Accretion Rates of Planetesimals by Protoplanets Embedded in Nebular Gas

When protoplanets growing by accretion of planetesimals have atmospheres, small planetesimals approaching the protoplanets lose their energy by gas drag from the atmospheres, which leads them to be captured within the Hill sphere of the protoplanets. As a result, growth rates of the protoplanets are enhanced. In order to study the effect of an atmosphere on planetary growth rates, we performed numerical integration of orbits of planetesimals for a wide range of orbital elements and obtained the effective accretion rates of planetesimals onto planets that have atmospheres. Numerical results are obtained as a function of planetesimals' eccentricity, inclination, planet's radius, and non-dimensional gas-drag parameters which can be expressed by several physical quantities such as the radius of planetesimals and the mass of the protoplanet. Assuming that the radial distribution of the gas density near the surface can be approximated by a power-law, we performed analytic calculation for the loss of planetesimals' kinetic energy due to gas drag, and confirmed agreement with numerical results. We confirmed that the above approximation of the power-law density distribution is reasonable for accretion rate of protoplanets with one to ten Earth-masses, unless the size of planetesimals is too small. We also calculated the accretion rates of planetesimals averaged over a Rayleigh distribution of eccentricities and inclinations, and derived a semi-analytical formula of accretion rates, which reproduces the numerical results very well. Using the obtained expression of the accretion rate, we examined the growth of protoplanets in nebular gas. We found that the effect of atmospheric gas drag can enhance the growth rate significantly, depending on the size of planetesimals.Comment: 41 pages, 14 figures, accepted for publication in Icaru

Special issue “Science of solar system materials examined from Hayabusa and future missions (II)”

Six years have passed since the first asteroid sample was returned from the S-type near-Earth asteroid 25143 Itokawa by the JAXA’s Hayabusa mission in 2010 (Yada et al. 2014). Considerable progress has been made in the study of surface regolith materials and the understanding of planetary surface processes such as space weathering (Noguchi et al. 2011), the chronology of Itokawa and its dynamic evolution processes (Nagao et al. 2011; Park et al. 2015), and the thermal alteration undergone in parent bodies (Nakamura T et al. 2011). Discussions of new findings from the Hayabusa-returned samples and from a large collection of meteorites, micrometeorites, and interplanetary dust particles have continued, especially at the annual international Hayabusa symposia of solar system materials (Okada et al. 2015). Progress in sample return science has driven the next stage of exploration. Now, two new sample return missions to primitive, volatile-rich asteroids, JAXA’s Hayabusa2 (Tsuda et al. 2013) and NASA’s OSIRIS-REx (Lauretta et al. 2012), are en route to their target bodies, C-type 162173 Ryugu and B-type 101955 Bennu, respectively. It is our great pleasure to present our second special issue of the journal Earth, Planets and Space, “Science of solar system materials examined from Hayabusa and future missions (II).” This special issue is based on discussions during the Hayabusa 2014 symposium, which featured new results from Hayabusa-returned samples and related studies, but was also open to any scientific results regarding primitive bodies and the early solar system, the results of laboratory experiments and ground-based observations, and reports of new instruments and methods. We will begin with a brief introduction to the missions of the Hayabusa and its successor Hayabusa2. In addition, all six manuscripts published in this special issue are reviewed below

Erratum to: Special issue “Science of solar system materials examined from Hayabusa and future missions (II)”

In the version of this article that was originally published (Okada et al. 2017), there was an error in the title. The phrase “Preface: The Earth, Planets and Space” has been removed from the beginning of the title and the original article has been updated. The publisher apologises for these errors

Distribution of Accreting Gas and Angular Momentum onto Circumplanetary Disks

We investigate gas accretion flow onto a circumplanetary disk from a protoplanetary disk in detail by using high-resolution three-dimensional nested-grid hydrodynamic simulations, in order to provide a basis of formation processes of satellites around giant planets. Based on detailed analyses of gas accretion flow, we find that most of gas accretion onto circumplanetary disks occurs nearly vertically toward the disk surface from high altitude, which generates a shock surface at several scale heights of the circumplanetary disk. The gas that has passed through the shock surface moves inward because its specific angular momentum is smaller than that of the local Keplerian rotation, while gas near the midplane in the protoplanetary disk cannot accrete to the circumplanetary disk. Gas near the midplane within the planet's Hill sphere spirals outward and escapes from the Hill sphere through the two Lagrangian points L$_1$ and L$_2$. We also analyze fluxes of accreting mass and angular momentum in detail and find that the distributions of the fluxes onto the disk surface are well described by power-law functions and that a large fraction of gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times the Hill radius. The nature of power-law functions indicates that, other than the outer edge, there is no specific radius where gas accretion is concentrated. These source functions of mass and angular momentum in the circumplanetary disk would provide us with useful constraints on the structure and evolution of the circumplanetary disk, which is important for satellite formation.Comment: 22pages, 17 figures, accepted for publication in Ap