Planet のバックアップ(No.69)

惑星セミナー2020 †

惑星セミナーは原則として毎週木曜日の14:00から開催しています。（連絡係：星野遥, 荒川創太, 古家健次, 荻原正博）
astro-phセミナーは毎週金曜日の12:00から開催しています。（連絡係：Carol Kwok）

Schedule & History †

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 4/9 15:00-	All members	Self-introduction	15:00	荻原
前期第2回 4/16 14:00-	Haruka Hoshino, Hirotaka Hohokabe	Small ASJ meeting		荒川
前期第3回 4/23 14:00-	Yuki Yoshida, Eiichiro Kokubo	Small ASJ meeting		星野
前期第4回 5/14 14:00-	Sota Arakawa	Thermal history and tidal evolution of trans-Neptunian satellite systems		古家
前期第5回 5/28 14:00-	Takuya Takarada (ABC)	Radial-velocity search and statistical studies for short-period planets in the Pleiades open cluster		荒川
前期第6回 6/4 16:00-	Beibei Liu (Lund Univ)	Pebble-driven planet formation around very low-mass stars and brown dwarfs	16:00	荻原
前期第7回 7/9 14:00-	Yuka Fujii	Detecting molecular lines of warm/temperate exoplanets with mid-infrared high-resolution spectroscopy		星野
前期第8回 7/21 14:00-	Masato Ishizuka (U. Tokyo)	Studies of exoplanets with high resolution spectroscopy		古家
後期第1回 10/16 14:00-	Makiko Ban	Free-floating planet research and perspective		荻原
後期第2回 10/30 14:30-	Yuki Tanaka (Tohoku Univ.)	Gap formation by a super-Jupiter-mass planet and its effects on the planetary mass accretion rate	14:30	荻原
後期第3回 11/20 14:00-	Shota Notsu (RIKEN)	The composition of hot Jupiter atmospheres assembled within chemically evolved protoplanetary discs		古家
後期第4回 11/27 14:00-	Kazuaki A. Homma (Tokyo Tech)	Vertical growth of dust particles with UV-irradiation and organic-synthesis in protoplanetary disks		荒川
後期第5回 12/17 15:00-	Naho Fujita (Kyoto Univ.)	短周期super-Earthの大気散逸に伴う軌道進化	Thu. 15:00	星野
後期第6回 1/12 14:00-	Jerome de Leon (Univ. Tokyo)	Discovery and validation of transiting exoplanets with diverse radii and ages	Tuesday	荻原
後期第7回 1/22 14:00-	Yuki Yoshida	銀河渦状腕形成の素過程ー重力散乱による軌道位相同期ー	金曜日 14:00	荒川
後期第8回 2/26 14:00-	Haruka Sakuraba (Tokyo Tech)	Element partitioning and atmospheric escape processes on accreting Venus		荻原
後期第9回 3/5 14:00-	Ayano Nakajima (Tokyo Tech)	Saturn’s rings formation through the tidal splitted disk		古家
後期第10回 3/26 14:00-	Masataka Aizawa (T.D.Lee Institute)	Application of sparse modeling to data analysis in astronomy		星野

6/4 Beibei Liu, Pebble-driven planet formation around very low-mass stars and brown dwarfs: We conduct a pebble-driven planet population synthesis study to investigate the formation of planets around very low-mass stars and brown dwarfs, in the (sub)stellar mass range between 0.01 M⊙ and 0.1 M⊙. Based on the extrapolation of numerical simulations of planetesimal formation by the streaming instability, we obtain the characteristic mass of the planetesimals and the initial masses of the protoplanets (largest bodies from the planetesimal size distributions), in either the early self-gravitating phase or the later non-self-gravitating phase of the protoplanetary disk evolution. We find that the initial protoplanets form with masses that increase with host mass, orbital distance and decrease with disk age. Around late M-dwarfs of 0.1 M⊙, these protoplanets can grow up to Earth-mass planets by pebble accretion. However, around brown dwarfs of 0.01 M⊙, planets do not grow larger than Mars mass when the initial protoplanets are born early in self-gravitating disks, and their growth stalls at around 0.01 Earth-mass when they are born late in non-self-gravitating disks. Around these low mass stars and brown dwarfs, we find no channel for gas giant planet formation because the solid cores remain too small. When the initial protoplanets form only at the water-ice line, the final planets typically have ≳15% water mass fraction. Alternatively, when the initial protoplanets form log-uniformly distributed over the entire protoplanetary disk, the final planets are either very water-rich (water mass fraction ≳15%) or entirely rocky (water mass fraction ≲5%).

10/16 Makiko Ban, Free-floating planet research and perspective: The free-floating planet (FFP) is a unique type of exoplanet. There have been very scarce discoveries about it because of the difficulty of observation. The up-comming space-based telescope missions (Euclid and Roman) are expected to boost the FFP research. Here, I'd like to introduce FFPs, the challenges about the research we are facing, and future perspectives that will be offered by those up-comming missions through my latest paper.

10/30 Yuki Tanaka, Gap formation by a super-Jupiter-mass planet and its effects on the planetary mass accretion rate: A giant planet embedded in a protoplanetary disk creates a gap structure along with its orbit by disk-planet interaction. Physical properties of the gap depend on several conditions such as mass of the planet and disk structures, and they affect both mass accretion rate onto the planet via the gap and migration rate of the planet. Therefore, the properties of the gap are important to investigate formation and evolution of planetary systems. Recently, numerical simulations of the disk-planet interaction have been done intensively, and the disk properties such as width and depth of the gap, and mass accretion rate have been studied. However, previous studies mainly focused on planets less massive than Jupiter. In addition, there are a discrepancy between several previous works on the mass accretion rate onto the planet heavier than Jupiter. Since a lot of super-Jupiter-mass planets have been found, formation and evolution of them in the protoplanetary disk should be investigated in more detail. We performed a set of hydrodynamic simulation of disk-planet interaction and investigated the properties of the gap and their parameter dependence. We varied the planetary mass from 1 to 10 Jupiter masses. We found that the gap becomes deeper as planet's mass increases up to around 3 Jupiter masses, but in more massive cases the outer edge of the gap shows significant eccentricity, which is consistent with several previous works. In this eccentric regime, the gap depth becomes shallower than an empirical relation between the depth and the planetary mass due to non-steady behavior of the gap outer edge. We also estimated the mass accretion rate onto the planet by using our result and found that the accretion rate can increase when the planet's mass is heavier because of the eccentricity of the gap.

11/20 Shota Notsu, The composition of hot Jupiter atmospheres assembled within chemically evolved protoplanetary discs: The radial-dependent positions of snowlines of abundant oxygen- and carbon-bearing molecules in protoplanetary discs will result in systematic radial variations in the C/O ratios in the gas and ice. This variation is proposed as a tracer of the formation location of gas-giant planets. However, disc chemistry can affect the C/O ratios in the gas and ice, thus potentially erasing the chemical fingerprint of snowlines in gas-giant atmospheres. We calculate the molecular composition of hot Jupiter atmospheres using elemental abundances extracted from a chemical kinetics model of a disc midplane where we have varied the initial abundances and ionization rates. The models predict a wider diversity of possible atmospheres than those predicted using elemental ratios from snowlines only. As found in previous work, as the C/O ratio exceeds the solar value, the mixing ratio of CH4 increases in the lower atmosphere, and those of C2H2 and HCN increase mainly in the upper atmosphere. The mixing ratio of H2O correspondingly decreases. We find that hot Jupiters with C/O>1 can only form between the CO2 and CH4 snowlines. Moreover, they can only form in a disc which has fully inherited interstellar abundances, and where negligible chemistry has occurred. Hence, carbon-rich planets are likely rare, unless efficient transport of hydrocarbon-rich ices via pebble drift to within the CH4 snowline is a common phenomenon. We predict combinations of C/O ratios and elemental abundances that can constrain gas-giant planet formation locations relative to snowline positions, and that can provide insight into the disc chemical history. This seminar talk is mainly based on our paper which was recently published, Notsu et al. (2020, MNRAS, 499, 2229). https://doi.org/10.1093/mnras/staa2944

11/27 Kazuaki A. Homma, Vertical growth of dust particles with UV-irradiation and organic-synthesis in protoplanetary disks: Refractory organic matter found in the solar-system would play important roles in life and planet formation. It is important to reveal how the organic-matter was formed. One possible scenario is that they form by UV irradiation and subsequent heating of volatile ices on the dust particles in protoplanetary disks (Ciesla & Sandford (2012)). Ciesla & Sandford (2012) simulated the motion of dust particles in the protoplanetary disk and calculated the total UV-photon dosage on dust particles. They also estimated how much amount of the organic-matter would be formed from the total UV-photon dosage. They found that µm-sized dust particles can absorb UV-irradiation efficiently and would contain 10 wt% of the organic matter. However, their model assumes the UV-irradiation strength without calculating the dust distribution. It is unclear whether large dust aggregates can absorb UV-radiation or not. We study how much organic-matter can be formed on dust particles in protoplanetary disks via UV-irradiation on dust aggregates if we consider the dust size distribution. We construct a model to simulate growth, vertical transport, and UV absorption of icy aggregates in a protoplanetary disk. We find that the large dust aggregates can contain the irradiated-dust particles, due to the fragmentation of dust aggregates and the turbulent transportation of dust particles. We also estimate the amount of organic-matter formed on the dust particles by the UV-irradiation dosage. Our results suggest that the non-sticky dust particle in strong turbulent disks can contain the same amount of organic-matter as interplanetary dust particles.

12/17 藤田菜穂　短周期super-Earthの大気散逸に伴う軌道進化: 近年Kepler宇宙望遠鏡などの活躍によって系外惑星探査が大きく進展し、特にsuper-Earthの発見数は飛躍的に増加している。発見されたsuper-Earthはその大気量や軌道分布において多様性に富んでいるが、この多様性の起源は明らかになっていない。ただし、特に中心星近傍を回る短周期super-Earthでは、形成直後の大気状態や軌道を現在も維持しているとは考えにくい。短周期super-Earthは、中心星からの強力なX線、EUV照射による大気の流体力学的散逸を経験して現在の大気量になっているはずである。このような惑星の大気散逸についてはこれまで多くの研究がなされているが、その際、大気散逸を経験する惑星の軌道は変化しないと仮定されてきた。しかし、中心星-惑星系全体の軌道角運動量保存を考慮すると、実際には大気散逸のみではなく、大気散逸による質量損失に伴って惑星が外側に移動するという軌道進化も起きると考えられる。本研究ではsuper-Earth系の多様性の起源を探ることを目的とし、惑星の大気散逸に伴う軌道進化が最終的な惑星系の構造に与える影響を調べた。計算の結果、惑星は大気散逸に伴って外側に移動することが分かり、この移動は軌道間隔の狭いコンパクトなsuper-Earth系では特に重要になってくるであろうという示唆が得られた。また今後は観測によってM型星周りのsuper-Earthが多数発見されることが期待されるため、本研究はそのような観測に先駆けた理論的な予測となる。さらに、将来的には観測データと計算結果を比較することで、この理論モデルを再検証することが可能になる。本講演ではこのような今後の系外惑星探査との関連についても議論したい。

1/12 Jerome de Leon, Discovery and validation of transiting exoplanets with diverse radii and ages: The Kepler, K2, and TESS missions have brought many exciting exoplanet discoveries that yield new insights into the occurrence rate, formation and evolution of exoplanets. This success was driven primarily by the sustained efforts to homogeneously analyse ensembles of light curves to detect new candidate systems and consequently statistically validate or confirm their planetary nature aided by follow-up data. Here, "validation" is different from "confirmation," wherein the former means that there is overwhelming evidence that the transits must be explained by a planet, through elimination of all false positive scenarios, whereas the latter involves determination that the planet's mass is in the substellar regime (Mp<13Mjup). Confirmation via radial velocity (RV) mass measurements have been conducted for planets around bright stars but is impractical for faint or magnetically active stars, and is observationally expensive for the large numbers of planet candidates detected by Kepler and K2. In this talk, I will present the discovery and validation of 37 new transiting planets with various radii and ages using various statistical techniques. Then I will present some insights on the emerging trends observed from young transiting planet population and their implications on the planets’ evolution during their first hundred million years.

2/26 Haruka Sakuraba, Element partitioning and atmospheric escape processes on accreting Venus: Due to limited information, the climate of early Venus has been poorly understood. Venus might have been in the runaway greenhouse state since its formation, in which water forms a steam atmosphere. On the other hand, a habitable climate with liquid oceans has been also suggested as another possibility. Present-day Venusian volatile budget shows a deficient in water by orders of magnitude and low C/N ratio by a factor of ~4 compared to the bulk silicate Earth. Hydrodynamic escape from the primordial atmosphere is thought to have contributed to the water loss, but another mechanism is needed to explain the C/N fractionation. In this study, we adapted a numerical model of element partitioning between the atmosphere, crust, mantle (magma ocean), and core as well as impact-induced atmospheric escape to accreting Venus. We estimated the final volatile inventories for both cases of the runaway greenhouse state and of Earth-like habitable condition. In the former case, all volatile species on the surface were assumed to be partitioned into the atmosphere in the former model. In the latter case, water and carbon were assumed to be partitioned into liquid oceans and sedimentary carbonates, respectively, after the magma ocean solidification. Finally, Venusian low C/N ratio was reproduced in the runaway greenhouse case, while Earth-like high C/N ratio was obtained in the habitable case. As a conclusion, we propose that the combination of the runaway greenhouse state and the impact-induced atmospheric escape on early Venus are required to explain the present-day volatile abundances.

3/5 Ayano Nakajima, Saturn’s rings formation through the tidal splitted disk: Cassini space craft gave us the detailed chemical composition and interesting features of Saturn’s rings and satellites. So, we can compare the observational data and simulation results derived from the physical process of ring formation. In this study, we focus on the rings formation process through the tidal disruption of a fly-by object. Hyodo et al. (2017) performed SPH simulation, and suggested that one large Kuiper-belt object is captured by Saturn’s gravity and disrupted into many small particles due to tide. They investigated only how the large object was destroyed, but not orbital evolution of the debris particles after the tidal disruption. Therefore, we explore the orbital evolution of the debris particles toward the current equatorial circular rings. The orbits of debris particles evolve into nearly circular orbits around the Roche limit radius due to the collisional energy damping and some fraction of the particles outside the Roche limit accrete as multiple seeds of satellites, forming the rings inside the Roche limit and the satellites simultaneously. In the previous model, rings formation and satellites formation are considered to occur separately. If the multiple satellites are directly formed in packed orbits beyond the Roche limit at the same time. it can easily explain the current compact orbital configuration of Saturn’s mid-sized satellites, We also check the collisional velocity during the whole rings formation event. As a result, the icy materials (volatiles) can be retained, because the collision velocity remains relatively small even in collisions between particles in highly eccentric orbits and hence the icy materials do not evaporate and the accreted satellites retain the primitive icy materials including the volatile spices, when the collisional timescale would be shorter than the orbital precession and randomization of the particles are still aligned during the collisional orbital evolution. Our new model may consistently reproduce the current orbital configuration of the satellites, and the ring particles retaining the primitive icy composition, which is suggested by the observational results brought by Cassini spacecraft.

3/26 Masataka Aizawa, Application of sparse modeling to data analysis in astronomy: Inverse problems, in which we estimate some parameters from given data set, frequently appear in astronomical data analyses. In some cases, however, the number of data is not enough to constrain model parameters, leading to　misinterpretation of the observations. Sparse modeling deals with such situations by setting a fraction of parameters in solutions to be zero a priori, and this method is quite powerful when the effective degree of freedom of parameters is small. In the talk, I will briefly introduce the concept and implementation of sparse modeling, and discuss its application to two particular problems in astronomy: imaging in radio interferometry and global mapping of directly-imaged planets.