Ta-Lab: Colloquium

Ta-Lab Colloquium

(LAST UPDATE on 11-May-2018)

Next Talk

Schedule for 2017

April 12 Dimitris Stamatellos (Univ. of Central Lancashire)
14 Shu-ichiro Inutsuka
19 Jiro Shimoda (Aoyama Gakuin Univ.)
21 Hiroshi Kobayashi
24 Kazuya Takahashi (Tohoku Univ.)
28 Yuri Fujii
May 11 Jonathan Tan (Univ. of Florida)
12 Tsuyoshi Inoue
16 Ryohei Nakatani (Univ. of Tokyo)
19 Keisuke Sugiura
June 2 Torsten Stamer
16 Ryosuke Tominaga
23 Koki Matsumoto
29 Yuhiko Aoyama (Univ. of Tokyo)
30 Kaori Kawamura
July 14 Kensuke Yokosawa
21 Rehearsal of presentations for the summer school by M1 students
28 Steve Vance (NASA & Caltech)
August 8 Daisuke Nakauchi(Tohoku University)
28 Hua-bai Li(The Chinese University of Hong Kong)
September 14-15 Interim Reports
October 2 Chat Hull (Harvard-Smithsonian Center for Astrophysics)
11 Kenji Kurosaki
12 Kate Pattle (UCLAN UK, NAOJ)
18 Ryosuke Tominaga
27 Doris Arzoumanian
November 8 Kensuke Yokosawa
20 Kaori Kawamura
December 1 Shinsuke Takasao
6 Koki Matsumoto
13 Kenta Nakashima
14 Interim report for graduation research
January 24 Shu-ichiro Inutsuka
29 Rehearsal for master defense (1)
31 Rehearsal for master defense (2)
February 16 Kohei Inayoshi (Columbia University)
23 Seminar for Graduation research
March 19 Pedro Palmeirim (Instituto de Astrofisica e Ciencias do Espaso, Porto, Portugal)

Previous Talks

Date/Room Apr. 12, 14:00- @ES606
Speaker Dimitris Stamatellos (Univ. of Central Lancashire)
Title The theory of the formation of brown dwarfs and low-mass stars
Abstract More than half of all stars (including brown dwarfs) have masses below 0.2 Msun. The formation mechanism of these objects is uncertain. I will review the four main theories for the formation of low-mass objects: turbulent fragmentation, ejection of protostellar embryos, disc fragmentation, and photo-erosion of prestellar cores. I will discuss the observational predictions of these models regarding the low-mass initial mass function, the brown dwarf desert, and the binary statistics of low-mass stars and brown dwarfs. I will further discuss whether observations may be used to distinguish between different formation mechanisms, and give a few examples of systems that strongly favour a specific formation scenario. Finally, I will argue that it is likely that all mechanisms may play a role in the formation of brown dwarfs and low-mass stars.
Date/Room Apr. 14, 14:00- @ES606
Speaker Shu-ichiro Inutsuka
Title The Physics in the Formation of Protostars and Protoplanetary Disks
Abstract I will give an introductory lecture about star formation in our Galaxy. In this first presentation I will focus on the physics of gravitational collapse of a dense molecular cloud core and the resulting formation of a protostar and a protoplanetary disk. A overall scenario of star formation in the Galaxy will be presented in the second lecture.
Date/Room Apr. 19, 14:00- @ES606
Speaker Jiro Shimoda (Aoyama Gakuin Univ.)
Title On synthetic measurements of the turbulent magnetic field spectral slope in SNR
Abstract The maximum energy and the production amounts of the cosmic-rays (CRs) in supernova remnants are open issues. In this seminar, we will introduce the method of measurements of turbulent magnetic filed spectral slope using the polarized radio synchrotron emission (Shimoda+ 2017 submitted to ApJ). In addition, we will discuss the CR production amounts, which can be measured by the polarized Balmer line emission (Shimoda+17 in preparation).
Date/Room Apr. 21, 13:00- @ES606
Speaker Hiroshi Kobayashi
Title From Dust to Planets via Collisions
Abstract I will talk about planet formation based on collisional coagulation.
Date/Room Apr. 24, 15:00- @ES606
Speaker Kazuya Takahashi (Tohoku Univ.)
Title Limb-brightening radio emission from the relativistic jet of M87
Abstract M87 is an active galactic nucleus that emits a relativistic radio jet. Thanks to the proximity and the large mass of the central super-massive black hole, VLBI observations have resolved the fine structures of the jet. One of the characteristic features is the limb-brightening structure, where the sheath of the jet is brighter than the spine in radio. Despite the remarkableness, previous studies have failed to reproduce the limb-bright structure. In this talk, we introduce a model to reproduce the limb-bright radio emission image of M87. We modeled the jet structure by using a force-free approximation and assumed synchrotron emission from relativistic lepton particles. Comparing the reproduced images and the observations, we put constraints on the jet structure of M87.
Date/Room Apr. 28, 15:00- @ES606
Speaker Yuri Fujii
Title Formation of Resonant Moons in Weakly Accreting Circumplanetary Disks
Abstract During the formation phase of gas giants, circumplanetary gaseous disk form around the planets. Circumplanetary disks are important not only for mass supply to gas giants but also for formation of regular satellites. Because of the comparatively small size-scale of the sub-disk, quick magnetic diffusion prevents the magnetorotational instability (MRI) from being well-developed at ionization levels that would allow MRI in the parent protoplanetary disk. In the absence of significant angular momentum transport, continuous mass supply from the parental protoplanetary disk leads to the formation of a massive circumplanetary disk. We have developed an evolutionary model for this scenario and have estimated the orbital evolution of satellites within the disk. In a certain temperature range, we find that inward migration of a satellite can be stopped by a disk structure due to the opacity transitions. We also find that the second and third migrating satellites can be captured in mean motion resonances. In this way, a compact system in Laplace resonance, which are similar to inner three bodies of Galilean satellites, can be formed in our disk models.
Date/Room May. 11, 15:00- @ES606
Speaker Jonathan C. Tan (Univ. of Florida)
Title A Light in the Dark - Massive Star Birth Through Cosmic Time
Abstract 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?
Date/Room May. 12, 14:00- @ES606
Speaker Tsuyoshi Inoue
Title Dynamical Evolution of the ISM by Shock Waves
Abstract The interstellar medium (ISM) is an interesting astrophysical fluid where a lot of physical processes (such as radiative cooling/heating, compressibility, magnetic field and so on) play important role. In this seminar I shall start from the review of basics physical properties of the ISM and then discuss its dynamical evolution triggered by shock compression based on our recent MHD simulations. I shall particularly focus on the ISM evolution from warm neutral medium to cold HI cloud, cold HI cloud to molecular cloud, and molecular cloud to dense star forming filaments.
Date/Room May. 16, 16:30- @ES606
Speaker Ryohei Nakatani (Univ. of Tokyo)
Title Radiation-hydrodynamical simulations of photoevaporating protoplanetary disks with various metallicities
Abstract It is observationally known that a protoplanetary disk disperses within a finite timescale. The timescale is referred to as the lifetime of a protoplanetary disk. From the observations of the nearby clusters, the lifetimes have been estimated to be 3-6 Myr (e.g., Haisch et al. 2001). On the other hand, recent observations suggest that protoplanetary disks have shorter lifetimes (~1Myr) in lower metallicity environments (e.g., Yasui et al. 2010). It is not well understood what mechanism(s) can act as the cause(s) to produce such metallicity dependence of the lifetimes. Photoevaporation is proposed as the key mechanism for disk dispersal. It can potentially explain the observational trends of disk dispersal. The opacity of the disk medium and the photo-heating rate depend on metallicity, so that photoevaporation rates can show metallicity dependence. We present our recent 2D radiation-hydrodynamic simulations of the photoevaporation of protoplanetary disks with various metallicities. We solve the time-dependent hydrodynamics with various chemical and thermal processes, also along with the transfer both of EUV and FUV irradiation from a central star. The grain temperature is determined by the balance between the irradiation and (re-)emission, which are solved with a hybrid scheme using the radial ray-tracing and flux-limited diffusion. Our simulations cover a broad range of the different metallicities, from 10^-4 Zsun to 10 Zsun. We show that the photoevaporation rate, or the disk lifetime, largely varies with different metallicities owing to different efficiencies of the FUV photoevaporation. Interestingly, the resultant photoevaporation rate does not show a monotonic trend of the metallicity dependence but has a peak around the sub-solar metallicity around 0.5 Zsun. Our results are in good agreement with the observations, and we predict that the disk lifetime should become longer with the even lower metallicities, Z < 0.1 Zsun.
Date/Room May. 19, 13:00- @ES606
Speaker Keisuke Sugiura
Title Ellipsoidal Figures of Equilibrium for Self-Gravitating Fluid and Rubble Pile Body
Abstract Recent direct observations by spacecraft and light curve observations by ground-based telescopes reveal the detailed shapes of asteroids existing in the Solar System. Observations by the spacecraft Hayabusa show that the asteroid Itokawa has an elongated shape like the sea-otter. Moreover, because of the small mean density of the Itokawa compared to that of rock, the asteroid Itokawa is considered as a rubble pile, which is consisting of numerous pieces of rock bounded by the gravity. Irregular shapes of rubble piles are mainly supported by friction between pieces of rock, rather than the rotation or material strength. Therefore, if we clarify permitted shape of rubble pile in the case of particular angle of friction, we can achieve some constraints on material property of asteroids. In this talk, firstly I will introduce the equilibrium shape of ellipsoid for rotating and self-gravitating fluid (Chandrasekhar 1969). I especially introduce solutions in the case that the length of major axis and intermediate axis is the same (Maclaurin spheroids), and solutions for tri-axial ellipsoids with high angular momentum (Jacobi ellipsoids). Secondly I will review Holsapple (2001), which states about permitted shape of ellipsoids for ruble piles.
Date/Room June 2 (Fri.), 14:00- @ES606
Speaker Torsten Stamer
Title Brown Dwarfs and the Question of their Formation
Abstract Brown dwarfs are substellar objects with masses between about 1 and 8 percent of solar mass. Their main formation channels are still unclear: For brown dwarfs to form in the same way as low-mass stars, i.e. through the gravitational collapse of a molecular cloud core, extremely high densities (2 to 3 orders of magnitude larger than typically observed) are required for the initial cloud core to be Jeans-unstable. Various theories have been put forward to explain this situation, among them the collapse of Jeans-stable cores triggered by turbulence, disk instabilities, and the accretion-and-ejection scenario. In my talk, I will review the basic characteristics of brown dwarfs and explain the process of protostellar collapse. I will then present the different theories concerning brown dwarf formation, and examine their advantages and disadvantages. I will also discuss similarities and differences between brown dwarfs and giant planets, and finally present some results of my own simulations relating to this problem.
Date/Room June 16 (Fri.), 13:30- @ES606
Speaker Ryosuke Tominaga
Title Non-Linear Development of secular Gravitational Instability in Protoplanetary Disks
Abstract Recent observations have revealed various structures in protoplanetary disks. Especially, we are interested in the origin of the multiple ring-like structures observed by ALMA. One of promising mechanisms for creating multiple ring structures is secular gravitational instability (Secular GI) which is an instability due to the gas-dust friction even in a self-gravitationally stable disk. Previous works, Takahashi & Inutsuka 2014 and 2016, have shown that the ring-like structures in HL Tau disk could form by Secular GI and suggested that the ring formation could lead to the planetesimal formation in the HL Tau disk. These works are based on the local linear analysis. To understand the actual process of the ring formation and the planetesimal formation, however, we need to investigate the non-linear evolution of Secular GI and determine the fate of the protoplanetary disk. The purpose of this study is to investigate the non-linear evolution by a numerical simulation. Since the timescale of the growth of the secular GI is much longer than the Keplerian rotation period, we develop a new numerical scheme for a long term calculation utilizing the concept of symplectic integrator. With our new scheme, we first investigate the non-linear development of the secular GI in a disk without a pressure gradient in the initial state. We find that the surface density of dust increases by more than a factor of one hundred while that of gas does not increase even a factor of two, which results in the formation of dust-dominant rings. A line mass of the dust ring tends to be very close to the critical line mass of a self-gravitating isothermal filament. Our results indicate that the non-linear growth of the secular GI provides a powerful mechanism for the planetesimal formation. We also find that the dust ring formed via the non-linear growth of the secular GI migrates inward with a low velocity, which is driven by the self-gravity of the ring itself. We also give a semi-analytical expression for the inward migration speed of the dusty ring.
Date/Room June 23 (Fri.), 14:00- @ES606
Speaker Kouki Matsumoto
Title 特殊相対論的流体力学を記述する高精度衝撃波捕獲数値計算法の開発
Abstract  天体物理学に現れる流体力学的問題の多くで,計算領域内の空隙 (密度が低く,物質がほとんど存在しない領域) が大きく,またその空隙の形が変形するような問題を取り扱う.例えば,ブラックホールによる潮汐破壊現象である.この現象が起きているブラックホールのevent horizon近傍では落下物質の速度が光速に近い値になる.さらに,ブラックホールによって光速近くまで加速されたジェットが放出される.また,ジェットなどの超高速流では強い衝撃波が生じる.  流体力学のシミュレーション技法の一つに Smoothed Particle Hydrodynamics法 (以下,SPH法) がある.これはカーネル関数を用いて表される広がりをもった質量分布の粒子を用いて流体力学を記述する計算法である.SPH法は,密度の高い領域に対して粒子をより多く注ぎ込む為,物理現象を見たい主な領域である高密度領域ほど空間分解能が高くなる.つまり,計算領域内の空隙が大きい場合に他の計算法に比べてSPH法が有利である.しかし,標準的なSPH法では強い衝撃波を精度良く記述出来ないという問題がある.そこでInutsuka (2002) は有限体積法で確立されたGodunov法を応用し,SPH法で強い衝撃波を精度良く記述できる計算法 (以下,Godunov的SPH法) を開発した.標準的なSPH法で相対論的な問題を扱うことが出来る計算法を開発した研究は存在するが (Rosswog 2010, 2015) ,上述のように標準的なSPH法では強い衝撃波を精度良く記述できない.そこで本研究では,Godunov的SPH法で相対論的な問題を扱うことが出来る計算法を開発した.本講演ではテスト計算結果を通して,本研究で開発した計算法の有効性について議論する.
Date/Room June 29 (Thur.), 16:30- @ES606
Speaker Yuhiko Aoyama (Univ. of Tokyo)
Title Theoretical estimate of intensity of hydrogen line emission from accreting gas giants
Abstract Proto-gas-giants play a key role in planet system formation. In order to detect and characterize the proto-gas-giants, excess photons in protoplanetary disks are seeded by direct imaging. Recently, a lot of IR excesses have been found and a few H-alpha excesses have been detected. IR excesses can be explained by radiation of porto gas giants and circum-planetary disks heated by viscosity. However, it is uncertain how a forming proto-gas-giant creates H line emission and how much information about the properties of the protoplanet and its surroundings we can derive from the observed emission intensity. In this study, we quantify the radiative emission from the hot gas heated at the shockwave front at which the vertically-falling gas clashes with the circumplanetary disk (i.e., a sub-nebula). To do so, we perform 1-D post-shock hydrodynamic simulations with chemical reactions, electronic transitions, and H line radiative transfer. We then calculate the intensity of hydrogen line emission during that process. We demonstrate that the Hα line emission from the circumplanetary-disk surface is significantly strong in some cases. We also find that the Hα intensity depends on the number density of the surrounding disk gas and the protoplanet’s mass. Comparing our theoretical estimate with the observed Hα intensity from LkCa15b, we constrain the properties of that protoplanet.
Date/Room June 30 (Fri.), 13:30- @ES606
Speaker Kaori Kawamura
Title フィラメント状分子雲におけるコア質量関数の理論的記述と数値実験による検証
Abstract Herschel 宇宙望遠鏡による Gould Belt 領域のサーベイ観測で,分子雲中には幅 0.1pc 程度のフィラメント状構造が多数存在することが明らかになった(André et al. 2010).また同観測では,分子雲コアや原始星がフィラメント状構造に沿って分布していること,星の初期質量関数と分子雲コア質量関数の形がよく似ていることも示された.これらの結果は星形成においてフィラメント状分子雲の分裂が重要であることを示唆する.Inutsuka(2001,以下I01と略記)は Press-Schechter の方法(以下 PS 法)を応用し,フィラメント状分子雲の線密度ゆらぎのパワースペクトルから分子雲コア質量関数を解析的に求める理論を提案している.例として,パワースペクトルが波数の -1.5 乗に比例する形の場合,観測結果と整合的な質量関数が得られることが示されていた.近年の観測で,実際のパワースペクトルは波数の -1.6 ± 0.3 乗に比例しているという結果が報告された(Roy et al. 2015).この観測結果は,I01 の理論で分子雲コア質量関数を説明できることを期待させる.しかし,一般に PS 法は幾つかの仮定を含んでおり,理論の正当性について検討する余地が残っている.そこで本研究では,具体的な数値実験を行うことで I01 の理論の正当性を検証する.計算機上にフィラメント状分子雲の線密度ゆらぎを作成して観測的にコアを同定し,I01との対応を詳しく調べた.今回の発表では,その結果を紹介し議論する.
Date/Room July 14 (Fri.), 13:00- @ES606
Speaker Kensuke Yokosawa
Title 太陽フレアループ内の電子-イオン2温度進化に関する流体シミュレーション
Abstract 太陽フレアはコロナと低層大気が熱伝導や高エネルギー電子によるエネルギー輸送によって動的にカップルする爆発現象である.太陽フレアの物理を理解する為, 熱伝導や彩層蒸発の効果を考慮したシミュレーションが行われてきた(Yokoyama & Shibata 2001, Takasao et al. 2015).こうした過去の太陽フレアのシミュレーション研究のほとんどでは, 電子とイオンは十分衝突して両者の温度は常に等しいという1温度近似が置かれている. しかし実際は何らかの原因によりイオンまたは電子が加熱されると両者は衝突を介して熱平衡へと向かう過程があるはずである. ここで注意すべき点は, フレアのような高温プラズマではイオンと電子が衝突緩和によって熱交換を行う時間スケールがフレアの力学的な時間スケールと同程度以上になりうる点である. 過去にフレアの2温度構造を調べた研究はあったが(Longcope & Bradshaw 2010) フレアループの温度・密度を決める上で重要な彩層の影響は考慮されていなかった. そこで我々はフレアループの熱構造進化を正しく取り扱うべく, 電子とイオンのそれぞれの温度を独立に扱い彩層蒸発の影響も考慮した初めての1次元2温度流体シミュレーションを行った. その結果, ループトップのガスが衝撃波を通じてイオンのみが加熱された場合はイオンの冷却が1温度近似のときより非効率になることがわかった. これはループトップのガスが急膨張して希薄になることで電子-イオンの衝突緩和時間スケールが長くなり, イオンと電子間での熱交換が非効率になるためである. さらにループ内の質量変化から, 彩層蒸発のプラズマ量を解析したところ、2温度系は1温度系に比べ彩層蒸発による質量増加が有意に小さくなることがわかった. これは2温度系では熱輸送が非効率的になることで, 彩層に運ばれる熱伝導フラックスが1温度系に比べて小さくなり, 彩層蒸発が弱まることが原因と考えられる. これらの結果は衝撃波で解放されたエネルギー輸送の理解や観測データの解釈に重要となる可能性がある. 本発表ではシミュレーション結果に基づき, 観測への示唆やフレアにおける電子とイオンの2温度構造について議論する.
Date/Room July 28 (Fri.), 14:00- @ES606
Speaker Steve Vance
Title Geophysical tests for habitability in ice-covered ocean worlds
Abstract I will describe the challenges of trying to understand whether deep extraterrestrial oceans in the solar system may harbor life. I will focus on geophysical measurements, particularly seismology, and the limitations in the associated thermodynamic data. Geophysical measurements can reveal the structure of icy ocean worlds and cycling of volatiles. The associated density, temperature, sound speed, and electrical conductivity of such worlds thus characterize their habitability. To explore the variability and correlation of these parameters, and to provide tools for planning and data analyses, we develop 1-D calculations of internal structure, which use available constraints on the thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3, water ices, and silicate content. Limits in available thermodynamic data narrow the parameter space that can be explored: insufficient coverage in pressure, temperature, and composition for end-member salinities of MgSO4 and NaCl, and for relevant water ices; and a dearth of suitable data for aqueous mixtures of Na-Mg-Cl-SO4-NH3 For Europa, ocean compositions that are oxidized and dominated by MgSO4, vs reduced (NaCl), illustrate these gaps, but also show the potential for diagnostic and measurable combinations of geophysical parameters. The low-density rocky core of Enceladus may comprise hydrated minerals, or anydrous minerals with high porosity comparable to Earth's upper mantle. Titan's ocean must be dense, but not necessarily saline, as previously noted, and may have little or no high-pressure ice at its base. Ganymede's silicious interior is deepest among all known ocean worlds, and may contain multiple phases of high-pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's likely near-eutectic ocean cannot be adequately modeled using available data. Callisto may also lack high-pressure ices, but this cannot be confirmed due to uncertainty in its moment of inertia.
Date/Room August 8 (Tue.), 15:00- @ES606
Speaker Daisuke Nakauchi (Tohoku University)
Title Constructing Helium-Star Models with Optically Thick Winds: Implications for the Internal Structures and Mass-Loss Rates of Wolf-Rayet Stars
Abstract Wolf-Rayet (WR) stars are post-main-sequence massive stars that are in core helium (He) burning phase. They show powerful stellar winds that make them lose their hydrogen-rich envelopes. While they are usually modeled by He-stars, their internal structures, especially those around the wind accelerating regions, are not well understood, owing to the optically thick winds. Here, we construct He-star models with optically thick winds, by smoothly connecting steady wind solutions to hydrostatic He-core solutions. We calculate stellar structures and derive mass-loss rates for models with various masses and chemical compositions. By comparing our models with the observed WR stars in the Hertzsprung-Russell diagram and in the mass-loss rate to luminosity plane, we discuss the implications for the internal structures and mass-loss rates of WR stars.
Date/Room August 28 (Mon.), 15:00- @ES606
Speaker Hua-bai Li (The Chinese University of Hong Kong)
Title Probing Star Formation from Hong Kong
Abstract I moved to Hong Kong right after the Protostars & Planets VI conference, where we contributed a review chapter about the role of magnetic fields (B-fields) in star formation [1]. The chapter concluded that B-field orientation should be quite ordered from the cloud to core scales. We are trying to understand the consequence of such kind of B-fields on cloud fragmentation [2], star formation rates [3] and turbulence behaviors (anisotropy [4] and ambipolar diffusion), based on both observations and numerical simulations. We are also building APol, the ASTE polarimeter. I will report these recent progress.
[1] https://arxiv.org/abs/1404.2024
[2] http://www.nature.com/nature/journal/v520/n7548/full/nature14291.html
[3] https://www.nature.com/articles/s41550-017-0158
[4] http://iopscience.iop.org/article/10.3847/1538-4357/836/1/95/meta
Date/Room September 14-15 @ES606
Title Interim Reports
Date/Room October 2 (Mon.), 14:00- @ES606
Speaker Chat Hull (Harvard-Smithsonian Center for Astrophysics)
Title Star formation, polarization, and magnetic fields in the ALMA era
Abstract The results from the ALMA polarization system have begun both to expand and to confound our understanding of the role of the magnetic field in low-mass star formation. After a brief motivation via CARMA, SMA, and other polarization observations from the prior decade, I will discuss new ALMA results, including the highest resolution and highest sensitivity polarization images made to date of two very young, Class 0 protostellar sources. These new observations achieve ~140 AU resolution, allowing us to probe polarization -- and thus magnetic field orientation -- in the innermost regions surrounding the protostars. First is a Class 0 protostellar source in Serpens known as Ser-emb 8, where a comparison with cutting-edge, moving-mesh AREPO simulations suggests that cloud-scale turbulence -- not a large-scale magnetic field preserved from the source's natal cloud -- is dictating the magnetic field morphology immediately surrounding the protostar. In contrast, in the second source -- known as Serpens SMM1 -- the magnetic field has clearly been shaped by the bipolar outflow emanating from the central source, a situation that is quite different from the turbulence-dominated morphology of Ser-emb 8. Finally, I will show very recent observations of polarization toward IM Lup, a much more evolved, Class II protoplanetary disk. In the case of IM Lup, consistent with previous observations of other disks, the polarization appears to be due to scattering by dust grains, thus complicating the search for magnetic fields in disks, but opening up a new window into dust grain growth and evolution.
Date/Room October 2 (Mon.), 15:40- @ES606
Speaker Yusuke Tsukamoto
Title Does magnetic field-angular momentum misalignment strengthens or weakens the magnetic braking ?
Abstract Magnetic braking during the cloud core collapse has been a long standing issues for the formation and early evolution of the circumstellar disks and It has been suggested that the misalignment of magnetic field and angular momentum changes the magnetic braking efficiency Mouschovias 85, Matsumoto+04, Henebelle 09, Joos12. First three-dimensional simulations is conducted by Matsumoto+04 and they found that the angular momentum of the central region is more efficiently removed when magnetic field and rotation vector are perpendicular. On the other hand, Henebelle+04 and Joos+12 reported that the efficiency of the magnetic braking decreases as the mutual angle between the magnetic field and the rotation axis increases and is minimum in the perpendicular configuration. Therefore, the results of the previous studies are qualitatively different. To solve the discrepancy, we conducted ideal/non-dieal MHD simulations with several initial conditions which mimics the previous studies. We essentially reproduced all results of previous studies and found that the discrepancy is caused by the difference of initial conditions. Furthermore, we found that three mechanisms i.e., (1)magnetic braking in isothermal collapse phase (2)magnetic braking (or backreaction of outflow) in the disk (3)non-spherical collapse caused by radial magnetic tension influence the angular momentum evolution of the fluid. These mechanisms have a different dependence on the initial conditions and, as a result, the apparent discrepancy of the previous studies are introduced.
Date/Room October 6 (Fri.), 16:00- @ES606
Speaker Kazuki Tokuda (Osaka Prefecture University)
Title Giant Molecular Clouds in LMC Observed by ALMA
Date/Room October 11 (Wed.), 14:00- @ES606
Speaker Kenji Kurosaki
Title Self-luminous Jupiter mass planet with the the ice-rich atmosphere
Abstract Progress of the observation enables us to observe the planetary luminosity from gas giants directly. Young gas giants possess high luminosity and the planetary luminosity decreases as the time passed. Combining the theoretical model of the thermal evolution, we can find the the mass of young gas giant. However, if the atmosphere is polluted by ice materials (e.g., water, ammonia, methane), the atmospheric temperature structure should be affected by the condensation of ices. Consequently, the luminosity evolution is changed compared to that without the effect of the condensation of ices. Here we calculate the thermal evolution of a Jupiter-mass planet with ice-rich atmosphere. We can find that condensation of ices affects the atmospheric structure after 10^{8} years, which causes the change of the effective temperature of the planet. Before 10^7 years, the atmospheric structure is also changed due to the heat capacity and then the effective temperature of the planet is also changed. Those results will be important to constrain the self-luminous gas giants by using high-precision direct imaging.
Date/Room October 12 (Thu.), 14:00- @ES635
Speaker Kate Pattle (UCLAN UK, NAOJ)
Title The Magnetic Field Strength and Energetics of OMC 1
Abstract We are entering a new era of astronomical polarimetry, in which new and stringent observational constraints are being placed on the role of magnetic fields in star formation. In this talk, I will present James Clerk Maxwell Telescope (JCMT) POL-2 polarimetric observations of the OMC 1 region of the Orion A filament, taken as part of the BISTRO (B-Fields in Star-Forming Region Observations) survey. I will discuss modifications to the Chandrasekhar-Fermi method which allow estimation of plane-of-sky magnetic field strengths in the presence of a magnetic field showing large-scale, ordered variation, and will derive an estimate of the magnetic field strength in OMC 1. I will discuss the energetic importance and evolutionary history of the magnetic field in the OMC 1 region, with particular emphasis on the interaction between the magnetic field and the explosive BN/KL outflow.
Date/Room October 18 (Thu.), 14:00- @ES606
Speaker Ryosuke Tominaga
Title ダストの拡散を含む永年重力不安定性の基礎方程式の再考
Abstract 近年のアルマ望遠鏡による観測によって、様々な天体に多重のリング構造が発見された(e.g., ALMA Partnership et al. 2015)。これらのリング構造は惑星形成との関連が示唆されているため、多重リング形成機構を明らかにすることは惑星形成の解明のために重要である。リング形成機構の候補として永年重力不安定性が考えられている(Takahashi & Inutsuka 2014, 2016)。これまでの永年重力不安定性の先行研究ではガス乱流によるダストの乱流拡散を、単にダストの連続の式に拡散項を加えることで模擬していた(Youdin 2011, Michikoshi et al. 2011, Takahashi & Inutsuka 2014, 2016)。しかしこのような定式化をすると系の全角運動量保存が破れるという問題がある。そこで我々は従来の基礎方程式を再検討し、全角運動量保存を破らずに摩擦が強い場合のダスト拡散を記述するモデルを基礎方程式に組み込んだ。モデル化した基礎方程式を線形解析した結果、先行研究で得られていた過安定モードとは異なり、指数関数的に不安定成長するモードが得られた。さらに成長率が先行研究で得られていた値よりも数倍程度大きくなり、不安定条件を決める乱流強度の上限が5倍程度大きくなることがわかった。
Date/Room October 27 (Fri.), 16:00- @ES635
Speaker Doris Arzoumanian
Title Dust polarization properties of the interstellar medium as revealed by Planck
Abstract I will summarize the main observational results derived from the whole sky Galactic dust polarization observations derived from Planck 850micron data. I will present the progress that we have made in understanding the link between magnetic field and (column) density structure, by correlating the variations and dispersions of polarization angle and fraction with intensity (column density), in the diffuse ISM as well as in molecular clouds.
Date/Room November 8 (Wed.), 16:30- @ES606
Speaker Kensuke Yokosawa
Title 太陽フレアループ内の電子-イオン2温度進化に関する流体シミュレーション
Abstract 太陽フレアはコロナと低層大気が熱伝導や高エネルギー電子によるエネルギー輸送によって動的にカップルする爆発現象である.太陽フレアの物理を理解する為, 熱伝導や彩層蒸発の効果を考慮したシミュレーションが行われてきた(Yokoyama & Shibata 2001, Takasao et al. 2015).こうした過去の太陽フレアのシミュレーション研究のほとんどでは, 電子とイオンは十分衝突して両者の温度は常に等しいという1温度近似が置かれている. しかし実際は何らかの原因によりイオンが加熱されると電子はイオンとの衝突を介して加熱されるはずである. ここで注意すべき点は, フレアのような高温プラズマではイオンと電子が衝突緩和によって熱交換を行う時間スケールがフレアの力学的な時間スケールと同程度以上になりうる点である. そこで我々はフレアループの熱構造進化を正しく取り扱うべく, 電子とイオンのそれぞれの温度を独立に扱い彩層蒸発の影響も考慮した初めての1次元2温度流体シミュレーションを行った. その結果, ループトップのガスが衝撃波を通じて加熱されたようなイオンのみが加熱された場合はイオンの冷却が1温度近似のときより非効率になることがわかった. これはループトップのガスが急膨張して希薄になることで電子-イオンの衝突緩和時間スケールが長くなり, イオンと電子間での熱交換が非効率になるためである. これにより、最大温度の増加や彩層蒸発量の減少など1温度系と2温度系とでは明確な違いを示した。さらに我々は、リコネクション加熱を考慮したフレア加熱モデルを導入する事で、様々な規模のフレアについてのパラメータ調査を行った. 本発表ではこのモデルを用いた幅広いパラメータのシミュレーション結果に基づき, 観測への示唆や様々なスケールのフレアにおける電子とイオンの2温度構造について議論する.
Date/Room November 20 (Mon.), 14:00- @ES606
Speaker Kaori Kawamura
Title フィラメント状分子雲におけるコア質量関数の理論的記述と数値実験による検証
Abstract Herschel 宇宙望遠鏡による観測で,分子雲中に多数存在するフィラメント状構造に沿って分子雲コアや原始星が分布していること,星の初期質量関数と分子雲コア質量関数がよく似た形をしていることが明らかになった(André et al. 2010).よって、フィラメント状構造を考慮して分子雲コア質量関数の起源を解明することが,星形成を理解するために重要である.分子雲コア質量関数の理論の一つに Inutsuka (2001) がある.この理論では Press-Schechter の方法を応用して,フィラメント状構造の線密度ゆらぎのパワースペクトルからコア質量関数を解析的に記述する.近年,Inutsuka (2001) の予言と整合的なパワースペクトルが実際に観測され(Roy et al. 2015),この理論でコア質量関数を説明できるという期待が高まった.しかし,Press-Schechter の方法には幾つかの仮定が含まれており,理論が数学的に正しいかどうか検証する余地が残っていた.そこで本研究では計算機上にフィラメントの線密度ゆらぎを再現し,Inutsuka (2001) と同様の定義で具体的にコアを同定して質量関数を求めるという数値実験を行った.今回の発表では,理論で解析的に記述される質量関数と数値実験で得られた質量関数を比較した結果を紹介し,理論の正当性と観測への応用の方法を議論する.
Date/Room December 1 (Fri.), 16:00- @ES606
Speaker Sinsuke Takasao
Title 3D MHD Simulations of Accretion onto a Star from a Surrounding Disk
Abstract Young stars in the main accretion phase evolve through interaction with magnetized accretion disks. It is believed that the structure of the accretion flows should be strongly influenced by magnetic fields. In a classical picture, the accretion disk is connected to the stellar surface before the development of the stellar magnetosphere. But after the development, the so-called magnetospheric accretion is expected. The 3D structure of the accretion flow can affect the observational signatures (e.g. X-ray emission from shocked material is expected in the case of the magnetospheric accretion) and the surrounding disk evolution through the change in the optical depth around the star. In spite of this importance, there have been only a few 3D magnetohydrodynamics (MHD) simulations of the accretion onto the star. We performed 3D MHD simulations by using the Athena++ code, and analyzed the accretion from an MRI (Magneto-Rotational Instability)-active disk onto a weakly magnetized star. As a result, we found that fast accretion to a high-latitude, which is similar to the magnetospheric accretion, is possible even without the stellar magnetosphere. The newly found accretion is caused by a combination of turbulence in the disk and strong magnetic torque by the large-scale coherent field above the disk. Our results suggest a possibility that stars without the magnetosphere can show a violent accretion behavior associated with X-ray activities. We will discuss the physics of the accretion on the basis of our simulations.
Date/Room December 6 (Wed.), 14:00- @ES606
Speaker Kouki Matsumoto
Title 特殊相対論的流体力学を記述する高精度衝撃波捕獲数値計算法の開発
Abstract 天体物理学に現れる流体力学的問題の多くで,計算領域内の空隙 (密度が低く,物質がほとんど存在しない領域) が大きく,またその空隙の形が変形するような問題を取り扱う.例えば,ブラックホールによる潮汐破壊現象である.この現象が起きているブラックホールのevent horizon近傍では落下物質の速度が光速に近い値になる.さらに,ブラックホールによって光速近くまで加速されたジェットが放出される.また,ジェットなどの超高速流では強い衝撃波が生じる.  このような現象に対して有効な流体力学のシミュレーション技法の一つに Smoothed Particle Hydrodynamics法 (以下,SPH法) がある.これはカーネル関数を用いて表される広がりをもった質量分布の粒子を用いて流体力学を記述する計算法である.SPH法は,密度の高い領域に対して粒子をより多く注ぎ込む為,物理現象を見たい主な領域である高密度領域ほど空間分解能が高くなる.つまり,計算領域内の空隙が大きい場合に他の計算法に比べてSPH法が有利である.しかし,標準的なSPH法では強い衝撃波を精度良く記述出来ないという問題がある.そこでInutsuka (2002)は有限体積法で確立されたGodunov法を応用し,SPH法で強い衝撃波を精度良く記述できる計算法 (以下,Godunov的SPH法) を開発した.標準的なSPH法で相対論的な問題を扱うことが出来る計算法を開発した研究は存在するが (Rosswog 2010, 2015),上述のように標準的なSPH法では強い衝撃波を精度良く記述できない.そこで本研究では,Godunov的SPH法で相対論的な問題を扱うことが出来る計算法を開発した.今回はテスト計算結果を通して,本研究で開発した計算法の有効性について議論する.
Date/Room December 13 (Wed.), 14:00- @ES606
Speaker Kenta Nakashima
Title 複数惑星系の重力相互作用による軌道進化と太陽系外惑星での観測可能性
Date/Room January 24 (Wed.), 13:00- @ES606
Speaker Shu-ichiro Inutsuka
Title Lecture for numerical simulation and debug
Date/Room February 16 (Fri.), 14:00- @ES606
Speaker Kohei Inayoshi (Columbia University)
Title Black hole evolution: from stellar to ultra-massive ones
Date/Room March 19 (Mon.), 14:00- @ES606
Speaker Pedro Palmeirim (Instituto de Astrofisica e Ciencias do Espaso, Porto, Portugal )
Title Star formation along filaments and ionized HII bubble regions
Abstract Star formation is a fundamental process for the enrichment of the InterStellar Medium (ISM), having driven the evolution of matter from the primordial conditions to the complex and chemically diverse conditions essential for life. This ongoing recycling process that continuously enriches the ISM is mainly regulated by the feedback from massive stars and supernovae blasts, that generates bubble-like structures in the ISM and triggers star formation. However, in the absence of strong feedback, molecular clouds tend to contract into star-forming filaments. This seminar will thus focus on the relevance of these two mechanisms of forming stars. First, the importance of the role of filamentary structures in the star-formation process will be shown in highlighted results from the Herschel Gould Belt Survey. In particular, a comprehensive study of the most prominent star-forming filament located in the B211/3 region of the Taurus Molecular cloud will be presented. Secondly, an unprecedented statistical analysis of star-forming objects located in the vicinity of 1360 bubble structures throughout the Inner Galactic Plane and their local environments will be shown. The compilation of nearly 70 000 star-forming sources, detected in both Hi-GAL and GLIMPSE surveys, revealed a clear evolutionary trend where more evolved sources are found spatially located near the center, while younger sources are found at the edge of the bubbles. A higher ratio of protostellar vs prestellar clumps is found inside the collected shells of the bubbles which could be a signature of acceleration of the star-formation process cause by the feedback. Moreover, comparison between fragmentation time of the shells and the dynamic ages of the bubbles advocate for the pre-existence of clumps in the medium prior to the bubble expansion. Finally, we will look into the collision between galactic bubbles and at how they potentially interact to generate star-forming filaments.

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