Planet のバックアップ(No.79)

惑星セミナー2021 †

惑星セミナーは原則として毎週木曜日の15:00から開催しています。（連絡係：星野遥, 荒川創太, 高橋実道，古家健次）
astro-phセミナーは毎週月曜日の12:00から開催しています。（連絡係：Carol Kwok，瀧哲朗）

Schedule & History †

2020年度 2019年度 2018年度 2017年度 2016年度 2015年度 2014年度

日程	発表	タイトル	Remarks	担当
前期第1回 4/15 15:00-	All members	Self-introduction		古家
前期第2回 4/22 15:00-	All members	Self-introduction		古家
前期第3回 5/13 14:00-	Teruyuki Hirano (ABC)	Near Infrared Spectroscopy as a Powerful Tool to Probe Exoplanetary Systems	14:00	古家
前期第4回 5/20 15:00-	Ryuki Hyodo (ISAS/JAXA)	Planetesimal formation -- Around the snow line and the "no-drift" mechanism		荒川
前期第5回 6/17 15:00-	Riouhei Nakatani (RIKEN)	Photoevaporation of Protoplanetary Disks: Revisiting the Underlying Physics and the Gravitational Radius		高橋
前期第6回 6/24 15:00-	Hiroaki Kaneko (titech)	Simultaneous evolution of rims around chondrules and accreting dust particles		星野
前期第7回 7/8 15:00-	Tatsuya Okamura (Nagoya University)	Collision Rate between a Planet and Small bodies in Protoplanetary Disks Perturbed by the Planetary Gravity		荒川
前期第8回 7/15 15:00-	Hidekazu Tanaka (Tohoku University)	New models for planetary gaps, type II migration, and giant planet formation		古家
後期第1回 9/30 15:00-	Takehiro Miyagoshi (JAMSTEC)	Numerical studies of mantle convection in super-Earths		高橋

5/20 Ryuki Hyodo (ISAS/JAXA), Planetesimal formation -- Around the snow line and the "no-drift" mechanism: Forming planetesimals in protoplanetary disks is a major challenge in our current understanding of planet formation. Icy pebbles mixed with silicate dust formed at the outer disk drift inward due to the gas drag. We performed 1D diffusion-advection simulations that include the back-reaction (the inertia) to radial drift and diffusion of icy pebbles and silicate dust, ice sublimation, the release of silicate dust, and their recycling through the recondensation and sticking onto pebbles outside the snow line. In this talk, I will present how icy pebbles and silicate dust pile up around the snow line. I also report a new mechanism, the “no-drift” runaway pile-up, that leads to a runaway accumulation of pebbles in disks, thus favoring the formation of planetesimals by streaming and/or gravitational instabilities. References: Hyodo et al. 2021 A&A, 646, A14; Ida et al. 2021 A&A, 646, A13; Hyodo et al. 2021 A&A, 645, L9

6/17 Riouhei Nakatani (RIKEN), Photoevaporation of Protoplanetary Disks: Revisiting the Underlying Physics and the Gravitational Radius: In a variety of astrophysical problems, we find a situation where a clump of gas is irradiated by ultraviolet and X-ray from radiation sources. An important outcome of this process is that excessive photon energy goes into the heat for the gas, which results in driving winds. This wind-driving process, termed photoevaporation, is essential to determine the fate of the irradiated objects. Protoplanetary disks are one of such objects. The stellar UV and X-ray can yield sufficiently high mass-loss rates that can disperse the disks within 10 Myr. The gravitational radius is often used as a criterial radius above which the photoheated gas is possible to escape from the gravitational binding of the host star. However, the gravitational radius is derived from dimensional analysis and thus does not provide a definite criterion regarding the escape capability. We have recently developed an analytic model for photoevaporation in a first-principles approach. It is of use to understand the basic physics operating in the vicinity of the wind-launching points. Our model naturally sets a gravitational-radius-like criterion, which is fundamentally different from the gravitational radius in origin. In this talk, I first present the analytic model. Then, the model aside, I introduce our recent numerical works regarding photoevaporation of protoplanetary disks hosted by intermediate-mass stars.

6/24 Hiroaki Kaneko (titech), Simultaneous evolution of rims around chondrules and accreting dust particles: Chondrules are the major components of primitive meteorites, i.e. chondrites. Chondrules are often surrounded by fine-grained dust rims (FGRs). FGRs are visibly distinct from interstitial matrix, and their origin has been debated so far but still an unsolved problem. Nebular accretion scenario is one of the possible solutions to the origin of FGRs. In this scenario, chondrules floating in a nebula capture small dust grains and aggregates to form rims on their surface. Xiang et al. (2019) examined the initial structures of FGRs formed in the nebular accretion scenario. They reported that the morphology of accreting dust, i.e. monomer grains or aggregates, affects the initial structures of FGRs. It was revealed that monomer-accreting rims show compact and layered structures with grain size coarsening from the bottom to the top. However, they did not consider which type of dust can accumulate onto the surface of chondrules to form rims in a nebular setting. In a nebula, dust grains quickly collide and coagulate into aggregates. To solve this issue, we track the collisional growth of dust grains and their accretion onto chondrules simultaneously. We find that to form monomer-accreting rims, the maximum grains size in the monomer grain population must be > 1μm in a moderately turbulent nebula ( α < 10-3 ) and ~ 10μm in a weakly turbulent nebula ( α < 10-5 ). Moreover, the monomer grain size distribution with larger mass fraction in the large grains compared to that of Inter Stellar Medium might be necessary for layered structures in FGRs.

7/8 Tatsuya Okamura (Nagoya University), Collision Rate between a Planet and Small bodies in Protoplanetary Disks Perturbed by the Planetary Gravity: Planets grow via the collisional accretion of small bodies in a protoplanetary disk. Such small bodies feel strong gas drag and their orbits are significantly affected by the gas flow and atmospheric structure around the planet. We investigate the gas flow in the protoplanetary disk perturbed by the gravity of the planet by three-dimensional hydrodynamic simulation. We then calculate the orbital evolutions of particles in the gas structure obtained from the hydrodynamic simulation. Based on the orbital calculations, we obtain the collision rate between the planet and centimeter to kilometer sized particles. Our results show that meter-sized or larger particles effectively collide with the planet due to the atmospheric gas drag, which significantly enhances the collision rate. On the other hand, the gas flow plays an important role for smaller particles. Finally, considering the effects of the atmosphere and gas flow, we derive the new analytic formula for the collision rate, which is in good agreement with our simulations.

7/15 Hidekazu Tanaka (Tohoku University), New models for planetary gaps, type II migration, and giant planet formation: Based on recent hydro-dynamical simulations, we constructed new models for planetary gaps, type II migration, and gas accretion onto giant planets. These models have renewed the previous paradigm of giant planet formation. We actually developed a simple model for giant planet formation, focusing on the runaway gas accretion stage. Our simple model gives universal evolution tracks in the diagram of planetary mass and orbital radius. We find that giant planets with a few Jupiter masses or less suffer only a slight radial migration in the runaway gas accretion stage. Our model successfully explains properties in the mass distribution of giant exoplanets with the mass distribution of observed protoplanetary disks.

9/30 Takehiro Miyagoshi (JAMSTEC), Numerical studies of mantle convection in super-Earths: Mantle convection governs tectonic activity on the surface of the planet and internal thermal evolution. It also drives plate motion, material cycles, and core convection. What the mantle dynamics inside super-Earths is and how they differ from the Earth’s one are interesting issues because mantle convection is one of key factors to understand the thermal evolution and surface environment of super-Earths. We have studied mantle convection in super-Earths by numerical simulations. One of the most important differences between the Earth’s and super-Earth’s interior is that there is a very large adiabatic temperature gradient (large dissipation number) in massive super-Earths. Usually, this effect is ignored in modelling the dynamics in the mantle of the Earth because the effect is small in the Earth. However, this effect becomes strong as the size of the planet increases so it cannot be ignored in large planets. In this seminar we talk about our results of numerical simulation studies of mantle convection in super-Earths (up to ten times the Earth’s mass) with this effect (Miyagoshi et al., 2014, 2015, 2017, 2018). We also take account for high Rayleigh number which is relevant in super-Earths, temperature-dependent viscosity, and depth-dependent thermal expansion coefficient. Our results are briefly summarized as follows. (1) The activity of ascending hot plumes is lowered as the planetary size increases. In contrast, the activity of cold plumes is not lowered even in large planets. The efficiency of heat transport by thermal convection is significantly lowered compared with the results with Boussinesq approximation in which the dissipation number is zero and which is often used in models for the Earth. We also found that the feature of lowered hot plume activity becomes substantial when the planetary mass exceeds about 4 times the Earth’s mass. The plate thickness increases and the convective velocity is almost constant as the planetary mass increases. These results suggest that the tectonic activities such as plate motion or hot spot volcanisms hardly occur as planetary size increases. (2) In massive super-Earths, thermal evolution process is very different from the Earth’s one and its time scale becomes significantly long. Transient layered convection continues as long as several billion years before it yields to a whole layer convection.