authors: Shugo Michikoshi, Eiichiro Kokubo, Shu-ichiro InutsukaAbstract:We perform a linear stability analysis of a dust layer in a turbulent gas disk. Youdin investigated the secular Gravitational Instability (GI) of a dust layer using hydrodynamic equations with a turbulent diffusion term. We obtain essentially the same result independently of Youdin. In the present analysis, we restrict the area of interest to small dust particles, while investigating the secular GI in a more rigorous manner. We discuss the time evolution of the dust surface density distribution using a stochastic model and derive the advection-diffusion equation. The validity of the analysis by Youdin is confirmed in the strong drag limit. We demonstrate quantitatively that the finite thickness of a dust layer weakens the secular GI and that the density-dependent diffusion coefficient changes the growth rate. We apply the results obtained to the turbulence driven by the shear Instability and find that the secular GI is faster than the radial drift when the gas density is three times as large as that in the minimum-mass disk model. If the dust particles are larger than chondrules, the secular GI grows within the lifetime of a protoplanetary disk.
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n body simulation of planetesimal formation through Gravitational Instability of a dust layer in laminar gas disk
The Astrophysical Journal, 2010Co-Authors: Shugo Michikoshi, Eiichiro Kokubo, Shu-ichiro InutsukaAbstract:We investigate the formation process of planetesimals from the dust layer by the Gravitational Instability in the gas disk using local N-body simulations. The gas is modeled as a background laminar flow. We study the formation process of planetesimals and its dependence on the strength of the gas drag. Our simulation results show that the formation process is divided into three stages qualitatively: the formation of wake-like density structures, the creation of planetesimal seeds, and their collisional growth. The linear analysis of the dissipative Gravitational Instability shows that the dust layer is secularly unstable although Toomre's Q value is larger than unity. However, in the initial stage, the growth time of the Gravitational Instability is longer than that of the dust sedimentation and the decrease in the velocity dispersion. Thus, the velocity dispersion decreases and the disk shrinks vertically. As the velocity dispersion becomes sufficiently small, the Gravitational Instability finally becomes dominant. Then wake-like density structures are formed by the Gravitational Instability. These structures fragment into planetesimal seeds. The seeds grow rapidly owing to mutual collisions.
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a two fluid analysis of the kelvin helmholtz Instability in the dusty layer of a protoplanetary disk a possible path toward planetesimal formation through Gravitational Instability
The Astrophysical Journal, 2006Co-Authors: Shugo Michikoshi, Shu-ichiro InutsukaAbstract:Weanalyzethestabilityofthedustlayerinprotoplanetarydiskstounderstandtheeffectofrelativemotionbetween gas and dust. Previous analyses not including the effect of the relative motion between gas and dust show that shearinducedturbulencemaypreventthedustgrainsfromsettlingsufficientlytobeGravitationallyunstable.Wedetermine the growth rate of the Kelvin-Helmholtz Instability in a wide range of parameter space and propose a possible path toward planetesimal formation through Gravitational Instability. We expect the density of the dust layer to become � d/� g � 100 if the dust grains can grow up to 10 m. Subject headingg instabilities — planetary systems: formation — turbulence
Sanemichi Z Takahashi - One of the best experts on this subject based on the ideXlab platform.
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an origin of multiple ring structure and hidden planets in hl tau a unified picture by secular Gravitational Instability
The Astronomical Journal, 2016Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:Recent ALMA observation has revealed multiple ring structures formed in a protoplanetary disk around HL Tau. Prior to the ALMA observation of HL Tau, theoretical analysis of secular Gravitational Instability (GI) described a possible formation of multiple ring structures with separations of 13 AU around a radius of 100 AU in protoplanetary disks under certain conditions. In this article, we reanalyze the viability of secular GI by adopting the physical values inferred from the observations. We derive the radial distributions of the most unstable wavelength and the growth timescale of secular GI and verify that secular GI can form the ring structures observed in HL Tau. When a turbulent viscosity coefficient $\alpha$ remains small in inner region of the disk, secular GI grows in the whole disk. Thus, the formation of planetary mass objects should occur first in the inner region as a result of Gravitational fragmentation after the non-linear growth of secular GI. In this case, resulting objects are expected to create the gaps at $r$ ~ 10 AU and ~ 30 AU. As a result, all ring structures in HL Tau can be created by secular GI. If this scenario is realized in HL Tau, the outer region corresponds to the earlier growth phase of the most unstable mode of secular GI, and the inner region corresponds to the outcome of the non-linear growth of secular GI. Therefore, this interpretation suggests that we are possibly witnessing both the beginning and end of planet formation in HL Tau.
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two component secular Gravitational Instability in a protoplanetary disk a possible mechanism for creating ring like structures
The Astrophysical Journal, 2014Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:The Instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated using linear analysis. In the case where the dust-to-gas ratio is enhanced and turbulence is weak, the Instability grows, even in Gravitationally stable disks, on a timescale of order 104-5 yr at a radius of order 100 AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the Instability reduces to the so-called "secular Gravitational Instability," which was investigated previously to be an Instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular Gravitational Instability in the simplified treatment. This may indicate that we should not neglect small terms in the equation of motion if the growth rate is small. The Instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the Instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the Instability is the accumulation of dust grains, and hence the Instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii ~100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.
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two component secular Gravitational Instability in a protoplanetary disk a possible mechanism for creating ring like structures
arXiv: Earth and Planetary Astrophysics, 2013Co-Authors: Sanemichi Z Takahashi, Shu-ichiro InutsukaAbstract:The Instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated by linear analysis. In the case where the dust to gas ratio is enhanced and turbulence is week, the Instability grows, even in Gravitationally stable disks, on a timescale of order $10^{4- 5}$yr at a radius of order 100AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the Instability reduces to the so-called "secular Gravitational Instability", which was investigated previously as an Instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular Gravitational Instability in the simplified treatment. This may indicate that we should not neglect small terms in equation of motion if the growth rate is small. The Instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the Instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the Instability is the accumulation of dust grains, and hence the Instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii $\sim$100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.
Reinaldo J Gleiser - One of the best experts on this subject based on the ideXlab platform.
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Gravitational Instability of einstein gauss bonnet black holes under tensor mode perturbations
Classical and Quantum Gravity, 2005Co-Authors: Gustavo Dotti, Reinaldo J GleiserAbstract:We analyse the tensor mode perturbations of static, spherically symmetric solutions of the Einstein equations with a quadratic Gauss–Bonnet term in dimension D > 4. We show that the evolution equations for this type of perturbation can be cast in a Regge–Wheeler–Zerilli form, and obtain the exact potential for the corresponding Schrodinger-like stability equation. As an immediate application we prove that for D ≠ 6 and α > 0, the sign choice for the Gauss–Bonnet coefficient suggested by string theory, all positive mass black holes of this type are stable. In the exceptional case D = 6, we find a range of parameters where positive mass asymptotically flat black holes, with regular horizon, are unstable. This feature is found also in general for α < 0.
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Gravitational Instability of einstein gauss bonnet black holes under tensor mode perturbations
arXiv: General Relativity and Quantum Cosmology, 2004Co-Authors: Gustavo Dotti, Reinaldo J GleiserAbstract:We analyze the tensor mode perturbations of static, spherically symmetric solutions of the Einstein equations with a quadratic Gauss-Bonnet term in dimension $D > 4$. We show that the evolution equations for this type of perturbations can be cast in a Regge-Wheeler-Zerilli form, and obtain the exact potential for the corresponding Schr\"odinger-like stability equation. As an immediate application we prove that for $D \neq 6$ and $\alpha >0$, the sign choice for the Gauss-Bonnet coefficient suggested by string theory, all positive mass black holes of this type are stable. In the exceptional case $D =6$, we find a range of parameters where positive mass asymptotically flat black holes, with regular horizon, are unstable. This feature is found also in general for $\alpha < 0$.
Seung Soo Hong - One of the best experts on this subject based on the ideXlab platform.
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Gravitational Instability of rotating pressure confined polytropic gas disks with vertical stratification
The Astrophysical Journal, 2012Co-Authors: Jeonggyu Kim, Woongtae Kim, Young Min Seo, Seung Soo HongAbstract:We investigate the Gravitational Instability (GI) of rotating, vertically stratified, pressure-confined, polytropic gas disks using a linear stability analysis as well as analytic approximations. The disks are initially in vertical hydrostatic equilibrium and bounded by a constant external pressure. We find that the GI of a pressure-confined disk is in general a mixed mode of the conventional Jeans and distortional instabilities, and is thus an unstable version of acoustic-surface-gravity waves. The Jeans mode dominates in weakly confined disks or disks with rigid boundaries. On the other hand, when the disk has free boundaries and is strongly pressure confined, the mixed GI is dominated by the distortional mode that is surface-gravity waves driven unstable under their own gravity and thus incompressible. We demonstrate that the Jeans mode is gravity-modified acoustic waves rather than inertial waves and that inertial waves are almost unaffected by self-gravity. We derive an analytic expression for the effective sound speed c eff of acoustic-surface-gravity waves. We also find expressions for the gravity reduction factors relative to a razor-thin counterpart that are appropriate for the Jeans and distortional modes. The usual razor-thin dispersion relation, after correcting for c eff and the reduction factors, closely matches the numerical results obtained by solving a full set of linearized equations. The effective sound speed generalizes the Toomre stability parameter of the Jeans mode to allow for the mixed GI of vertically stratified, pressure-confined disks.
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Gravitational Instability of rotating pressure confined polytropic gas disks with vertical stratification
arXiv: Astrophysics of Galaxies, 2012Co-Authors: Jeonggyu Kim, Woongtae Kim, Young Min Seo, Seung Soo HongAbstract:We investigate Gravitational Instability (GI) of rotating, vertically-stratified, pressure-confined, polytropic gas disks using linear stability analysis as well as analytic approximations. The disks are initially in vertical hydrostatic equilibrium and bounded by a constant external pressure. We find that GI of a pressure-confined disk is in general a mixed mode of the conventional Jeans and distortional instabilities, and is thus an unstable version of acoustic-surface-gravity waves. The Jeans mode dominates in weakly confined disks or disks with rigid boundaries. When the disk has free boundaries and is strongly pressure-confined, on the other hand, the mixed GI is dominated by the distortional mode that is surface-gravity waves driven unstable under own gravity and thus incompressible. We demonstrate that the Jeans mode is gravity-modified acoustic waves rather than inertial waves and that inertial waves are almost unaffected by self-gravity. We derive an analytic expression for the effective sound speed c_eff of acoustic-surface-gravity waves. We also find expressions for the gravity reduction factors relative to a razor-thin counterpart, appropriate for the Jeans and distortional modes. The usual razor-thin dispersion relation after correcting for c_eff and the reduction factors closely matches the numerical results obtained by solving a full set of linearized equations. The effective sound speed generalizes the Toomre stability parameter of the Jeans mode to allow for the mixed GI of vertically-stratified, pressure-confined disks.
