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Register a new userRotational Disruption of Porous Dust Aggregates due to Gas Flow in Protoplanetary Disks
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We introduce a possible disruption mechanism of dust grains in planet formation by their spinning motion. This mechanism has been discussed as rotational disruption for the interstellar dust grains. We theoretically calculate whether porous dust aggregates can be disrupted by their spinning motion and if it prohibits dust growth in protoplanetary disks. We assume radiative torque and gas-flow torque as driving sources of the spinning motion, assume that dust aggregates reach a steady-state rigid rotation, and compare the obtained tensile stress due to the centrifugal force with their tensile strength. We model the irregularly-shaped dust aggregates by introducing a parameter, $gamma_mathrm{ft}$, that mimics the conversion efficiency from force to torque. As a result, we find that porous dust aggregates are rotationally disrupted by their spinning motion induced by gas flow when their mass is larger than $sim10^8$ g and their volume filling factor is smaller than $sim 0.01$ in our fiducial model, while relatively compact dust aggregates with volume filling factor more than 0.01 do not face this problem. If we assume the dust porosity evolution, we find that dust aggregates whose Stokes number is $sim0.1$ can be rotationally disrupted in their growth and compression process. Our results suggest that the growth of dust aggregates may be halted due to rotational disruption or that other compression mechanisms are needed to avoid it. We also note that dust aggregates are not rotationally disrupted when $gamma_mathrm{ft}leq0.02$ in our fiducial model and the modeling of irregularly-shaped dust aggregates is essential in future work.
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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. Recently, it has been proposed that they can grow as porous aggregates when their monomer radius is smaller than $sim$ 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 of this dust layer. To evaluate the disk stability, we calculate Toomres 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 gravitational instability 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 $alphalesssim10^{-5}$. If the dust-to-gas ratio is increased twice, the gravitational instability occurs for $alphalesssim10^{-4}$. 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.
Comets are thought to have information about the formation process of our solar system. Recently, detailed information about comet 67P/Churyumov-Gerasimenko has been found by a spacecraft mission Rosetta. It is remarkable that its tensile strength was estimated. In this paper, we measure and formulate the tensile strength of porous dust aggregates using numerical simulations, motivated by porous dust aggregation model of planetesimal formation. We perform three-dimensional numerical simulations using a monomer interaction model with periodic boundary condition. We stretch out a dust aggregate with a various initial volume filling factor between $10^{-2}$ and 0.5. We find that the tensile stress takes the maximum value at the time when the volume filling factor decreases to about a half of the initial value. The maximum stress is defined to be the tensile strength. We take an average of the results with 10 different initial shapes to smooth out the effects of initial shapes of aggregates. Finally, we numerically obtain the relation between the tensile strength and the initial volume filling factor of dust aggregates. We also use a simple semi-analytical model and successfully reproduce the numerical results, which enables us to apply to a wide parameter range and different materials. The obtained relation is consistent with previous experiments and numerical simulations about silicate dust aggregates. We estimate that the monomer radius of comet 67P has to be about 3.3--220 $mathrm{mu m}$ to reproduce its tensile strength using our model.
Theoretical models of the ionization state in protoplanetary disks suggest the existence of large areas with low ionization and weak coupling between the gas and magnetic fields. In this regime hydrodynamical instabilities may become important. In this work we investigate the gas and dust structure and dynamics for a typical T Tauri system under the influence of the vertical shear instability (VSI). We use global 3D radiation hydrodynamics simulations covering all $360^circ$ of azimuth with embedded particles of 0.1 and 1mm size, evolved for 400 orbits. Stellar irradiation heating is included with opacities for 0.1- to 10-$mu$m-sized dust. Saturated VSI turbulence produces a stress-to-pressure ratio of $alpha simeq 10^{-4}$. The value of $alpha$ is lowest within 30~au of the star, where thermal relaxation is slower relative to the orbital period and approaches the rate below which VSI is cut off. The rise in $alpha$ from 20 to 30~au causes a dip in the surface density near 35~au, leading to Rossby wave instability and the generation of a stationary, long-lived vortex spanning about 4~au in radius and 40~au in azimuth. Our results confirm previous findings that mm size grains are strongly vertically mixed by the VSI. The scale height aspect ratio for 1mm grains is determined to be 0.037, much higher than the value $H/r=0.007$ obtained from millimeter-wave observations of the HL~Tau system. The measured aspect ratio is better fit by non-ideal MHD models. In our VSI turbulence model, the mm grains drift radially inwards and many are trapped and concentrated inside the vortex. The turbulence induces a velocity dispersion of $sim 12$~m/s for the mm grains, indicating that grain-grain collisions could lead to fragmentation.
Planet formation is thought to begin with the growth of dust particles in protoplanetary disks from micrometer to millimeter and centimeter sizes. Dust growth is hindered by a number of growth barriers, according to dust evolution theory, while observational evidence indicates that somehow these barriers must have been overcome. The observational evidence of dust traps, in particular the Oph IRS 48 disk, with the Atacama Large Millimeter/submillimeter Array (ALMA) has changed our view of the dust growth process. In this article I review the history of dust trapping in models and observations.
We present a novel mechanism for the outward transport of crystalline dust particles: the outward radial drift of pebbles. The dust ring structure is frequently observed in protoplanetary disks. One of the plausible mechanisms of the formation of dust rings is the accumulation of pebbles around the pressure maximum, which is formed by the mass loss due to magnetically driven disk winds. In evolving protoplanetary disks due to magnetically driven disk winds, dust particles can migrate outwardly from the crystallization front to the pressure maximum by radial drift. We found that the outward radial drift process can transport crystalline dust particles efficiently when the radial drift timescale is shorter than the advection timescale. Our model predicts that the crystallinity of silicate dust particles could be as high as 100% inside the dust ring position.
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