arXiv subjects

Satoshi Okuzumi

Publications and source records attributed to Satoshi Okuzumi.

118 records · Page 7Linked to original sources

arXiv2012

Geometrical Cross Sections of Dust Aggregates and a Compression Model for Aggregate Collisions

Geometrical cross sections of dust aggregates determine their coupling with disk gas, which governs their motions in protoplanetary disks. Collisional outcomes also depend on geometrical cross sections of initial aggregates. In the previous paper, we performed three-dimensional N-body simulations of sequential collisions of aggregates composed of a number of sub-micron-sized icy particles and examined radii of gyration (and bulk densities) of the obtained aggregates. We showed that collisional compression of aggregates is not efficient and that aggregates remain fluffy. In the present study, we examine geometrical cross sections of the aggregates. Their cross sections decreases due to the compression as well as their gyration radii. It is found that a relation between the cross section and the gyration radius proposed by Okuzumi et al. is valid for the compressed aggregates. We also refine the compression model proposed in our previous paper. The refined model enables us to calculate the evolution of both gyration radii and cross sections of growing aggregates and reproduces well our numerical results of sequential aggregate collisions. The refined model can describe non-equal-mass collisions as well as equal-mass case. Although we do not take into account oblique collisions in the present study, oblique collisions would further hinder compression of aggregates.

astro-ph.EP↗

arXiv2011

Induced Turbulence and the Density Structure of the Dust Layer in a Protoplanetary Disk

We study the turbulence induced in the dust layer of a protoplanetary disk based on the energetics of dust accretion due to gas drag. We estimate turbulence strength from the energy supplied by dust accretion, using the radial drift velocity of the dust particles in a laminar disk. Our estimate of the turbulence strength agrees with previous analytical and numerical research on the turbulence induced by Kelvin-Helmholtz and/or streaming instabilities for particles whose stopping time is less than the Keplerian time. For such small particles, the strongest turbulence is expected to occur when the dust-to-gas ratio of the disk is ~C_eff^(1/2) (h_g / r) ~ 10^(-2), where C_eff ~ 0.2 represents the energy supply efficiency to turbulence and h_g / r ~ 5 x 10^(-2) is the aspect ratio of the gas disk. The maximum viscosity parameter is alpha_max ~ C_eff T_s (h_g / r)^2 ~ 10^(-4) T_s, where T_s (<1) is the non-dimensional stopping time of the dust particles. Modification in the dust-to-gas ratio from the standard value, 10^(-2), by any process, results in weaker turbulence and a thinner dust layer, and consequently may accelerate the growth process of the dust particles.

astro-ph.EP↗

arXiv2011

A Fast and Accurate Calculation Scheme for Ionization Degrees in Protoplanetary and Circumplanetary Disks with Charged Dust Grains

We develop a fast and accurate calculation method for ionization degrees in protoplanetary and circumplanetary disks including dust grains. We apply our method to calculate the ionization degree of circumplanetary disks. It is important to understand the structure and evolution of protoplanetary/circumplanetary disks since they are thought to be the sites of planet/satellite formation. The turbulence that causes gas accretion is supposed to be driven by magnetorotational instability (MRI) that occurs only when the ionization degree is high enough for magnetic field to be coupled to gas. We calculate the ionization degrees in circumplanetary disks to estimate the sizes of MRI-inactive regions. We properly include the effect of dust grains because they efficiently capture charged particles and make ionization degree lower. Inclusion of dust grains complicates the reaction equations and requires expensive computation. In order to accelerate the calculation of ionization reactions, we develop a semianalytic method based on the charge distribution model proposed previously. This method enables us to study the ionization state of disks for a wide range of model parameters. For a previous model of circum-Jovian disk, we find that an MRI-inactive region covers almost all regions even without dust grains. This suggests that the gas accretion rates in circumplanetary disks are much smaller than previously thought.

astro-ph.EP↗

arXiv2011

Modeling Magnetorotational Turbulence in Protoplanetary Disks with Dead Zones

Turbulence driven by magnetorotational instability (MRI) crucially affects the evolution of solid bodies in protoplanetary disks. On the other hand, small dust particles stabilize MRI by capturing ionized gas particles needed for the coupling of the gas and magnetic fields. To provide an empirical basis for modeling the coevolution of dust and MRI, we perform three-dimensional, ohmic-resistive MHD simulations of a vertically stratified shearing box with an MRI-inactive "dead zone" of various sizes and with a net vertical magnetic flux of various strengths. We find that the vertical structure of turbulence is well characterized by the vertical magnetic flux and three critical heights derived from the linear analysis of MRI in a stratified disk. In particular, the turbulent structure depends on the resistivity profile only through the critical heights and is insensitive to the details of the resistivity profile. We discover scaling relations between the amplitudes of various turbulent quantities (velocity dispersion, density fluctuation, vertical diffusion coefficient, and outflow mass flux) and vertically integrated accretion stresses. We also obtain empirical formulae for the integrated accretion stresses as a function of the vertical magnetic flux and the critical heights. These empirical relations allow to predict the vertical turbulent structure of a protoplanetary disk for a given strength of the magnetic flux and a given resistivity profile.

astro-ph.EP↗

arXiv2011

Electrostatic Barrier against Dust Growth in Protoplanetary Disks. II. Measuring the Size of the "Frozen" Zone

Coagulation of submicron-sized dust grains into porous aggregates is the initial step of dust evolution in protoplanetary disks. Recently, it has been pointed out that negative charging of dust in the weakly ionized disks could significantly slow down the coagulation process. In this paper, we apply the growth criteria obtained in Paper I to finding out a location ("frozen" zone) where the charging stalls dust growth at the fractal growth stage. For low-turbulence disks, we find that the frozen zone can cover the major part of the disks at a few to 100 AU from the central star. The maximum mass of the aggregates is approximately 10^{-7} g at 1 AU and as small as a few monomer masses at 100 AU. Strong turbulence can significantly reduce the size of the frozen zone, but such turbulence will cause the fragmentation of macroscopic aggregates at later stages. We examine a possibility that complete freezeout of dust evolution in low-turbulence disks could be prevented by global transport of dust in the disks. Our simple estimation shows that global dust transport can lead to the supply of macroscopic aggregates and the removal of frozen aggregates on a timescale of 10^6 yr. This overturns the usual understanding that tiny dust particles get depleted on much shorter timescales unless collisional fragmentation is effective. The frozen zone together with global dust transport might explain "slow" (\sim 10^6 yr) dust evolution suggested by infrared observation of T Tauri stars and by radioactive dating of chondrites.

astro-ph.EP↗

arXiv2011

Electrostatic Barrier against Dust Growth in Protoplanetary Disks. I. Classifying the Evolution of Size Distribution

Collisional growth of submicron-sized dust grains into macroscopic aggregates is the first step of planet formation in protoplanetary disks. These grains are expected to carry nonzero negative charges in the weakly ionized disks, but its effect on their collisional growth has not been fully understood so far. In this paper, we investigate how the charging affects the evolution of the dust size distribution properly taking into account the charging mechanism in a weakly ionized gas as well as porosity evolution through low-energy collisions. To clarify the role of the size distribution, we divide our analysis into two steps. First, we analyze the collisional growth of charged aggregates assuming a monodisperse (i.e., narrow) size distribution. We show that the monodisperse growth stalls due to the electrostatic repulsion when a certain condition is met, as is already expected in the previous work. Second, we numerically simulate dust coagulation using Smoluchowski's method to see how the outcome changes when the size distribution is allowed to freely evolve. We find that, under certain conditions, the dust undergoes bimodal growth where only a limited number of aggregates continue to grow carrying the major part of the dust mass in the system. This occurs because remaining small aggregates efficiently sweep up free electrons to prevent the larger aggregates from being strongly charged. We obtain a set of simple criteria that allows us to predict how the size distribution evolves for a given condition. In Paper II (arXiv:1009.3101), we apply these criteria to dust growth in protoplanetary disks.

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arXiv2009

Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates

Porosity evolution of dust aggregates is crucial in understanding dust evolution in protoplanetary disks. In this study, we present useful tools to study the coagulation and porosity evolution of dust aggregates. First, we present a new numerical method for simulating dust coagulation and porosity evolution as an extension of the conventional Smoluchowski equation. This method follows the evolution of the mean porosity for each aggregate mass simultaneously with the evolution of the mass distribution function. This method reproduces the results of previous Monte Carlo simulations with much less computational expense. Second, we propose a new collision model for porous dust aggregates on the basis of our N-body experiments on aggregate collisions. We first obtain empirical data on porosity changes between the classical limits of ballistic cluster-cluster and particle-cluster aggregation. Using the data, we construct a recipe for the porosity change due to general hit-and-stick collisions as well as formulae for the aerodynamical and collisional cross sections. Simple coagulation simulations using the extended Smoluchowski method show that our collision model explains the fractal dimensions of porous aggregates observed in a full N-body simulation and a laboratory experiment. Besides, we discover that aggregates at the high-mass end of the distribution can have a considerably small aerodynamical cross section per unit mass compared with aggregates of lower masses. We point out an important implication of this discovery for dust growth in protoplanetary disks.

astro-ph.EP↗

arXiv2009

Electric charging of dust aggregates and its effect on dust coagulation in protoplanetary disks

Mutual sticking of dust aggregates is the first step toward planetesimal formation in protoplanetary disks. In spite that the electric charging of dust particles is well recognized in some contexts, it has been largely ignored in the current modeling of dust coagulation. In this study, we present a general analysis of the dust charge state in protoplanetary disks, and then demonstrate how the electric charging could dramatically change the currently accepted scenario of dust coagulation. First, we describe a new semianalytical method to calculate the dust charge state and gas ionization state self-consistently. This method is far more efficient than previous numerical methods, and provides a general and clear description of the charge state of gas-dust mixture. Second, we apply this analysis to early evolutionary stages where the dust has been thought to grow into fractal ($D \sim 2$) aggregates with a quasi-monodisperse (i.e., narrow) size distribution. We find that, for a wide range of model parameters, the fractal growth is strongly inhibited by the electric repulsion between colliding aggregates and eventually "freezes out" on its way to the subsequent growth stage involving collisional compression. Strong disk turbulence would help the aggregates to overcome this growth barrier, but then it would cause catastrophic collisional fragmentation in later growth stages. These facts suggest that the combination of electric repulsion and collisional fragmentation would impose a serious limitation on dust growth in protoplanetary disks. We propose a possible scenario of dust evolution after the freeze-out. Finally, we point out that the fractal growth of dust aggregates tends to maintain a low ionization degree and, as a result, a large magnetorotationally stable region in the disk.

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arXiv2008

Possible discovery of a nonlinear tail and second-order quasinormal modes in black hole ringdown

We investigate the nonlinear evolution of black hole ringdown in the framework of higher-order metric perturbation theory. By solving the initial-value problem of a simplified nonlinear field model analytically as well as numerically, we find that (i) second-order quasinormal modes (QNMs) are indeed excited at frequencies different from those of first-order QNMs, as predicted recently. We also find serendipitously that (ii) late-time evolution is dominated by a new type of power-law tail. This ``second-order power-law tail'' decays more slowly than any late-time tails known in the first-order (i.e., linear) perturbation theory, and is generated at the wavefront of the first-order perturbation by an essentially nonlinear mechanism. These nonlinear components should be particularly significant for binary black hole coalescences, and could open a new precision science in gravitational wave studies.

gr-qc↗

arXiv2007

Quasinormal ringing of acoustic black holes in Laval nozzles: Numerical simulations

Quasinormal ringing of acoustic black holes in Laval nozzles is discussed. The equation for sounds in a transonic flow is written into a Schrödinger-type equation with a potential barrier, and the quasinormal frequencies are calculated semianalytically. From the results of numerical simulations, it is shown that the quasinormal modes are actually excited when the transonic flow is formed or slightly perturbed, as well as in the real black hole case. In an actual experiment, however, the purely-outgoing boundary condition will not be satisfied at late times due to the wave reflection at the end of the apparatus, and a late-time ringing will be expressed as a superposition of "boxed" quasinormal modes. It is shown that the late-time ringing damps more slowly than the ordinary quasinormal ringing, while its central frequency is not greatly different from that of the ordinary one. Using this fact, an efficient way for experimentally detecting the quasinormal ringing of an acoustic black hole is discussed.

gr-qc↗