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Planet のバックアップ(No.89)


惑星セミナー2019

惑星セミナーは原則として毎週木曜日の14:00から理論部セミナー室で開催しています。(連絡係:荻原 正博)
astro-phセミナーは毎週金曜日の12:00から理論部セミナー室で開催しています。(連絡係:Carol Kwok)

Schedule & History

2018年度 2017年度 2016年度 2015年度 2014年度

日程発表タイトルRemarks
前期 第1回 4/11 14:30-Shinsuke Takasao3D MHD simulations of Inner Protoplanetary Disks14:30
前期 第2回 4/18 14:00-Carina Heinreichsberger (Universität Wien)Terrestrial or Gaseous? A classification of exoplanets according to density, mass and radius
前期 第3回 4/25 14:00-Kazunari IwasakiChemistry in Debris Disks
前期 第4回 5/16 14:30-Yuhito Shibaike (Tokyo Tech)A new formation scenario for the Galilean satellites14:30
前期 第5回 5/23 15:30-Yuji Matsumoto (ASIAA)The orbital stability of planets in resonances considering the evolution of mass ratio15:30
前期 第6回 6/13 15:00-Hiroaki Kaneko (Tokyo Tech)ストリーミング不安定の発生機構の物理的解釈In Japanese 15:00
前期 第7回 6/20 14:00-Yuhiko Aoyama (University of Tokyo)Theoretical modeling of hydrogen line emission from accreting gas giants: How gas flows around LkCa15b and PDS70b
前期 第8回 6/27 15:00-Shoji Mori (University of Tokyo)Inefficient Magnetic Accretion Heating in Protoplanetary Disks15:00
前期 第9回 7/19 14:00-Takaya NozawaTBAFriday
4/18 Carina Heinreichsberger, Terrestrial or Gaseous? A classification of exoplanets according to density, mass and radius
When looking at Exoplanet Archives the class of a planet is not given. Therefore I tried to find an easy and fast way to classify exoplanets using only density, mass and radius. In this talk I will discuss the formation theory of Planets to explain the boundaries between the different classes (gas, terrestrial) and show the results of my empirical study.
5/16 Yuhito Shibaike, A new formation scenario for the Galilean satellites
It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter’s formation. I will discuss a new formation scenario for the Galilean satellites, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. Our slow-pebble-accretion scenario can reproduce the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites.
6/13 Hiroaki Kaneko, ストリーミング不安定の発生機構の物理的解釈
Youdin & Goodman (2005)(YG05)が発見したダスト・ガスの二流体の線形不安定 “Streaming Instability” (SI)によるダスト密度上昇とそれに続く自己重力崩壊が微惑星形成の一つの可能性として考えられている。SIについては線形および非線形の数値計算がされていて不安定が存在することは確かであるものの、メカニズムははっきりしていない。今回は各物理量の摂動の位相差に注目して、ガスが支配的な場合におけるSIのメカニズムを提案する。それは以下の通りである。 1. 基本場としてガスはダストから角運動量を受け取り、中心星より遠方へと運動する。ダスト密度に濃淡があるとすると、ダスト密度の違いによってガスへの角運動量の供給に違いが生じ、半径方向にガス圧力の摂動ができる。 2. 高圧部からは鉛直方向にガスが流出し、低圧部にはガスが流入する。ダストもこれに引きずられ、同様に高圧部から流出、低圧部に流入する。 3. ガスが支配的な状態ではダスト密度の摂動の位相は中心星方向に進む。ダスト密度の極大はガスの低圧部へと向かい、そこで鉛直方向から流入するダストと合流し、ダスト密度の摂動の振幅が大きくなる。
6/20 Yuhiko Aoyama, Theoretical modeling of hydrogen line emission from accreting gas giants: How gas flows around LkCa15b and PDS70b
Observation of growing protoplanets is key to understand planet formation. Planets are thought to form in the gaseous disk around pre-main-sequence stars, so-called proto-planetary disk. So far, a few planets embedded in such disks are detected. Among them, LkCa15b and PDS70b are particularly interesting planets. These planets are reported to be bright not only in the infra-red but also in Hα. Hydrogen line emission such as Hα needs hot hydrogen gas with a temperature higher than ~104 K as their source. Such a high temperature is unlikely to be realized in the protoplanet nor circum-planetary disk. Also, observationally, other wavelengths observations suggest a few thousand K of gas temperature, which is too cool to emit observable Hα. The physical mechanism for the Hα emission is poorly understood in planet formation context. In this study, we focus on two accreting flows. One flows from a proto-planetary disk to a circum-planetary disk. According to recent 3D hydrodynamic simulations, the accreting gas almost vertically onto and collides with the surface of the circumplanetary disk at a super-sonic velocity. And the other flow comes from circum-planetary disks to a planetary surface. In stellar accretion context, both theoretical model and observational data suggest surrounding gaseous disk is truncated around the central object and gas falls from the disk to the object with a supersonic velocity, when the object has a strong magnetic field. In both cases, the gas passes through a strong shock wave and gets hot enough to emit hydrogen lines. However, the gas instantly cools. So we have to study whether the shock-heated gas can emit significant Hα faster than the cooling. Here, we have developed a 1D radiative hydrodynamic model of the flow after the shock with detailed calculations of chemical reactions and electron transitions in hydrogen atoms. Then, we quantify the hydrogen line emission from the shock surface. Our model concluded the shock-heated gas can emit Hα at significant intensity. Comparing our theoretical Hα intensity with the observed ones from LkCa15b and PDS70b, most of the gas going to the planet have to pass through a strong shock and contribute to Hα emission. From this, gas flow around the two gas giants is discussed.
6/27 Shoji Mori, Inefficient Magnetic Accretion Heating in Protoplanetary Disks
The gas temperature in the inner region of protoplanetary disks is thought to be determined by accretion heating, which is conventionally attributed to turbulent dissipation. However, recent studies have suggested that the inner disk (a few AU) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation. We perform local stratified MHD simulations including all three non-ideal MHD effects (Ohmic, Hall, and ambipolar diffusion), and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that in the inner disk, as Ohmic and ambipolar diffusion strongly suppress electrical current around the midplane, Joule heating primarily occurs at several scale heights above the midplane, making midplane temperature much lower than that with the conventional viscous heating model. Including the Hall effect, Joule heating is enhanced/reduced when magnetic fields threading the disks are aligned/anti-aligned with the disk rotation, but is overall ineffective. Our results suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating. We will also discuss the evolution of the water snow line based on our results and the formation process of the Earth.