天文学教室談話会(2017年度)

last-update:2017/07/27

最新の情報はこちらをご覧ください。

  第1621回 2017年4月11日(火)

ALMAで探る惑星形成(Planet Formation Explored by ALMA)
秋山永治 (国立天文台 チリ観測所)
近年の赤外線および電波観測技術が飛躍的に向上したことで、惑星系の
前駆天体である原始惑星系円盤内のギャップ、渦状腕、非対称構造など、
惑星の形成過程を理解する上で大変重要な観測結果が得られている。
しかし、そのような構造の起源については様々な解釈がされており、
統一した理解に至っていない。さらに、以前から近赤外線観測によって、
これまで標準理論として認識されていた惑星系形成理論の修正が予見され、
世界最大のミリ波サブミリ波望遠鏡ALMAを用いた高感度・高空間分解能観測
では、複数の形成過程の検討が要請される観測結果が数多く得られている。
従って、惑星の形成過程を理解するには、より詳細に原始惑星系円盤を観測
し、惑星系スケールの領域で実際に何が起こっているのか明らかにする必要
がある。
我々のグループでは、すばる望遠鏡の戦略枠であるStrategic Explora-
tions of Exoplanets and Disks with Subaru(SEEDS)プロジェクトで
得られた近赤外線による原始惑星系円盤の観測結果を基に、太陽と同規模の
中心星を有し、且つ原始惑星系円盤の中心で惑星系の基となる構造が存在す
ると考えられる天体を3つ選定し、ALMAを用いて中心星から数100AUの円盤
外縁部から半径30AU以内の領域について詳細な観測を行った。その結果、
中心星近傍領域のガスとダストの分布や、理論モデルで予想されるブリッジ
構造の存在が明らかとなり、ガスの運動と併せて解析することで惑星系形成
時の物質供給過程についての観測結果が得られた。さらに、すばる望遠鏡で
得られた近赤外線の観測結果と比較することで陰影効果によるダスト温度の
影響についても新たな知見が得られた。また、太陽と同程度の質量を持つ若
い星と褐色矮星程度の天体からなる連星系において、両者間で物質の供給を
示す結果が得られた。本結果は、褐色矮星の形成過程の一つとして、褐色矮
星は連成系の枠組みで主星と共進化していくことを示し、褐色矮星の形成過
程を説明する貴重な観測的証拠である。
本談話会では、SEEDSプロジェクトからALMA観測に至るまでの研究の流れ
を示し、ALMAで得られた最新の原始惑星系円盤の観測結果について紹介する。
また上述の研究と関連し、主に日本のグループが主導で行っている最近の原始
惑星系円盤の観測的研究についても併せて紹介する。
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  第1622回 2017年4月25日(火)

Technical Challenges in Scaling up to an Extremely Large Telescope
Matthew William Johns (The University of Arizona / The University of Tokyo)
The coming generation of Extremely Large Telescopes (ELTs) will be a factor 2.5
to 4 larger in diameter than the current generation of 6-10 meter ground-based
optical/infrared telescopes. Their design and construction poses a set of technical
challenges in scaling up from their smaller predecessors. This talk discusses
a few of those challenges from the speaker’s experience with one such project,
the Giant Magellan Telescope (GMT). Many of the remarks apply to other ELT projects
as well.
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  第1623回 2017年5月2日(火)

A Light in the Dark – Massive Star Birth Through Cosmic Time
Jonathan C. Tan (Univ. of Florida)
Massive stars have played a dominant role in shaping our universe since its earliest times,
but there is still no consensus on the mechanism by which they form. I review the physics
important for massive star formation and the intimate connection this process has with star
cluster formation. I then focus on a particular theoretical model, Turbulent Core Accretion,
which assumes the initial conditions are massive, turbulent, magnetized cores of gas and dust
that are reasonably close to virial equilibrium. Our group has been exploring this scenario
via analytic models and numerical simulations of the physics and chemistry of the interstellar
medium. Crucially, these models can now be tested in detail with ALMA and I present the latest
results from multiple projects that are zooming in to massive star birth in the darkest shadows
of giant molecular clouds. Extension of this work has the potential to also determine how the
full stellar initial mass function is established across different Galactic environments.
I then switch to the protostellar accretion phase and the emergence of feedback processes,
presenting both theoretical predictions and observational tests. Finally, I discuss an
application of massive star formation theory to the early universe: how massive were the
first stars and could they have been the progenitors of supermassive black holes?
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  第1624回 2017年5月9日(火)

Astrochemistry in Star-Forming Regions
星・惑星系形成領域の星間化学:分子組成と同位体比
相川祐理 (東京大学)
In star formation, gas, dust and icy material accrete onto a circumstellar disk, where they will eventually be incorporated to planetary material. In order to reveal the evolution of interstellar material from clouds to planetary systems, we simulate the evolution of molecular abundances and isotope ratios in star formation processes, and compare the results with observations. In interstellar clouds, the chemical timescale is often longer than the dynamical timescale. The molecular evolution thus needs to be calculated along the dynamics. Several groups, including us, are currently working on astrochemical modeling of dense cores, protostellar cores, and protoplanetary disks. It is also getting clear that the ‘initial condition’ is important in such models; e.g protostellar core is characterized by the sublimation of ices which originate in prestellar phase. I will present models covering from cloud formation to protoplanetary disks, and compare them with observations.

(分子雲中のガス、氷、ダストは星形成過程において円盤に取り込まれ、惑星系の材料物質となる。我々は
星・惑星系形成過程における分子組成・同位体比進化の数値計算を行い、その結果を天文観測と比較する
ことで、星間物質から惑星系物質への進化の解明を目指している。分子雲での化学反応の時間スケールは、
多くの場合、系の動的進化の時間スケールと同程度か長い。よって化学進化は非平衡であり、重力収縮な
どの動的進化に沿って反応速度方程式を解く必要がある。現在では、高密度分子雲コア、原始星コア、
原始惑星系円盤等について、それぞれ系の動的な進化や物理構造を考慮した星間化学の研究が盛んに行わ
れている。一方、最近これら各天体の組成進化を解く際の初期組成の重要性も認識されてきている。例え
ば、原始星コアの組成は、その前段階である高密度分子雲コアで生成された氷の組成に大きく影響される。
よって、分子雲の形成から原始惑星系円盤までの進化全体を視野にいれて研究を進めることも重要である。
講演では、分子雲形成から、高密度分子雲、原始星、原始惑星系円盤までの組成進化モデルを概観し、こ
れをALMAなどで得られている観測結果と比較する。)
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  第1625回 2017年5月16日(火)

Early Universe and Dust in Galaxies
Denis Burgarella (Laboratoire d’Astrophysique de Marseille, Aix-Marseille Universite, CNRS)
The average dust attenuation of galaxies in the universe increases from z = 0 to z = 1 – 2. This result has been known for quite a long time thanks to ISO, Spitzer and Akari. To gather some information on what happened earlier in the history of the universe, we had to wait for Herschel, ALMA in the Southern Hemisphere and now IRAM in the Northern Hemisphere. With Herschel, we observe a maximum in the amount of cosmic dust attenuation followed by a decrease at z ~ 2.5. At higher redshifts, we observe a decrease and, by z ~ 4, we reach about the same level than at z ~ 0 i.e. 50% of the star formation in UV and 50% in the IR. At even larger redshifts, we expect a further decrease which means that statistically, galaxies in the early universe at z > 6 will have low dust content.

Does it mean that we can forget about dust? The answer is probably “no” for at least two reasons: 1) we know that there are massive and dusty galaxies up to the highest redshifts. Most of them being detected via a lensing amplification and 2) even a small amount of dust can impact the stellar emission in galaxies via the UV dust attenuation.

What do we know about dust in the first billion year of the universe? In this seminar, we will try to collect some information and ask questions about how progress can be made to better understand this still unknown early dust and its origin.
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  第1626回 2017年5月30日(火)

Mean-field dynamos on late-type stars
Valery Pipin (Russian Academy of Sciences, Moscow)
The talk reviews results of the recent and ongoing theoretical works
about dynamo models of the large-scale magnetic activity on the Sun and
the fully convective stars. I make an overview on the observational
results about magnetic activity on the late-type stars. The basic ideas
of the mean-field models will be introduced, as well. I discuss results
of the mean-field dynamo models for the Sun and some M-dwarfs stars. The
discussion concerns similarities and differences of the dynamo processes
on the stars. It touches the number of topics, such as, the role of the
differential rotation in the dynamo, excitation the non-axisymmetric
magnetic field, the magnetic feedback on the global flows and the
typical topology of the dynamo generated magnetic field. The topology
and strength of the global magnetic field affect the coronal activity. I
hope that this discussion can shed some light on the origin of the
enhanced coronal and flare activity on the young solar analogs.
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  第1627回 2017年6月19日(月)

Exploring the early evolution of the Milky Way with LAMOST and Subaru
Haining Li (National Astronomical Observatories, Chinese Academy of Sciences: NAOC)
Surveys of very metal-poor (VMP) stars and follow-up spectroscopic studies for them in the past decade have provided with abundant information on the nature of first stars and early chemical evolution of the Milky Way. LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) will soon accomplish its first 5-year spectroscopic survey, and has already observed over 6 million Galactic stars. Such huge database will provide an unprecedented chance to enlarge the currently limited VMP star sample. Since 2014, a joint project on searching for VMP stars has been initiated based on the LAMOST survey and Subaru follow-up observation. So far, the project has obtained a success rate of >90% in searching for VMP stars, obtaining chemical abundances for about 230 VMP stars including more than 70 EMP stars and a number of chemically interesting objects such as: (1) Three UMP (ultra metal-poor) stars with [Fe/H] ~ -4.0. One of them is the second UMP turnoff star with Li detection. (2) A dozen Li-rich VMP/EMP stars, including six super Li-rich (A(Li) > +3) stars. They distribute in wide range of evolutionary stage and metallicity (-3.1 < [Fe/H] < -1.7). (3) Other peculiar VMP/EMP stars including r-process enhanced stars, etc. The project has also observed a number of member stars of halo moving groups and low-alpha abundance halo stars, which would shed light on the merging history of our Galaxy. Statistics of the large sample of VMP stars, togerther with abundance patterns of these peculiar objects, will be of great interest and importance to probe the early Galaxy and low-mass star evolution with very low metallicities. PDF

  第1628回 2017年6月27日(火)

High Energy, Cool Transients: Investigating Infrared-Luminous Outbursts with Spitzer
Ryan M. Lau (NASA JPL)
The Spitzer Space Telescope is pioneering the exploration of infrared (IR) luminous transients. In our Spitzer InfraRed Intensive Transients Survey (SPIRITS), we are conducting a systematic search of 200 nearby galaxies for IR-luminous outbursts that elude detection in traditional optical surveys. In this talk, I will present two puzzling discoveries from SPIRITS and discuss open questions and the plans to address them.
SPIRITS has revealed a new class of obscured, red transients with mid-IR luminosities between novae and supernovae that do not exhibit optical counterparts. We call them SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). The key to disentangling the various possible physical origins of SPRITEs is IR spectroscopy, which can only be obtained with the unprecedented sensitivity of the upcoming James Webb Space Telescope (JWST). I will discuss possible SPRITE origins and the JWST Guaranteed Time Observation (GTO) program I am leading to determine their nature.
In SPIRITS, we serendipitously discovered variable mid-IR counterparts of ultraluminous X-ray sources (ULXs). A ULX is an off-nuclear point source showing extreme, super-Eddington X-ray luminosities thought to be driven by accretion onto a compact object in a close binary undergoing Roche Lobe overflow. We attribute the mid-IR emission to a circumbinary dust disk produced by the outflow from the supergiant donor star, but the nature of the variability is still uncertain. I will discuss the work I have lead on dusty ULXs and highlight the importance of coordinated multi-wavelength observations.
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  第1629回 2017年7月4日(火)

Neutrino Oscillations in Core Collapse Supernovae
Yamaç Pehlivan Deliduman (Mimar Sinan University)
Neutrinos are the second most abundant particle species in the universe after
the photons. Their sheer numbers make them major players in many astrophysical
phenomena despite their minuscule properties. Neutrinos also hold a great
potential as a new observational window to the Universe since, due to their
small cross sections, their last point of scattering (and hence their memory)
lies deep within dense astrophysical objects.
In this talk, I will focus on the neutrinos emitted by a proto-neutron star
which forms at the center of a core collapse supernovae. Here, in the deep
regions, self interactions of neutrinos turn their flavor evolution into a
nonlinear many-body phenomenon. Tiny neutrino properties such as their mass
hierarchy and magnetic dipole moments, can be amplified by emergent many-body
effects. I will discuss the consequences of these many-body effects on
supernova dynamics, and various nucleosynthesis processes that take place in
the outer layers of the supernova.
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  第1630回 2017年7月11日(火)

(Sub)millimeter Observations for the Study of Star Formation and the Formation and Evolution of Dust Through Cosmic Times
Johannes Staguhn (The Johns Hopkins University/NASA’s Goddard Space Flight Center)
Understanding the emergence and evolution of star formation, the interstellar medium (ISM) in galaxies, as traced by dust emission, and how these two quantities relate to the underlying dark matter and larger scale structures is a major stated goal of extragalactic astronomy in the US Astronomy 2010 decadal survey. The primary tracers of star formation and the ISM are observable at FIR through (sub)millimeter wavelengths, yet most analysis on early Universe galaxy evolution has been done using optical/near­infrared studies because comparably sensitive Far­Infrared (FIR) through millimeter survey instruments have not existed, so few constraints exist on the rest-frame FIR properties of z>3 galaxies. To bridge this and other gaps in our understanding, the US decadal survey recommended building the CCAT observatory, which was not realized due to funding constraints. In order to compensate for this lack of observational capabilities, we are planning to install our millimeter continuum cameras GISMO and GISMO­2 at the LMT in Mexico, which will allow us to carry out a large fraction of the high­redshift survey science proposed for CCAT.
We will illustrate the anticipated scientific capabilities of our instruments by presenting scientific results we have already obtained from observations with GISMO at the IRAM 30 m telescope in Spain. These observations include deep millimeter surveys and spatially resolved Sunyaev-Zel’dovich effect observations of a high-redshift galaxy cluster.
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  第1631回 2017年7月18日(火)

Astrophysics with Weyl Gravity
Cemsinan Deliduman (Mimar Sinan University)
This talk will introduce an attempt to describe the diverse astrophysical phenomena via Weyl gravity.
In the first part I will review my work on the resolution of the flat galactic rotation curve problem via geometry instead of assuming the existence of dark matter. Motivation for this work came from the observation that the scale independence of the rotational velocity in the outer region of galaxies could point out to a possible existence of local scale symmetry and therefore the gravitational phenomena inside such regions should be described by the unique local scale symmetric theory, namely Weyl’s theory of gravity. Solution to field equations of Weyl gravity determines the special geometry of the outer region of galaxies.
In the second part of the talk it will be conjectured that this special geometry could be valid up to the scale of galaxy clusters. Then one challenge of this approach will be to explain gravitational lens characteristics of galaxy clusters by Weyl geometry without assuming existence of dark matter. Research in this direction will be summarized.
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  第1632回 2017年7月25日(火)

 
ミリ波・サブミリ波ディープサーベイで探る遠方銀河
廿日出文洋 (東京大学 天文学教育センター)
ダストに隠された星形成活動や、宇宙赤外線背景放射の起源を探るためには、ダ
ストからの再放射をとらえるミリ波・サブミリ波観測が重要である。過去の広視
野サーベイにより、ミリ波・サブミリ波で明るい銀河(サブミリ波銀河)が遠方
宇宙 (z > 1) に多数存在することが明らかになった。サブミリ波銀河は、ダス
トに厚く覆われ、大規模な星形成活動(星形成率~100-1000 Msun/yr)を行う爆
発的星形成銀河であり、宇宙における星形成活動や銀河の形成・進化を理解する
上で重要な種族である。一方で、サブミリ波銀河の背景放射(ミリ波・サブミリ
波帯)への寄与は10%-20%程度、宇宙全体の星形成率密度への寄与も10%-20%程度
であることも分かってきた。ダストに隠された活動の全容を明らかにするには、
サブミリ波銀河よりも暗い一般的な星形成銀河をとらえるような深い観測が必要
である。
ALMAの登場により、過去の単一鏡によるサーベイと比較して1桁以上も深い観測
が可能になった。その結果、遠方宇宙における一般的な星形成銀河も検出できる
ようになった。現状ではまだ観測視野が非常に限られているため、より広視野の
サーベイが複数進行中である。本講演では、現在行われているミリ波・サブミリ
波ディープサーベイと、その初期成果を紹介する。
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  第1633回 2017年8月1日(火)

 
Recent highlights of high-energy astrophysical neutrino observations from the IceCube project
Masaaki Hayashida (Chiba University)
IceCube is a cubic-kilometer Cherenkov telescope buried in the ice sheet at the South Pole to detect neutrinos from tens of GeV to several PeV. IceCube is now in operation with the full configuration of detectors (86 strings).
Since the first discovery in 2012, several tens of high-energy astrophysical neutrino candidates have been detected, and provided the first astrophysical neutrino spectrum. On the other hand, despite many efforts of follow-up observations in electro-magnetic wave, no individual astrophysical sources have been identified as the neutrino origins so far.
In this seminar, I’ll present some recent highlights from the IceCube project, including the topics of the neutrino diffuse spectrum, cosmogenic neutrinos and comparisons with the current high-energy gamma-ray results. Some of them have been just updated in the International Cosmic-Ray Conference held in this month (July). If time allows. I’ll also introduce our recent work of a hadronic origin model for a high-energy gamma-ray flare in a blazar (AGN with jet) as a possible neutrino source.
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