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Detection and characterisation of planets transiting M dwarfs : How to pick up Earth-size planets with small ground based telescopes?
Anael Wunsche (Institut de Planétologie et d’Astrophysique de Grenoble)
Discovering telluric planets and searching for biomarkers in their atmospheres are among the main objectives of the 21st century. Hence, our interest is focused on finding and characterising planets located in the habitable zone of their host star. On one hand, the method known as radial velocities (RV) consists in the measure of the star’s reflex motion induced by orbiting planets. On the other hand, thanks to photometry, we can measure the decrease in flux when a planet transits in front of its host star. These techniques are complementary to better understand extrasolar systems. However, in order to reach the precision necessary to detect an Earth-like planet or a super-Earth, we need very stable ground instruments as well as the understanding and removal of earth’s atmosphere systematic effects. My work aimed on improving the detection and characterisation of planets in the habitable zone of M dwarfs with HARPS and ExTrA. This seminar will summarize the work I have been doing with the ExTrA project which uses spectrophotometry at middle-range resolution as a brand new method to detect planets from the ground. As the next space missions and the next generation RVs instruments will discover many more planets, photometry follow-up from the ground with small telescopes as well as amateur observatories will be key to validate the detections and characterise these systems.
(Presentation file is prepared in English, spoken in English)
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光度曲線とAO撮像を用いたマイクロレンズ系外惑星の研究
越本 直季 (天文学教室/ABC)
標準的な惑星形成論であるコア集積モデルは、原始惑星系円盤中のスノーラインの少し外側で、木星質量の巨大ガス惑星から地球質量の氷惑星まで様々な質量の惑星の形成が活発になると予言する。これまでに3000個以上の太陽系外惑星が発見されているが、そのほとんどがスノーラインの内側の惑星である。スノーライン外側領域の探査は惑星形成過程の解明にとって非常に重要であり、重力マイクロレンズ法は他の手法では発見しにくいこの領域で地球質量の惑星を検出できる唯一の方法である。
重力マイクロレンズは天球上でソース天体の手前をレンズ天体が横切る時に起こる増光現象である。レンズ天体に惑星が付随する場合、増光曲線から主星と惑星の質量比が測定されるが、それらの質量の決定には、レンズ天体の明るさの測定が一つの鍵となる。観測領域は星が密集しているため、宇宙望遠鏡か補償光学(AO)装置を用いた高空間分解能撮像により、イベント(ソース天体+レンズ天体)と他の星を分離できれば、光度曲線から求められるソース天体の明るさを差し引くことで、レンズ天体の明るさを検出できる。
本講演では、惑星が検出された二つの重力マイクロレンズイベント、OGLE-2012-BLG-0950とMOA-2016-BLG-227の観測結果について議論する。両イベントともにKeck望遠鏡のAO装置によって高空間分解能撮像観測され、天球上のイベントの発生位置にソース天体以外に明るさを持つ天体があることがわかった。この超過分にはレンズ天体の明るさがある割合で含まれる。我々は、この割合を求めるための従来の解析法の統計的取り扱いが正しくないことを指摘するとともに、新たにベイズ推定を用いた正しい解析法を開発した。この方法を用いた結果、OGLE-2012-BLG-0950の場合は、超過の明るさが全てレンズ天体のものである確率が高い一方で、MOA-2016-BLG-227では他の星の明るさが大きい割合で含まれている確率が高いことがわかった。これらの解析の結果、OGLE-2012-BLG-0950のレンズ天体はM型またはK型の主星とそのスノーラインの外側にある海王星と土星の間の質量の惑星からなる系であることがわかった。コア集積モデルによればこのような惑星系はできやすいとされているが、検出感度不足のためか既発見数は少ない。また、MOA-2016-BLG-227のレンズ天体は、M型またはK型の主星周りの巨大ガス惑星からなる系であることがわかった。このような惑星系はコア集積モデルでは説明が難しいため、他の惑星形成過程で生まれた可能性を示唆している。
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初代星連星の観測可能性
衣川 智弥 (天文学教室)
2015年9月14日、アメリカの重力波観測器advanced LIGOは世界で初めて重力波(GW150914)の直接観測に成功した。日本でも重力波観測器KAGRAがテスト稼働を終え、本格的な観測に向け動き出している。今はまさに重力波天文学の黎明期に位置している。重力波観測のメインターゲットはコンパクト連星の合体である。コンパクト連星とは、星が寿命を終え超新星爆発や重力崩壊を起こしてできるコンパクト星(ブラックホール、中性子星)同士の連星である。コンパクト連星は重力波放出により軌道が縮まり、いずれ合体する。重力波によるエネルギー放出は弱いため、合体までのタイムスケールは数億年から宇宙年齢以上と非常に長い。したがって、宇宙初期にできたコンパクト連星でも現在で合体するものがあるはずである。そこで我々は宇宙最初の星である初代星に注目し、重力波源として研究を行ってきた。その結果、初代星起源の連星は典型的に約30太陽質量程度の連星ブラックホールになることを2014年に示した。一方で従来観測されてきたX線連星内にあるブラックホール候補天体は10太陽質量程度であり、そのような重いブラックホールはほとんど存在しないだろうと思われていた。しかし、LIGOによる重力波の初検出はまさに約30太陽質量の重い連星ブラックホールの合体によるものであった。これにより、宇宙には従来考えられていなかった重い連星ブラックホールが多く存在することが示唆されており、それらは宇宙初期にできたものかもしれない。本セミナーでは初代星の連星進化の概要とその手法についても述べる。また、連星ブラックホール以外での初代星連星の観測可能性についても時間の許す限り議論する。
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すばる/HSCを用いた遠方宇宙の稀少天体探査
柏川 伸成 (天文学教室)
すばるHSCは8m望遠鏡に取り付けられた超広視野撮像装置であり、現在行われている
HSC-SSPサーベイ観測は、広さ、深さともにこれまでのサーベイを凌駕するものである。
従来機であるSuprime-Camもすばるに多くの科学成果をもたらしたが、サーベイ面積を
大きく上回るHSCではこれまでに発見が難しかった、宇宙に稀にしか存在しない天体も
見つかるはずである。われわれのグループでは、遠方クェーサーと遠方銀河団の探査を
行っている。前者は、いつどのように最初のブラックホールが生まれたか、という謎を
解く鍵であり、後者はいつどのように構造形成が始まったか、という問いに直結する
観測対象である。また両者とも初期宇宙におきた宇宙再電離に少なからず関与し、詳細
なプロセスを理解する上では重要な天体となっている。われわれは、SSP初期データに
おいて、すでにこれまでのサンプル数に匹敵、あるいは大きく上回る個数のサンプルを
獲得している。本講演ではその現状と初期成果について紹介する。
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星の年齢速度分散関係から探る天の川銀河進化史
熊本 淳 (天文学教室)
太陽近傍の星々の観測から星の年齢と速度分散の間には正の相関があることが知られて
いる(年齢速度分散関係)。理論的研究からは巨大分子雲、渦状腕などの銀河の内部構造
による重力的散乱の効果に加え、銀河同士の相互作用や合体などの外的要因の効果なども
考慮し、年齢速度分散関係の理解が試みられている。しかしながら、年齢速度分散関係の
起源に対しては未だに多くの議論がなされており、その詳細は未解明である。
そこで、我々は天の川銀河を想定した高分解能なN体/SPHシミュレーションを行い年齢
速度分散関係の起源を解明するための研究を行った。本シミュレーションでは、星間ガス
の自己重力、輻射冷却、星形成、超新星フィードバック、ガス降着を考慮している。我々
のシミュレーション結果は年齢速度分散関係と単純な星の速度分散の時間進化が異なるこ
とを示す。 星の速度分散は (1) 星形成時に材料となるガスの速度分散、(2) 散乱源とな
る高密度ガスクランプの総質量に依存して多様な進化を示すことを発見した。ガスの物理
状態はガス降着史に依存して時々刻々と変化するため、星の速度分散の時間進化は星が形
成された時刻によって振る舞いが大きく異なる。本講演では、これらの結果が天の川銀河
進化史研究に及ぼす可能性についても議論を行う。
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Diffuse Lyman-alpha halos around star forming galaxies
百瀬 莉恵子 (天文学教室)
Theoretical studies have suggested that the Lyα emission of high-z
star-forming galaxies is more spread than their UV emission radiating
from star-forming regions (e.g., Laursen & Sommer-Larsen 2007, Zheng
et al. 2011, Dijkstra & Kramer 2012, Verhamme et al. 2012). This
extended Lyα emission around galaxies is so called Lyα halos (LAHs).
Because LAHs are too diffuse to detect from a single galaxy, the
stacking analysis is often used to find the evidence of LAHs from
observational data (e.g., Steidel et al. 2011, Matsuda et al. 2012).
However, there is an argument that artifacts by the analysis are
possible to produce artificial LAHs. In order to investigate the
presence of LAHs, we carefully examined potential artifacts mimicking
LAHs by using large star-forming galaxy, Lyα emitters (LAEs), samples
at z = 2-7, and found LAHs around them (Momose et al. 2014). In this
talk, I show results of LAHs over redshifts and discuss their size
evolution. Additionally, I also discuss dependences between LAHs’ size
and physical properties of their host LAEs at z=2.2 (Momose et al.
2016). If I have more time, I introduce recent studies of LAHs (e.g.,
Wisotzki et al. 2016, Leclercq et al. 2017, Matthee et al. 2017).
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天の川銀河・太陽円内側領域のセファイド探査
柳澤 顕史 (国立天文台)
セファイドは、銀河系の構造を知る上で有用な、若い星のトレーサである。その実体は中質量星であり、主系列を離れ、巨星・超巨星に至った進化段階にある。セファイドは、厳密に規則的な周期的光度変化を示すため、数多の変光星からのなかから見分けやすい。その上、周期光度関係や周期年齢関係を持つため、観測から対象の距離と年齢を知ることができる。天の川銀河をモニタ観測してセファイドを見つければ、若い星の空間分布を知ることができて、20-100Myr の星形成史を遡ることができる。
セファイドをプローブとした天の川銀河の構造を探る試みは多くの研究者が進めてきた。しかし、太陽円内側については 3kpc より遠方のセファイド分布は殆ど知られていない。これまでの探査は主として可視光線で行われてきたのが原因である。この状況を打開する方法は、星間吸収の影響を受けにくい近赤外線での探査が有効である。そこで、我々は岡山天体物理観測所の91cm反射望遠鏡を広視野近赤外線カメラに改造し、銀河面をKsバンド(波長2ミクロン)の広域探査を開始した。これまでに、銀経26度を中心とする30平方度をモニタした。本講演では本探査の現状と初期成果について紹介する。
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Nearby lethal supernovae may be responsible for the mysteriously small dark energy
戸谷 友則 (天文学教室)
The observed value Lambda_obs of the cosmological constant Lambda is extremely smaller than theoretical expectations, and the anthropic argument has been proposed as a solution to this problem because galaxies do not form when Lambda >> Lambda_obs. However, the contemporary galaxy formation theory predicts that stars form even with a high value of Lambda / Lambda_obs ~ 50, which makes the anthropic argument less persuasive. Here we calculate the probability distribution of Lambda using a model of cosmological galaxy formation, considering extinction of observers caused by radiation from nearby supernovae. The life survival probability decreases in a large Lambda universe because of higher stellar density. Using a reasonable rate of lethal supernovae, we find that the mean expectation value of Lambda can be close to Lambda_obs, and hence this effect may be essential to understand the small but nonzero value of Lambda. It is predicted that we are located on the edge of habitable regions about stellar density in the Galaxy, which may be tested by future exoplanet studies.
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北天で発見された最近傍重力マイクロレンズ系外惑星のフォローアップ観測
福井 暁彦 (国立天文台ハワイ観測所岡山分室)
恒星と惑星が引き起こす重力レンズ効果を利用して系外惑星を発見する「重力マイクロレンズ法」は、1) 主星の明るさに依存せず惑星発見が可能であり、2) 主星から数AU離れた雪線以遠の惑星に最も感度が高いという特徴をもつ。同手法による惑星探索は、専ら重力マイクロレンズ現象の発生頻度が最大となる銀河中心方向で行われている。これまでに約50個の系外惑星が発見され、雪線以遠では海王星質量の惑星が最も豊富であることなどが明らかとなった。一方、同手法には、発見される惑星系の大半は太陽系から非常に遠く、フォローアップ観測がほぼ不可能という欠点がある。
2017年10 月、日本のアマチュア天文家(小嶋正氏)によって、おうし座方向で距離約 700 pc の F型星を光源とする重力マイクロレンズ現象が偶然発見された。さらに、海外のアマチュア天文家らのフォローアップ観測によって増光ピーク付近で光度曲線の逸脱が捉えられ、後の解析からレンズ星に惑星が付随する事が判明した(Nucita et al. 2018)。我々は天体発見の一報を受け、直ちに岡山188cm望遠鏡/MuSCAT、カナリア諸島1.5m望遠鏡/MuSCAT2、岡山91cm望遠鏡/広視野赤外カメラなどを使って多バンドでの測光フォローアッ プ観測を開始した。その結果、高精度かつ密な光度曲線が得られ、その解析を通して主星の放射光を検出することに初めて成功した。それにより、惑星系の物理量が正確に求まり、主星は距離約500 pcのM/K型矮星であり、惑星は海王星質量をもつことが判明した。この惑星系は重力マイクロレンズ法で発見された惑星系の中で最も近傍にあり、主星が比較的明るいため、将来的に様々なフォローアップ観測が期待される。
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すばる広視野観測で探る高赤方偏移クェーサーの性質
尾上 匡房 (MPIA)
赤方偏移z>6の時代に存在するクェーサーは宇宙再電離期を探る上で重要な観測対象の一つである。これまで広視野サーベイ観測により200 個以上のz>6クェーサーが発見されており、宇宙再電離がz=6で終了したこと、ビッグバン後10億年未満の時代にモンスター級(10億太陽質量)の超巨大ブラックホールが存在し非常に高い質量降着率で成長中であることなどが解明されてきた。一方で既存のz>6クェーサーの大半はSDSSクラスの非常に明るいものばかりであるため、クェーサー光度関数の低光度側やz>6での典型的な超巨大ブラックホール成長の様子などは不明であった。そこで我々は広視野観測を得意とするすばる望遠鏡を用いてz>6低光度クェーサーの探査に取り組んでいる。まずSuprime-Camを用いて6.5平方度の領域でz=6クェーサーを探査したところ合計2つのクェーサーを発見し、得られたクェーサー光度関数からクェーサーから放出される電離光子の量が再電離に必要な量の1-12% (2σ)しかないことを示した。さらに現在進行中のHSCすばる戦略枠サーベイ(HSC-SSP)においても既に60個の発見に成功しており、z=6-7低光度クェーサーの巨大サンプルを構築しつつある。これらのうち6天体をVLT/XSHOOTER、Gemini/GNIRSで近赤外追観測を行ないBH質量測定を行なった結果、BH質量とエディントン比にばらつきが見られた。特に10億太陽質量、エディントン比0.1の超巨大ブラックホールはz>>6で既に成長を止めてしまった種族だと考えられる。またターゲットの1つが母銀河の爆発的星形成によって生まれたダストが晴れていく途上段階にあることもわかった。本講演では、これらのプロジェクトの結果に加えて高赤方偏移クェーサー探査の今後の展望、さらに自身の海外生活についても時間が許す限りお話したい。
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Galactic Dynamics with Gaia DR2
河田 大介 (University College London)
European Space Agency’s Gaia mission has made the 2nd data release (Gaia DR2) on 25th April 2018. Gaia DR2 provides position, parallax and proper motions for more than one billion stars with unprecedented accuracy and radial velocity for about 7 million bright stars. In this talk, I will summarise our Galactic dynamics studies with Gaia DR2. First, I will show that for the first time Gaia DR2 has revealed the velocity structures of the Galactic stellar disk in the large range of radii, 5 < Rgal < 12 kpc. We found many diagonal ridges in the Rgal vs. Vrot map, which are expected from the influence of the bar and spiral arms. Second, I will talk about our search for dwarf galaxies in Fermi-LAT extended unassociated source fields, and discuss implications for annihilating dark matter. Finally, I will present our measurement of Galactic rotation at the Sun’s location from the cross-matched data of the existing Cepheids catalogue and the Gaia DR2 proper motion, and discuss the influence of the bar and spiral arms on such measurements.
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Filamentary accretion flows around massive protoclusters
Vivian Chen (清華大学)
Although filamentary structures are ubiquitous in molecular clouds, basic observational constraints are needed to clarify the role of filaments in the mass assembly process. Using ALMA Band 3, we have performed full-synthesis imaging of the N2H+ (1-0) emission in the remarkable IRDC complexes, M17 SWex, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. We derive gas kinematics by fitting the hyperfine components of N2H+ spectra and measure velocity gradients along 11 identified filaments with principal component analysis. The mass accretion rates along the filaments are fairly large and significant to affect the hub dynamics within one free-fall time. The N2H+ filaments are likely to be in equilibrium with lower virial parameters in more massive filaments. These filaments are gravitationally bound with possible support from magnetic fields. Comparison study of virial parameters measured in the NH3 filament, N2H+ filament, NH3 clumps, and 3mm continuum dense cores will be discussed. Our observation results are consistent with the hierarchical gravitational collapse scenario.
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Three-body dynamics close to supermassive black holes: on the origin of the S-stars in the Galactic center
Alessandro Trani (天文学教室)
The environment close to supermassive black holes (SMBHs) has stellar densities that can reach 107 Msun/pc3. An example is the SMBH in our galaxy, which features a density cusp composed of old and young stars and even compact remnants. Moreover, a high number of binaries is expected to be present close to SMBHs in galactic nuclei, due to either dynamical friction or star formation. These binaries can efficiently interact dynamically with other stars, undergoing close encounters that can trigger tidal disruptions and mergers.
We investigated the outcome of 3-body encounters between binary and single stars while in orbit around a SMBH, by means of high-accuracy N-body simulations performed with the new state-of-art code TSUNAMI.
Three-body encounters can ionize the binaries so that their member are scattered into highly eccentric orbits about the SMBH. Via this scenario, we can explain the origin of 11 S-stars, the eccentric stars observed orbiting around the SMBH in our galaxy. In addition, binaries that survive the encounters become more eccentric. This effect can be very important for compact binaries that can coalesce via gravitational wave emission.
First infrared interferometric mapping of the gas motion in the atmosphere of stars in late evolutionary stages
大仲 圭一(北部カトリカ大学, チリ) / Keiichi Ohnaka (Universidad Católica del Norte, Antofagasta, Chile)
Studies of the mass loss from stars in late evolutionary stages are of utmost importance for improving our understanding of not only stellar evolution but also the chemical enrichment of galaxies. Despite such importance, the mass loss from cool evolved stars is one of the long-standing problems in stellar astrophysics. Milliarcsecond-resolution achieved by optical/infrared long-baseline interferometry provides a unique opportunity to spatially resolve the innermost key region, where the mass loss is considered to be initiated. We have recently succeeded not only in imaging the surface of the red supergiant supernova progenitor Antares in the 2.3 micron CO lines in unprecedented detail but also in witnessing, for the first time, the complex gas dynamics over the surface and atmosphere of the star. Our two-dimensional velocity field map of Antares reveals vigorous upwelling and downdrafting motions of large gas clumps in the atmosphere extending out to 1.7 stellar radii. This suggests that the mass loss in red supergiants may be launched in a turbulent, clumpy manner. We will also present the preliminary results of the next step of this velocity-field imaging: 3-D tomographic velocity-field imaging throughout the stellar atmosphere. Our work opens an entirely new window to observe stars just like in observations of the Sun, bringing stellar astrophysics to a new dimension.
A new parameter in the fundamental metallicity relation of star-forming galaxies
橋本 哲也 (台湾国立清華大学) / Tetsuya Hashimoto (National Tsing Hua University, Taiwan)
Star-forming galaxies display a close relation among stellar mass, metallicity, and star formation rate (or molecular-gas mass). This is known as the fundamental metallicity relation (FMR) (or molecular-gas FMR), and it has a profound implication on models of galaxy evolution. However, there still remains a significant residual scatter around the FMR. We show here that a fourth parameter, the surface density of stellar mass, reduces the dispersion around the molecular-gas FMR. In a principal component analysis of 29 physical parameters of 41,338 star-forming galaxies, the surface density of stellar mass is found to be the fourth most important parameter. The new 4D fundamental relation forms a tighter hypersurface that reduces the metallicity dispersion to 50 per cent of that of the molecular-gas FMR. We suggest that future analyses and models of galaxy evolution should consider the FMR in a 4D space that includes surface density. The dilution time-scale of gas inflow and the star-formation efficiency could explain the observational dependence on surface density of stellar mass.
Massive stars formed in atomic hydrogen reservoirs
Michał Michałowski (Adam Mickiewicz University, Poland)
I will discuss the process of gas inflow on galaxies and subsequent fuelling of star-formation. Using ATCA HI observations I will show that galaxies with anomalous local metallicity decrements (gamma-ray burst host galaxies) have substantial atomic gas reservoirs, and are possibly deficient in molecular gas. This suggests that star formation in these galaxies may be fuelled by recent inflow of metal-poor atomic gas. This is controversial, but can happen in low-metallicity gas near the onset of star formation because cooling of gas (necessary for star formation) is faster than the HI-to-H2 conversion.
Future Prospects for Helioseismology
Michael J. Thompson (NCAR, USA)
Helioseismology has been spectacularly successful at revealing many aspects of the Sun’s internal structure and dynamics. What then are the new challenges and opportunities for the field? In this talk I will review the current state of helioseismology, and will then discuss science opportunities and future directions for observations from the ground and space.
Spiral Structure in Galactic Disk
江草 芙実 (天文学教育研究センター) / Fumi Egusa (Institute of Astronomy)
In the local universe, more than half of galaxies are categorized as a disk galaxy, which often exhibit a spiral structure. Meanwhile, the fraction of disk (and spiral) galaxies is lower in the distant universe. It it thus likely that the fraction has increased as the universe evolves. However, how disk galaxies and their spiral structure have evolved and reached to the current status is not fully understood yet.
Theoretical models on spiral structure can be divided into two in terms of its lifetime: long-lived (density wave) or short-lived (swing amplification, tidal arms). In this talk, I will present how these models are compared to observational results on nearby galaxies.
Observational studies on spiral structures in distant galaxies are still limited. I will briefly review the current status and future prospects of researches on the evolution of spiral galaxies.
Superluminous supernovae and their origin
守屋 尭 (国立天文台) / Takashi Moriya (NAOJ)
Superluminous supernovae are a newly-recognized class of core-collapse supernovae that often become more than 10 times brighter than standard core-collapse supernovae. Among them, the energy source and progenitors of hydrogen-poor (Type Ic) superluminous supernovae are not well understood. It is likely that there is some sort of central energy source powering hydrogen-poor superluminous supernovae. After introducing the observational properties of hydrogen-poor superluminous supernovae, I will discuss their possible central power sources. Although their main power source is likely at the center, I will argue that several lines of evidence indicates the existence of dense massive circumstellar media around hydrogen-poor superluminous supernovae. I will also discuss the possible origin of such dense circumstellar media.
Connecting the dots: disk formation around low-mass stars to Solar System formation
Daniel Harsono (Leiden University)
Stars and planet-forming disks form out of cold and dense clouds. One of the major questions in astrochemistry is whether or not the disk inherits the material of the protostellar envelope. Models of the young solar nebula, the disk that formed the Solar System, suggest that the chemical composition has been reset on the way to the disk. Some of these evidences are provided by the meteoritic studies. However, it has been only recently that it is possible to study the early stages of star formation in the spatial detail that matches the young solar nebula. I will present the on-going projects to understand the disk formation process and its implications on the early stages of planet formation.
宇宙人存在問題 / Question on Alien Civilization Existence
福島 登志夫 (国立天文台) / Toshio Fukushima (NAOJ)
(Japanese)
宇宙人存在問題とは、「なぜ、我々は宇宙人と出会っていないのか?」という素朴だが難解な問題を指し、フェルミの矛盾とも呼ばれる。これに関して度々引用されるのがドレイク方程式である。しかしながら、自然科学の範疇内でドレイク方程式の精密化を試みても不毛な議論となる可能性が高い。その大きな理由は、「知的生命は自然科学的原理だけでは行動しない」からである。大航海時代を引用するまでもなく、文明が未知域への探検に巨額の投資を行う背景には、軍事・政治・経済・宗教あるいは知的好奇心など何らかの文化科学上の行動原理がカギとなりうる。一方、最近のAIの進展を見ると、知的生命が生物である必然性は薄らいでいるようにも思える。実際、「光速の壁」を克服するためには自己複製可能なクローンあるいはロボットのみが広い宇宙を席巻できるとすると、もっと違った文化的考察が必要かもしれない。いずれにしろ、宇宙人存在問題を深く議論するためには、文理双方の知識・技能・方法論を総動員することが必要であろう。本講演では、これまでの研究を概括するとともに文科系の研究者と議論する機会により得られた新しい回答案をいくつか紹介する。
(English)
This talk focuses on the Fermi paradox, namely “Why do we not meet (or have we not met) an alien?”. When talking about this issue, almost always quoted is the Drake equation. However, the elaboration of the Drake equation within the framework of natural sciences is more likely to become the sterile argument even if it is precise. The main reason is that the intellectual life does not act only on a principle of the natural sciences. Needless to say for the Age of Geographical Discovery, the motivation upon which any civilization based in investing a lot of labors and assets in the exploration to an unknown place, consists of the military affairs, politics, economy, religion, or the intellectual curiosity as ground rules. Having said that, the recent progress of the artificial intelligence makes us to feel that the intellectual life may not be necessary a biological creature. Actually, the so-called light-speed-barrier requires a time much longer than the typical lifetime of all biological species in completing the interstellar voyage. Stephen Webb published a couple of interesting books (Webb, 2002, 2015) assembling the known and his answers to the Fermi paradox. Following him, I dare raise 10 possible solutions of mine.
Reference: Webb (2015), If the Universe is teeming with aliens … Where is everybody?, Springer, Berlin
CLASP projects: a window to explore the upper solar atmosphere
鹿野 良平 (国立天文台) / Ryouhei Kano (NAOJ)
The Sun has been observed for a long-long time. However, we have not yet well understood its activities. Hot (>1MK) plasma in the corona widely extends above the cool (~6000K) solar surface “photosphere”. This is so called the “coronal heating problem”. In the chromosphere, which is the interface layer between the photosphere and the corona, many energetic phenomena like waves or jets were found by the Hinode satellite. We believe that the chromosphere and many phenomena in it play an important role to transfer the photospheric energy to the corona, and eager to know physical quantities in the chromosphere, especially magnetic fields.
To reveal the upper-chromospheric magnetic fields, we have been carrying out the CLASP projects, which is sounding rocket experiments to perform something during a ballistic flight. The 1st flight was performed in September of 2015 as the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP), and successfully observed the linear-polarization in the Lyman-alpha line (121.6nm) from the solar chromosphere and/or transition region with the high precision (<0.1%). We got a hint for magnetic fields in the chromosphere. To establish the way to reveal the magnetic fields in the upper solar chromosphere, the 2nd flight has been already scheduled in the spring of 2019 as the Chromospheric LAyer Spectro-Polarization (CLASP2), which will take linear and circular polarization of the Sun in Magnesium lines around 280 nm.
In the seminar, I would like to introduce the results in the 1st flight and also the progress in the preparation for the 2nd flight.
The nature of Lyα emitters: SFR, stellar mass, and dark matter halo mass
日下部 晴香 (天文学教室) / Haruka Kusakabe (Department of Astronomy)
Lya emitters (LAEs) are a representative dwarf galaxy population at high redshift characterized by strong Lya emission. However, the stellar mass (Ms), SFR, and dark halo mass (Mh) of LAEs have not been estimated with high enough accuracy to place them in the framework of structure formation. Moreover, the origin of diffuse Lya halos (LAHs) has not been identified in spite of the fact that the Lya luminosity of LAHs is comparable to or even higher than that of the main body of LAEs. To address these issues, we study the stellar and dark matter halo properties of LAEs using a large sample of z~2 LAEs for which LAH luminosities are available by stacking analysis. We find that while these LAEs lie on star formation main sequence (SFR-Ms relation), they have relatively high Ms/Mh and SFR/Baryon accretion rate ratios. It implies that LAEs have been converting baryons into stars efficiently in spite of their low Mh. We will discuss the physical mechanisms of their star formation activity. With regard to their LAHs, we find a nearly constant LAH luminosity with Ms and Mh. This suggests that LAHs are caused by scattering of Lya photons in the circum-galactic medium, not by in situ gas cooling or star formation in satellite galaxies, because the latter two origins predict a roughly linear scaling of LAH luminosity with these masses.
若い惑星の直接撮像と特徴付け / Direct Imaging and Characterizations of Young Exoplanets
鵜山 太智 (天文学教室) / Taichi Uyama (Department of Astronomy)
1995年に太陽と他の恒星を回る惑星が初めて発見されて以来、数千もの系外惑星が現在までに報告されている。その中では太陽系の惑星とは大きく異なる形態を持つものが多数報告されており、これらを説明できる惑星形成・進化のメカニズムは未だに謎のままである。本研究では惑星形成・進化の理論モデルに対して観測的な見地から議論を行うため、若い惑星に感度の高い直接撮像法を用いた系外惑星探査と特徴付けを行った。大きく分けて二つの議論を行っており、本講演ではそれぞれの観測手法を紹介し、また得られた結果について考察する。
Rethinking the dynamical paradigm of low-mass stellar systems
Anna Lisa Varri (天文学教室)
The blooming era of ‘precision astrometry’ for Galactic studies opened by Gaia DR2 and the revolutionary beginnings of ‘gravitational wave astronomy’ enabled by aLIGO truly bring the rich internal dynamics of collisional stellar systems to the centre stage. But the current interpretative picture of the role of ‘small scales’ in the assembly of cosmic structures cannot cope with such new-generation data and fresh theoretical ambitions, therefore a modern view of low-mass stellar systems within the rapidly evolving landscape of near-field cosmology is urgently needed. In this context, I will present the highlights of a more realistic dynamical paradigm for globular clusters, with emphasis on the role of angular momentum, velocity anisotropy, external tidal field, and their non-trivial interplay. Such a fundamental understanding of the emerging phase space complexity of globulars and other low-mass stellar systems will allow me to address three puzzles at the core of the progressively blurrier ‘star cluster-galaxy divide’: 1) How did the first stellar aggregates form at high redshift? 2) Is there a ‘missing link’ between stellar and super-massive black holes? 3) What is the limit of the presence of dark matter on small astrophysical scales?
The production and escape of ionising photons from early star-forming galaxies
中島 王彦 (コペンハーゲン大学) / Kimihiko Nakajima (University of Copenhagen)
The ionising output of early star-forming galaxies is key to understanding their role in cosmic reionisation. Major uncertainties include the number of ionising photons per UV luminosity (xi_ion) and the fraction that can escape (f_esc). Since neither can be directly observed for sources beyond z=6, it is important to measure these parameters for suitable analogues at lower redshift. We have thus embarked a detailed survey of a large sample of z=3 Lyman alpha emitters (LAEs) as useful analogues of galaxies in the early universe. In this talk, I will present our on-going programme to characterise LAEs in terms of the ionising photon production and escape, discuss the implications for cosmic reionisation if the metal-poor intensely star-forming systems like LAEs are representative examples of those at much higher redshift.
Discovery and characterization of an ultra-short-period planet from K2
John Livingston (天文学教室)
Of the various types of planets not represented in the Solar system, planets with orbital periods less than one day, known as ultra-short-period (USP) planets, are one of the most mysterious. In addition to enabling statistical studies by increasing the sample size, the discovery of new USP planets amenable to characterization studies can yield important insights into the processes of planet formation. Thanks to its wide survey, the NASA K2 mission has enabled the discovery of planets orbiting stars brighter than those studied by the original Kepler mission, providing unique opportunities for planet characterization using ground-based high precision spectrographs. We identified a candidate USP planet in the K2 Campaign 10 photometry, which we subsequently analyzed in detail using follow-up high resolution speckle imaging and transit photometry. The results of these analyses enabled us to statistically validate the planet, and we subsequently obtained high precision radial velocity (RV) measurements of the host star using the HARPS and HARPS-N spectrographs. The RVs reveal not only the mass (and density) of the transiting USP planet, but also a second planet in the system; the presence of a non-transiting outer planet may provide important clues about USP planet formation pathways.
NIR insights into Classical Cepheids with WINERED
Scarlet S. Elgueta (天文学教室)
Estimating an accurate and precise distance to any astrophysical object is key to understand it. Cepheids have proven to be of crucial importance in setting and constraining the (extra)-galactic distance scale but remain intriguing due to the phenomena occurring in their pulsating photospheres leading to implications on the distance determination, i.e., the Baade-Wesselink method.
I will present my ongoing study performed with time-series spectroscopic data for Cepheids provided by WINERED NIR high-resolution spectrograph; the main goal is to characterize the variations of velocity and profile of absorption lines in the NIR for the first time, focusing on two special targets: delta Cep and Polaris. The former is a prototype widely studied “well behaved” s-Cepheid while the latter, is a complicated object which raises puzzling issues described in previous studies that show discrepancies regarding various features, i.e., its pulsation mode, evolutionary stage and distance. I’d also like to discuss GAIA DR2 results compared to previous studies considered as geometrical and quasi-geometrical distances.
New insights on the high-z universe with MUSE
Edmund Christian Herenz (Stockholm University)
The Multi Unit Spectroscopic Explorer (MUSE) is a 2nd generation instrument at ESOs Very Large Telescope UT4. It is an integral field spectrograph with a large field of view that is sampled at unprecedented spatial and spectral resolution. We have used this instrument both as a survey machine for high-redshift galaxies, as well as for detailed studies of individual nearby galaxies which are analogues to high-redshift galaxies. I will present in detail one of the MUSE survey campaigns – MUSE-Wide – that surveys 100 arcmin2 in fields where deep Hubble Space Telescope imaging is available. I will detail how we constructed the luminosity function of Lyman alpha emitting galaxies from this data. I will then outline how Lyman alpha luminosity function studies can be used to constrain the hydrogen ionising photon budget in the early universe. Moreover, I will show recent MUSE results from two nearby high-z analogues (SBS 0335-052E and ESO338-IG04) that offer clues on the physical mechanisms that enable hydrogen ionising radiation to escape from galaxies.
The early generations of rotating massive stars and the origin of Carbon-Enhanced Metal-Poor stars
Arthur Choplin (甲南大学)
The study of the long-dead early generations of massive stars is crucial in order to obtain a complete picture of the chemical evolution of the Universe, hence the origin of the chemical elements. The nature of these stars can be inferred indirectly by investigating the origin of low-mass metal-poor stars observed in our Galaxy, some of which are almost as old as the Universe. The peculiar extremely iron-poor Carbon-Enhanced Metal-Poor (CEMP) stars, whose precise origin is still debated, are thought to have formed with the material ejected by only one or very few previous massive stars. After introducing the topic and reviewing some effects of axial rotation on massive stars, I will discuss how the physics — especially rotation — and the nucleosynthesis of the early generations of massive stars can be explored by combining stellar evolution modeling with observations of CEMP stars.