Papers by Tetsuo Yamamoto
We analyzed the mechanism of low-temperature crystallization of an amorphous silicate grain due t... more We analyzed the mechanism of low-temperature crystallization of an amorphous silicate grain due to exothermic chemical reactions in the surface layer of the grains as demonstrated experimentally (Kaito et al. 2007, ApJ. 666, L57). We constructed a model of the crystallization and derived the crystallization conditions of the amorphous silicate core of the grain. The analysis enables us to obtain the crystallization conditions in protoplanetary disks. Our results suggest that silicate crystallization occurs in a wider variety of conditions than hitherto considered.
Dust particles in orbit around a star drift toward the central star by the Poynting-Robertson eff... more Dust particles in orbit around a star drift toward the central star by the Poynting-Robertson effect and pile up by sublimation. We analytically derive the pile-up magnitude, adopting a simple model for optical cross sections. As a result, we find that the sublimation temperature of drifting dust particles plays the most important role in the pile-up rather than their optical property does. Dust particles with high sublimation temperature form a significant dust ring, which could be found in the vicinity of the sun through in-situ spacecraft measurements. While the existence of such a ring in a debris disk could not be identified in the spectral energy distribution (SED), the size of a dust-free zone shapes the SED. Since we analytically obtain the location and temperature of sublimation, these analytical formulae are useful to find such sublimation evidences.
Publications- Astronomical Society of Japan
ABSTRACT
Astronomy and Astrophysics
We study collisions between dust aggregates to construct a model of their structural evolution in... more We study collisions between dust aggregates to construct a model of their structural evolution in protoplanetary disks. We carry out three-dimensional simulations of aggregate collisions and examine their compression and disruption processes following our previous two-dimensional simulations. We take clusters of ballistic cluster-cluster aggregation (BCCA) formed by a hit-and-stick process as initial structures and study their head-on collisions with the use of realistic binding forces. Our numerical results indicate that the energy criteria for compression and disruption of BCCA clusters are consistent with previous two-dimensional simulations. For aggregate compression at a collision, we succeed in obtaining a scaling law in which the gyration radius of the resultant aggregate is proportional to E À0:10 imp , where E imp is the impact energy. Furthermore, we derive an ''equation of state'' of aggregates which reproduces the scaling law for compression. The equation of state is useful for describing the density evolution of dust aggregates during their growth.
Astronomy and Astrophysics
Context. The observed sodium abundance of meteoroids in meteor showers might differ from the orig... more Context. The observed sodium abundance of meteoroids in meteor showers might differ from the original abundance because of processing in interplanetary space after ejections from their parent bodies. Among various processes, thermal alteration of alkali silicate is most likely the major process of Na depletion. Aims. We clarify at which perihelion distances the thermal desorption alters the Na content of meteoroids that are observed as meteor showers. Methods. We compile Na abundances of meteoroids in meteor showers at each perihelion distance and compare them to the sublimation temperatures of alkali silicates. Results. Na abundances of meteoroids do not depend on their perihelion distances at 0.14 ≤ q ≤ 0.99 AU. No Na depletion in these distances constrains the temperature of meteoroids at q = 0.14 AU to be lower than the sublimation temperature of alkali silicates ∼900 K. Conclusions. Meteoroid particles are characterized as large, compact, blackbody-like particles. On orbit with perihelion distances q < 0.1 AU, meteoroids would show evidence of thermal desorption of metals, in particular, Na.
Growth and disruption process of dust aggregates by their mutual collisions is important for unde... more Growth and disruption process of dust aggregates by their mutual collisions is important for understanding planetesimal formation in protoplanetary disks. We perform numerical simulations of aggregate collisions using two kinds of aggregates of submicrometer-sized spheres. First, we carry out numerical simulations of head-on collisions between aggregates having ballistic cluster-cluster aggregation (BCCA) structures to reveal their compression process. We obtain a scaling law on the compressed structure of the BCCA clusters and find that the compressed aggregates still remain fluffy with a fractal dimension ∼ 2.5. Second, we simulate collisions between aggregates having ballistic particle-cluster aggregation (BPCA) structures to examine their disruption process. Our results show that the BPCA clusters consisting of ice particles are able to grow at collision velocities up to 60 m s −1 , even if offset collisions are taken into account. This supports a scenario of planetesimal formation through collisions of dust aggregates in protoplanetary disks.
The impact of solar wind particles exerts a drag force on interplanetary dust grains in the same ... more The impact of solar wind particles exerts a drag force on interplanetary dust grains in the same manner as electro-magnetic drag so called the Poynting-Robertson effect. We study this plasma drag for fluffy aggregates particles with assumptions for the shape structure, and composition of the aggregates plausible for dust in the solar system. First, we calculate the plasma drag for single spherical grains taking into account the penetration of incident solar wind ions as a basis of evaluating the drag for fluffy aggregates consisting of spherical monomers. We find that the size dependence of the plasma drag is analogous to the electro-magnetic drag. Namely, 1) for the monomers smaller than the penetration depth of the solar wind ions, the plasma drag is proportional to the volume of the grain, whereas 2) for the monomers larger than the penetration depth, the drag is proportional to the geometrical cross section of a monomer. Although the penetration of solar wind ions through a mono...
In protoplanetary disks, dust aggregates consisting of submicron particles grow through their mut... more In protoplanetary disks, dust aggregates consisting of submicron particles grow through their mutual collisions. In the early growth stage, dust aggregates collide with each other at extremely low velocities (< 1 mm/s) due to strong coupling with disk gas. Because of such low-velocity collisions, the growing dust aggregates are thought to have a structure similar to the ballistic cluster-cluster aggregation (BCCA) clusters, which are formed by a series of hit-and-sticks of comparable aggregates and have a fluffy structure with fractal dimension 2. Such BCCA-like aggregates would be compressed with increasing collision energy and may end up with disruption at high-velocity collisions. We perform direct N-body simulations of aggregate collisions with the use of realistic binding forces in order to investigate the collisional growth and disruption processes of dust aggregates. As a result, we find that aggregates formed through collisions between BCCA clusters have a fractal dimensi...
We carry out numerical simulations of collisions of aggregates with various coordination numbers ... more We carry out numerical simulations of collisions of aggregates with various coordination numbers to investigate the bouncing conditions and collosional growth possibility of dust in protoplanetary disks.
Proceedings of the International Astronomical Union, 2005
Dust particles in the solar system are produced from the small bodies: asteroids, comets, meteoro... more Dust particles in the solar system are produced from the small bodies: asteroids, comets, meteoroids and Kuiper belt objects. A further source of dust is provided by the warm interstellar medium that the Sun is currently embedded in and that streams into the solar system. We review the physical properties of solar system dust and trace back its interrelation with the small solar system bodies. Comets contain relatively pristine material that they transport to the inner solar system. The alteration of dust in the vicinity of comets is complex and connected to the gas evolution, but a significant part of the organic dust material survives these alterations. The optical properties of cometary dust are best described with a mixture of silicate and carbon bearing materials. As far as the darkness of the cometary material is concerned, according to recent models, this is not a result of the porosity, but rather of the darkness of the carbon bearing component. This does not contradict the observation of silicate features in the thermal emission brightness of cometary dust, since porous mixtures of silicate and carbon bearing dust can produce the observed polarization and albedo characteristics, as well as the silicate features. The carbon-bearing component is most likely an organic refactory component. The relative contributions of different sources change within the solar system dust cloud and depend as well on the measurement technique considered. In particular, the dust from asteroids, which provides a large component of the dust near Earth orbit, is also preferably seen with most of the detection methods. The majority of dust inward from 1 AU is produced from cometary dust and meteoroids. Dust material evaporation induced by collisions inward from 1 AU produces a minor heavy ion component in the solar wind plasma known as inner source pick-up ions.
Conference on Electromagnetic and Light Scattering, 2007
Polarimetry of comets reveals significantly higher polarization in jets than in circumnucleus hal... more Polarimetry of comets reveals significantly higher polarization in jets than in circumnucleus halos. We hypothesize that this difference arises from distinction in the velocity of dust: particles in jets move out of a nucleus much faster than those in a halo and thus we may observe jet particles in the early stage of their evolution when they are large agglomerates of small grains fastened by a certain amount of ice; velocity of particles in a circumnucleus halo is low enough to be observed in the latter stage of evolution when ice already sublimated totally and agglomerates have been disrupted into constituent grains. Using the discrete dipole approximation we study the influence of disruption of agglomerated dust particles due to ice sublimation on their angular dependence of degree of linear polarization. We found that in a wide range of phase angles the linear polarization of agglomerates of three grains comparable to wavelength covered by ice is visibly higher than polarization of independently scattering constituent grains. This supports our interpretation of the polarimetric observations of comets.
It is proposed that planetesimals perturbed by Jovian mean-motion resonances are the source of sh... more It is proposed that planetesimals perturbed by Jovian mean-motion resonances are the source of shock waves that form chondrules. It is considered that this shock-induced chondrule formation requires the velocity of the planetesimal relative to the gas disk to be on the order of 7 kms −1 at 1 AU. In previous studies on planetesimal excitation, the effects of Jovian mean-motion resonance together with the gas drag were investigated, but the velocities obtained were at most 8 kms −1 in the asteroid belt, which is insufficient to account for the ubiquitous existence of chondrules. In this paper, we reexamine the effect of Jovian resonances and take into account the secular resonance in the asteroid belt caused by the gravity of the gas disk. We find that the velocities relative to the gas disk of planetesimals a few hundred kilometers in size exceed 12 kms −1 , and that this is achieved around the 3:1 mean-motion resonance. The heating region is restricted to a relatively narrow band between 1.5 AU and 3.5 AU. Our results suggest that chondrules were produced effectively in the asteroid region after Jovian formation. We also find that many planetesimals are scattered far beyond Neptune. Our findings can explain the presence of crystalline silicate in comets if the scattered planetesimals include silicate dust processed by shock heating.
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Papers by Tetsuo Yamamoto