arXiv subjects

Shigeru Wakita

Publications and source records attributed to Shigeru Wakita.

22 records · Page 2Linked to original sources

arXiv2016

Forming Chondrites in a Solar Nebula with Magnetically Induced Turbulence

Chondritic meteorites provide valuable opportunities to investigate the origins of the solar system. We explore impact jetting as a mechanism of chondrule formation and subsequent pebble accretion as a mechanism of accreting chondrules onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the solar nebula is $\le 5$ times more massive than the minimum-mass solar nebula at $a \simeq 2-3$ AU and parent bodies of chondrites are $\le 10^{24}$ g ($\le$ 500 km in radius) in the solar nebula, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, the formation timescale of chondrules, and the magnetic field strength of the nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies' masses leads to the following implications: primordial asteroids that were originally $\ge 10^{24}$ g in mass were unlikely to contain chondrules, while less massive primordial asteroids likely had a chondrule-rich surface layer. The scenario developed from impact jetting and pebble accretion can therefore provide new insights into the origins of the solar system.

astro-ph.EP↗

arXiv2015

Chondrule Formation via Impact Jetting Triggered by Planetary Accretion

Chondrules are one of the most primitive elements that can serve as a fundamental clue as to the origin of our Solar system. We investigate a formation scenario of chondrules that involves planetesimal collisions and the resultant impact jetting. Planetesimal collisions are the main agent to regulate planetary accretion that corresponds to the formation of terrestrial planets and cores of gas giants. The key component of this scenario is that ejected materials can melt when the impact velocity between colliding planetesimals exceeds about 2.5 km s$^{-1}$. The previous simulations show that the process is efficient enough to reproduce the primordial abundance of chondrules. We examine this scenario carefully by performing semi-analytical calculations that are developed based on the results of direct $N$-body simulations. As found by the previous work, we confirm that planetesimal collisions that occur during planetary accretion can play an important role in forming chondrules. This arises because protoplanet-planetesimal collisions can achieve the impact velocity of about 2.5 km s$^{-1}$ or higher, as protoplanets approach the isolation mass ($M_{p,iso}$). Assuming that the ejected mass is a fraction ($F_{ch}$) of colliding planetesimals' mass, we show that the resultant abundance of chondrules is formulated well by $F_{ch}M_{p,iso}$, as long as the formation of protoplanets is completed within a given disk lifetime. We perform a parameter study and examine how the abundance of chondrules and their formation timing change. We find that the impact jetting scenario generally works reasonably well for a certain range of parameters, while more dedicated work would be needed to include other physical processes that are neglected in this work and to examine their effects on chondrule formation.

astro-ph.EP↗

arXiv2015

Probing the Physical Conditions of Supernova Ejecta with the Measured Sizes of Presolar Al2O3 Grains

A few particles of presolar Al2O3 grains with sizes above 0.5 mum are believed to have been produced in the ejecta of core-collapse supernovae (SNe). In order to clarify the formation condition of such large Al2O3 grains, we investigate the condensation of Al2O3 grains for wide ranges of the gas density and cooling rate. We first show that the average radius and condensation efficiency of newly formed Al2O3 grains are successfully described by a non-dimensional quantity "Lambda_on" defined as the ratio of the timescale with which the supersaturation ratio increases to the collision timescale of reactant gas species at dust formation. Then, we find that the formation of submicron-sized Al2O3 grains requires at least ten times higher gas densities than those presented by one-dimensional SN models. This indicates that presolar Al2O3 grains identified as a SN origin might be formed in dense gas clumps, allowing us to propose that the measured sizes of presolar grains can be a powerful tool to constrain the physical conditions in which they formed. We also briefly discuss the survival of newly formed Al2O3 grains against the destruction in the shocked gas within the SN remnants.

astro-ph.SR↗

arXiv2014

Thermal and Impact History of the H Chondrite Parent Asteroid during Metamorphism: Constraints from Metallic Fe-Ni

We have studied cloudy taenite, metallographic cooling rates, and shock effects in 30 H3-6 chondrites to elucidate the thermal and early impact history of the H chondrite parent body. We focused on H chondrites with Ar-Ar ages greater than 4.4 Gyr and unshocked and mildly shocked H chondrites, as strongly shocked chondrites with such old ages are very rare. Cooling rates for most H chondrites at 500 C are 10-50 C/Myr and do not decrease systematically with increasing petrologic type as predicted by the onion-shell model in which types 3 to 5 are arranged in concentric layers around a type 6 core. Some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types, contrary to the onion-shell model. Cloudy taenite particle sizes, which range from 40 to 120 nm, are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating. The three H4 chondrites that were used to develop the onion-shell model, Ste. Marguerite, Beaver Creek, and Forest Vale, cooled through 500 C at ~5000 C/Myr. Our thermal modeling shows that these rates are 50 times higher than could be achieved in a body that was heated by 26Al and cooled without disturbance by impact. Published Ar-Ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism. Metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating. Impacts during metamorphism created Portales Valley and two other H6 chondrites with large metallic veins, excavated the fast-cooled H4 chondrites around 3-4 Myr after accretion, and mixed petrologic types.

astro-ph.EP↗