Planet のバックアップ(No.88)

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
後期第2回 10/28 14:00-	Tadahiro Kimura (UTokyo)	Theoretical prediction of water contents of exoplanets with effects of water production in the primordial atmosphere	以降14時開始に変更	星野
後期第3回 11/4 14:00-	Haruka Hoshino (UTokyo)	Orbital structure of planetary systems formed by giant impacts: stellar mass dependence	論文紹介
後期第4回 11/18 14:00-	Kenji Kurosaki (Naogoya University)	Atmospheric escape induced by the giant impact		古家
後期第5回 11/30 16:00-	Philipp Umstätter (Technical University of Kaiserslautern)	Granular mechanical and Molecular Dynamical insights into the first stage of planet formation	火曜日16時から	荒川
後期第6回 12/2 14:00-	Yuji Matsumoto (CfCA)	Size evolution of close-in super-Earths through giant impacts and photoevaporation	論文紹介
後期第7回 12/16 14:30-	Yusuke Tsukamoto (Kagoshima University)	形成期の原始惑星系円盤におけるダストの成長とその三次元的なダイナミクス	14:30	荒川
後期第8回 1/20 14:00-	Kazuhiro Kanagawa (Ibaraki University)	Dust rings as a footprint of planet formation in a protoplanetary disk		高橋
後期第9回 1/27 14:00-	Ko Arimatsu (Kyoto University)	Shadows and flashes in the outer solar system peering through OASES and PONCOTS		荒川
後期第10回 2/24 14:00-	Hiroyuki Tako Ishikawa (Astrobiology Center)	Elemental Abundances of nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey		星野

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.

10/28 Tadahiro Kimura (UTokyo), Theoretical prediction of water contents of exoplanets with effects of water production in the primordial atmosphere: Because of the increasing number of detected small exoplanets, there is growing interest in the abundance of exoplanets with Earth-like water contents. Previous theoretical studies are based on the assumption that icy (or water-rich) planetesimals beyond the snow line are the source of water and predict that planets in the habitable zone around M dwarfs, which are the main target of the recent exoplanet observations, do not have much water. On the other hand, since planets are generally formed in protoplanetary disks, they naturally acquire disk gas to form primordial atmospheres. In this study, we focus on water production by oxidation of atmospheric hydrogen with oxides in the magma ocean as another water capture process. When this reaction occurs efficiently, it is known that even sub-Earth-mass planets can acquire more water than the Earth's ocean mass. However, when combined with other formation processes, it is not known to what extent this water production process affects the final planetary water abundance distribution. Therefore, we developed a planetary population synthesis model that incorporates the effect of water production in the primordial atmosphere and predicted the water content distribution of exoplanets. As a result, we found that the water amount in the primordial atmosphere has a great impact on the planetary water content distribution. In particular, our results suggest that the terrestrial planets with Earth-like water contents can be found in the habitable zone of M dwarfs, in contrast to the previous predictions.

11/18 Kenji Kurosaki (Nagoya Univeristy), Atmospheric escape induced by the giant impact: Recent observation reveals that many kinds of exoplanets whose masses are Earth to Neptune-mass while those radii are more extensive than Earth-radius. Those planets possess a significant atmosphere whose mass fractions are several to several ten percent, which means a diversity of the atmospheric mass fraction. Such diversities are caused by the diversities of the formation processes of planets. In the late stage of the formation process, planets experience giant impact events that cause atmospheric escape. We perform the smoothed particle hydrodynamic simulation to reveal the impact-induced atmospheric escape. We find that the kinetic energy of escaped atmospheric mass is simply proportional to the sum of the kinetic impact energy and the released energy from the merged core. We demonstrate the relationship between the kinetic energy of the escaped mass and the escaped atmospheric mass fraction. Combining the relationships among the kinetic impact energy, kinetic escape energy, and the escaped atmospheric mass, we can derive an analytic expression for the atmospheric escape as a function of the impact energy. Finally, we discuss the implication for the formation of a planet with a massive atmosphere. Our study provides constraints on the formation scenarios of observed rocky planets. Since the giant impact removes the primordial atmosphere efficiently, observed rocky planets that were expected to have atmospheres should have formed before the protoplanetary disk dissipation.

11/30 Philipp Umstätter (Technical University of Kaiserslautern), Granular mechanical and Molecular Dynamical insights into the first stage of planet formation: The first stage of planet formation starts with the agglomeration of dust particles to form larger aggregates. The dynamics of this process is not yet completely understood. In this talk, I will present two contributions to a broader understanding of this process from simulations. First, I will briefly introduce a model implementation of granular mechanics. I will present results from simulations of collisions of several different types of aggregates, e.g. chondritic aggregates. A layer of dust around chondrules acts as a sticking agent. Energy will mainly be dissipated due to friction in the normal and sliding motions. In collisions of aggregates with several chondrules we find chondrules to mostly be contained by the largest fragments of dust grains. I will also present results of collisions of monodisperse clusters and compare the fragmentation behavior with collisions of clusters that consist of particles with sizes distributed according to the MRN distribution. Results show that the MRN clusters can often be replaced by monodisperse clusters without significantly changing the fragmentation behavior. Second, I will present Molecular Dynamics simulations of contacts and collisions of spherical dust particles. Certain aspects of the granular mechanical model can be observed already for particle sizes of roughly 100 nm, while some aspects deviate from the continuum model. In collisions of silica nanoparticles predictions of the JKR model describe collisional behavior well until the point of maximum compression. The second (outgoing) phase shows significant deviations. Collisions of pure ice grains exhibit melting in the collision interface. This leads to the prevention of bouncing. On the other hand, hydroxilated silica spheres exhibit lower bouncing threshold velocities.

12/16 Yusuke Tsukamoto (Kagoshima University), 形成期の原始惑星系円盤におけるダストの成長とその三次元的なダイナミクス: 原始星の形成進化過程におけるダストの成長とその運動は近年大きな注目を集めている。一方でこれまでの円盤内でのダストダイナミクスの理論研究はワンゾーン近似や 1D シミュレーションによって主に研究されてきた。しかしながらこれらの手法ではアウトフローといった原始星形成期の多様なガス運動におけるのダストの運動への影響を取り扱うことが困難であった。そこで我々はダスト成長を取り込んだ 3D 非理想 MHD ダスト-ガス 2 流体シミュレーションアルゴリズムとコードの開発を行ってきた。本講演では、このアルゴリズムに用いられる近似とその妥当性、アルゴリズムの詳細を説明したのち、このコードを用いて発見された新しい現象、「原始惑星系円盤への降灰現象」について発表する。「原始惑星系円盤への降灰現象」とは以下の様なものである。円盤内縁で成長した数mm 程度のダストがアウトフローで巻きあげられると、アウトフロー内でガスとすみやかに decouple する。その結果、成長ダストはアウトフロー内から遠心力によって弾きだされ、エンベロープを経由して、円盤外縁に再降着することを発見した。この現象は火山噴火によるガスと灰の大気中への放出と大気中での火山ガスと灰の decouplingと地上への降灰に類似している。このことから我々はこの現象を原始星における降灰現象と名付けた。本講演ではシミュレーション結果について説明するとともに、降灰現象が惑星形成理論に与える影響、さらに、ダスト成長が円盤進化に与える影響などについても考察する。

1/20 Kazuhiro Kanagawa (Ibaraki University), Dust rings as a footprint of planet formation in a protoplanetary disk: Relatively large dust grains (referred to as pebbles) accumulate at the outer edge of the gap induced by a planet in a protoplanetary disk, and a ring structure with a high dust-to-gas ratio can be formed. Such a ring has been thought to be located right outside of the planet orbit. We examined the evolution of the dust ring formed by a migrating planet, by performing two-fluid (gas and dust) hydrodynamic simulations. We found that the initial dust ring does not follow the migrating planet and remains at the initial location of the planet in the cases with a low viscosity of alpha ~ 0.0001. The initial ring is gradually deformed by viscous diffusion, and a new ring is formed in the vicinity of the migrating planet, which develops from the trap of the dust grains leaking from the initial ring. During this phase, two rings co-exist outside the planet orbit. This phase can continue over ~1 Myr for a planet migrating from 100 au. After the initial ring disappears, only the later ring remains. This change in the ring morphology can provide clues as to when and where the planet was formed and is the footprint of the planet. We also carried out simulations with a mass-growing planet. These simulations show more complex asymmetric structures in the dust rings. The observed asymmetric structures in the protoplanetary disks may be related to a migrating and mass-growing planet. In this talk, I will introduce the above result and discuss implications on planet formation and evolution in the protoplanetary disk.

1/27 Ko Arimatsu (Kyoto University), Shadows and flashes in the outer solar system peering through OASES and PONCOTS: Monitoring short-timescale astronomical events (stellar occultations and impact flashes on planets) is a powerful tool for exploring small objects in the outer solar system. I have developed two small and low-cost observation systems dedicated to time-domain solar system surveys; Organized Autotelescopes for Serendipitous Event Survey (OASES) and Planetary ObservatioN Camera for Optical Transient Surveys (PONCOTS). These observation systems have detected a stellar occultation event candidate by a kilometer-sized trans-Neptunian object (Arimatsu et al., Nature Astronomy, 3, 301, 2019) and an impact flash event on Jupiter (CBET 5059, 2021). I introduce the latest results of these discoveries.

2/24 Hiroyuki Tako Ishikawa (ABC), Elemental Abundances of nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey: Detailed chemical analyses of M dwarfs are scarce but necessary to constrain the formation environment and internal structure of planets being found around them. I will talk about our abundance analyses of 13 M dwarfs (2900 < Teff < 3500 K; mid- to late-M dwarfs) observed in the Subaru/IRD planet search project (IRD-SSP survey). We use the high-resolution (∼70,000) near-infrared (970–1750 nm) spectra to measure the abundances of Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Sr by the line-by-line analysis based on model atmospheres, with typical errors ranging from 0.2 dex for [Fe/H] to 0.3–0.4 dex for other [X/H]. We measure radial velocities from the spectra and combine them with Gaia astrometry to calculate the Galactocentric space velocities UVW. The resulting iron abundances agree with previous estimates based on medium-resolution K-band spectroscopy, showing a wide distribution of metallicity (－0.6 < [Fe/H] < +0.4). The abundance ratios of individual elements [X/Fe] are generally aligned with the solar values in all targets. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin-disk population, the most metal-poor object, Barnard's Star, could be a thick-disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and the Galactic stellar population of nearby M dwarfs.