| Date/Room | Mar. 22 (Wed.) 16:00- @ES606 |
| Speaker | Alexandre Lazarian (Univ. of Wisconsin-Madison) |
| Title | New Techniques of Tracing Magnetic Fields: Theory and Application to Data |
| April | 13 | Shu-ichiro Inutsuka |
| 19 | Kohei Inayoshi (Columbia University) | |
| 20 | Hiroshi Kobayashi | |
| 27 | Shinsuke Takasao | May | 11 | Torsten Stamer |
| 18 | Masanobu Kunitomo | 25 | Break (JpGU) | June | 8 | Hiroshi Kimura |
| 10 | Kenji Kurosaki | |
| 15 | Yansong Guo | |
| 16 | Hitoshi Miura (Nagoya City University) | |
| 20 | Jianchun Wang (Nagoya Institute of Technology) | |
| 22 | Keisuke Sugiura | |
| 29 | Break (International Congress on Plasma Physics) | July | 5 | Youhei Masada (Aichi University of Education) |
| 6 | Kensuke Kakiuchi | |
| 13 | Yutaro Sato | |
| 15 | Kento Masuda (University of Tokyo) | |
| 20 | Rehearsal for Summer School | August | 5 | Zahed Wahhaj (ESO, Chile) | September | 9 | Zahed Wahhaj (ESO, Chile) | October | 3 | Andreas Schreiber (MPIA, Heidelberg, Germany) |
| 6-7 | Interim Reports | |
| 12 | Doris Arzoumanian | |
| 19 | Torsten Stamer | November | 1 | Simon Portegies Zwart (Leiden Observatory) | 8 | Tomohiro Ono (Kyoto Univ.) |
| 9 | Yutaro Sato | |
| 10 | Yansong Guo | |
| 16 | Kensuke Kakiuchi | January | 18 | Rehearsal for master defense (1) |
| 24 | Alvaro Hacar (Leiden Observatory) | |
| 25 | Rehearsal for master defense (2) | |
| February | 1 | Rehearsal for master defense (3) |
| 13 | Jungyeon Cho (Chungnam National University) | |
| March | 1 | Tristan Guillot (Observatoire de la Cote d’Azur) |
| 22 | Alexandre Lazarian (Univ. of Wisconsin-Madison) |
| Date/Room | Apr. 13, 14:45- @ES606 |
| Speaker | Shu-ichiro Inutsuka |
| Title | The Formation of Molecular Clouds and the Onset of Star Formation |
| Abstract | I will give an introductory explanation about star formation. It includes observational information about star formation on various spatial scales and basic physics in gravitational dynamics. I will highlight phase transition dynamics in cloud formation. Our picture of star formation in the Galaxy is also presented. |
| References | (1) "The Formation and Destruction of Molecular Clouds and Galactic Star Formation: An Origin for The Cloud Mass Function and Star Formation Efficiency" Shu-ichiro Inutsuka, Tsuyoshi Inoue, Kazunari Iwasaki, & Takashi Hosokawa (2015), Astronomy and Astrophysics, Volume 580, A49 (7pp) (arXiv:1505.04696) (2) "Phase Transition Dynamics of the Interstellar Medium: Theory, Methodology, and Implications" S. Inutsuka, T. Inoue, K. Iwasaki, J. M. Stone, T. K. Suzuki, Y. Tsukamoto, & M. Takamoto (2015), Invited Review in "Astronum 2014", ASP Vol.498, p.75 (3) “Formation of Molecular Clouds” (in Japanese) Tsuyoshi Inoue, Tenmon Geppo 2014 Vol.1 Eureka |
| Date/Room | Apr. 19, 16:15- @ES606 |
| Speaker | Kohei Inayoshi (Columbia University) |
| Title | Hyper-Eddington accretion flows onto massive black holes |
| Abstract | How fast can black holes (BHs) grow? The existence of bright quasars at high-redshift provides a challenging puzzle about the origin of supermassive BHs. To form such massive objects within a billion year, rapid growth of seed BHs is required. We study very-high rate, spherically symmetric accretion flows onto massive BHs embedded in dense metal-poor clouds. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback. Accretion rates in this regime are steady, and larger than 5000 L_Edd/c^2. At lower rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. The hyper-Eddington accretion solution is maintained as long as the emergent luminosity is limited to < (10-30) L_Edd because of photon trapping due to electron scattering. We apply our result to the rapid formation of massive BHs in protogalaxies. Once a seed BH forms at the center of the galaxy, it can grow to a maximum ~ 10^5 Msun via gas accretion independent of the initial BH mass. |
| Date/Room | Apr. 20, 14:00- @ES640 |
| Speaker | Hiroshi Kobayashi |
| Title | Collisional history for planet formation |
| Abstract | I will talk about planet formation, which happens in protoplanetary disks formed as by-product of star formation. Planet formation is the growth of solid bodies from dust (sub-micron) to planets (1000 km). The formation of planetesimals, which are believed to be building blocks, has several difficulties: Recently some mechanisms are found to overcome the difficulties. Collisions among planetesimals produce a larger body, called a planetary embryo, in each annulus of the disk, which grow via collisions with remnant planetesimals. The stage is controlled by gravitational interaction among bodies, more simple than the planetesimal formation stage; however massive embryos can be formed in limited conditions due to collisional fragmentation of planetesimals. Finally, once embryos exceed about 10 Earth masses, rapid gas accretion happens to form gas giants. I will briefly explain these processes from beginning to the completion of planets. |
| Date/Room | Apr. 27, 14:00- @ES606 |
| Speaker | Shinsuke Takasao |
| Title | Oscillatory nature of solar flares: What can we learn from observations of oscillations? |
| Abstract | Solar flares are an explosive phenomenon powered by magnetic reconnection, and are believed to be a prototype of various kinds of astrophysical explosions. Solar and many other astrophysical flares can be recognised as a sudden increase in a wide range of electromagnetic waves (from radio to gamma ray). It has been known that flares intrinsically show oscillations as pulsations in light curves and compressional waves propagating away from the flaring regions. Under the assumption that the oscillations are caused by magnetohydrodynamics (MHD) waves excited in flares, people have tried to derive physical parameters of solar and stellar flares which are difficult to directly measure. However, the oscillation mechanisms remain puzzling, and therefore the validity of the assumption is uncertain. To reveal the intrinsic oscillatory nature of solar flares, we carried out MHD simulations of solar flares. As a result, we found a new oscillation mechanism which has never been expected. In this talk, I will begin with a brief introduction of the solar physics, and then will introduce our results. I will also discuss the importance of this finding in the astrophysical context. |
| Date/Room | May 11, 14:00- @ES606 |
| Speaker | Torsten Stamer |
| Title | Addressing the brown dwarf formation problem using a new method of radiative transfer calculation in spherical symmetry |
| Abstract | I am developing a fast and accurate method of radiative transfer calculation in spherically symmetric systems. While such a method can be useful in many areas of astrophysics, the main motivation for this work is its application to protostellar collapse simulations, especially in the context of brown dwarf formation. The main formation mechanism of these substellar objects is still an open question. It is now considered likely that brown dwarfs can form in a manner analogous to low-mass stars, i.e. through the gravitational collapse of a molecular cloud core. However, accurate simulations of this process are needed in order to determine the properties of the objects formed in this manner, and to compare them to actual observations. One problem in carrying out such simulations is the complexity of the radiative transfer physics. The system starts out very optically thin, but changes to becomes extremely optically thick during the course of the collapse. This is very challenging because approximate methods of radiative transfer calculations tend to only work well in either very thin or very thick systems, but not in both at the same time, nor in intermediate cases. I will explain the basics of a new method of radiative transfer calculation that works for all regimes of optical thickness, assuming spherical symmetry. I will also show the preliminary results of collapse calculations performed by combining this scheme with a hydrodynamics module. |
| Date/Room | May 18, 14:00- @ES606 |
| Speaker | Masanobu Kunitomo |
| Title | Orbital evolution of planets around intermediate-mass red giants |
| Abstract | Since the first extrasolar planet was detected in 1995, more than 3000 planets have been found so far. Whereas most of them are orbiting around solar-type main-sequence stars, recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ~1.5–3 Msun stars. However, no planets have been found inside 0.6 AU around clump giants unlike in the case of solar-mass main-sequence stars, some of which harbor close-in giant planets called “hot Jupiters”. In this study we examine the possibility that close-in planets were engulfed by the host stars expanding on their red-giant branch (RGB) phase. For that purpose, we integrate numerically the post-main-sequence evolution of host stars and the orbital evolution of planets around the evolving stars including the effects of stellar tide and mass loss. We then derive the survival limit inside which the planets are eventually engulfed by their host stars after tidal decay of their orbits. We compare the survival limit to the observed distributions and discuss the adequacy of this model. We find that all the detected planets are beyond the survival limit, which is consistent with the planet-engulfment hypothesis. However, on the high-mass side (> 2.1 Msun), the detected planets are orbiting significantly far from the survival limit, which suggests that engulfment by host stars may not be the main reason for the observed lack of short-period giant planets. Furthermore, I will discuss the possibility that close-in planets are rarely formed around ∼ 1.5–3 Msun stars and introduce our recent observational results. |
| Date/Room | June 10 (Fri.), 14:30- @ES606 |
| Speaker | Kenji Kurosaki |
| Title | he thermal evolution of ice giants with the effect of the condensation of ice constituents in the atmosphere |
| Abstract | Though Uranus and Neptune are similar in mass and radius, the former is significantly fainter than the latter. As previous theoretical studies of thermal evolution of the ice giants demonstrated, the faintness of Uranus is not explained by simple three-layer models that are composed of a H/He-dominated envelope, an ice mantle and a rocky core. Namely, the observed effective temperature of Uranus is lower than theoretically predicted (e.g., Fortney et al., 2011; Nettelmann et al., 2013). Since the speed of the thermal evolution is determined by how efficiently the planetary atmosphere radiates energy, the atmospheric structure is important. If the atmosphere contains ice materials such as water, ammonia and methane, those materials have been condensed and removed from the atmosphere during the cooling. In this study, we quantify the impact of the condensation of ice components in the atmosphere on the thermal evolution, which previous studies ignore, to explain the current luminosity of Uranus. To do so, we simulate the thermal cooling of ice giants, based on three layer models with a relatively ice-component-rich, H/He-dominated atmosphere on top of a water mantle that surrounds a rocky core. We demonstrate that the effect of the condensation makes the timescale of the thermal cooling of the planet shorter by an order of magnitude than in the case without condensation. Such accelerated cooling is shown to be fast enough to explain the current faintness of Uranus. We also discuss what caused the difference in current luminosity between Uranus and Neptune. |
| Date/Room | June 8, 13:00- @ES606 |
| Speaker | Hiroshi Kimura |
| Title | Missing organic materials from interstellar dust inside the Solar System |
| Abstract | We tackle the conundrums of organic materials missing from interstellar dust when measured inside the Solar System, while undoubtedly existing in the Local Interstellar Cloud (LIC), which surrounds the Solar System. We solve the mysteries by demonstrating that organic compounds sublimate almost instantaneously by exothermic reactions, when solar insolation triggers recombination of free radicals or rearrangement of carbon bonds in the compounds. It turns out that the triggering temperature lies in the range of 20—50 K, by considering that sublimation of organic materials takes place beyond the so-called filtration region of interstellar neutral atoms. We find that in-situ measurements of LIC dust in the Solar System result in an overestimate for the gas-to-dust mass ratio of the LIC, unless sublimation of organic materials is taken into account. We also find that previous measurements of interstellar pickup ions have determined the total elemental abundances of gas and organic materials, instead of interstellar gas alone. We conclude that one must await a future exploration mission to the inner edge of the Oort cloud for a thorough understanding of organic substances in the LIC. |
| Date/Room | June 15 (Wed.), 14:00- @ES606 |
| Speaker | Yansong Guo |
| Title | Underlying mechanisms of TDE event: a review |
| Abstract | A tidal disruption event (TDE), which occurs when a star is destroyed by the gravitational field of a supermassive black hole, produces a stream of debris. The typical signatures of TDE are 1. The time dependency of bound material return rate (Rees,1988) 2. The “Smoking Gun”-like geometry of the debris stream. The above features only considered simplified models without incorporating full hydrodynamic effects.Recently, detailed numerical simulations have been employed to understand the complex hydrodynamic effect son the stream. I would like to review the paper Coughlin et al. (2016), where they derived an analytic self-similar solution for the velocity profile and radial density profile of the stream. The analysis performed in their paper was done with the specific application of stars disrupted by SMBHs. However, their results can be applied to many other tidally interacting systems, especially the tidal encounter of intermediate-mass black hole (IMBH) and molecular cloud. |
| Date/Room | June 16 (Thu.), 14:00- @ES606 |
| Speaker | Hitoshi Miura (Nagoya City University) |
| Title | Thermal desorption of molecules from grain surface by accretion shock |
| Abstract | Gravitational collapse of a molecular cloud is a transient process to form protostars and protoplanetary disks. Recent ALMA observations suggested that materials in the molecular cloud are significantly processed before being delivered into the disk. The dust heating by accretion shocks is responsible for the material processing. We investigate the thermal desorption of molecules from grain surfaces at the passage of the accretion shock. A molecule adsorbed on a grain surface is required to overcome a potential barrier (desorption energy) to be desorbed from there. We notice that the desorption energies of molecules cannot be represented by a single value on the surface of amorphous interstellar grains. We assume that the desorption energy has a Gaussian distribution and assess how the desorption fraction of molecules depends on the distribution using a numerical method. We found that the distribution of the desorption energy significantly affects the desorption fraction of molecules. Our numerical results indicate that the enhanced molecular line emissions around protostars observed by ALMA can be explained by the thermal desorption induced by the accretion shock. |
| Date/Room | June 20 (Mon.), 15:00- @ES606 |
| Speaker | Jianchun Wang (Nagoya Institute of Technology) |
| Title | Shocklets, spectrum and intermittency in compressible isotropic turbulence |
| Abstract | We perform numerical simulations of stationary compressible isotropic turbulence at the turbulent Mach number up to 1.0 and at the Taylor Reynolds number up to 250, by using a compact-WENO hybrid numerical method. We study the statistics of compressible turbulence driven by a large-scale solenoidal force. We report that there is a power law scaling region for the probability density functions (PDF) of the shocklet strength Mn-1 (Mn is the normal shock Mach number), as well as for the PDF of velocity divergence, at different turbulent Mach numbers. We also investigate the scaling behaviors of structure functions of compressive velocity, density, temperature and pressure. We present that the scaling exponents are saturated at high orders with the saturation exponents close to 1.0. This observation is similar to Burgers turbulence, which can be attributed to the effect of shocklets. We also study the kinetic energy transfer in compressible turbulence driven by a large-scale compressive force. Numerical simulation reveals that the compressive component of the density-weighted velocity has major contribution to the kinetic energy flux, due to the presence of large-scale shocks. We further show that the kinetic energy flux from both solenoidal and compressive components are nearly constant over the inertial range. The cascade rate of compressive mode is much faster than that of solenoidal mode, leading to the dominant of solenoidal kinetic energy over its compressive counterpart at high wavenumber. |
| Date/Room | June 22 (Wed.), 14:00- @ES606 |
| Speaker | Keisuke Sugiura |
| Title | Toward the understanding of the outcome of planetesimal collision: review of SPH method for elastic dynamics and models describing rocky material |
| Abstract | Recent in situ observations of some asteroids by spacecrafts reveal the detail of asteroids’ shapes. As a result of in situ observations, some asteroids, e.g. Churyumov-Gerasimenko comet, have complex shapes apart from round sphere. To make such complex shapes, planetesimal collision plays an important role. If we understand the detail of collision (e.g., collisional velocity, mass of impactor) to make those asteroids, we can obtain clues to assume the environment planetesimals experienced. To understand the outcome of planetesimal collision, some researches conduct numerical simulations using Smoothed Particle Hydrodynamics (SPH) method (e.g., Benz and Asphaug 1999, Jutzi and Asphaug 2015). Recently, SPH method is extended to elastic dynamics, and some models describing rocky material are implemented to SPH method to treat the collision of small planetesimals. In this talk, I will review the models for rocky material (fracture (Benz and Asphaug 1995), plastic deformation and friction (Jutzi 2015), porosity (Jutzi et al. 2008)) after the introduction of SPH method for elastic dynamics. Moreover, I will introduce Jutzi and Asphaug (2015), which simulate low-velocity collision of small planetesimals (~1km) to clarify formation condition of bi-lobe shape asteroids. |
| Date/Room | July 5 (Tue.), 16:30- @ES606 |
| Speaker | Yohei Masada (Aichi University of Education) |
| Title | Spontaneous Formation of Surface Magnetic Structure from Dynamo-maintained Magnetic Flux: The Impact of Strong Stratification |
| Abstract | A longstanding goal of the solar interior physics is to reproduce ``Sunspots" self-consistently from dynamo-maintained magnetic fluxes. However, there is still a large gap between the sunspot formation at the solar surface and the dynamo in its interior. Aiming to bridge the gap between them, we have conducted numerical studies on convective dynamos in a strongly-stratified atmosphere expected in the actual Sun. In my talk, I will present our recent results (c.f., Masada & Sano 2016, ApJL), successful simulations of spontaneous formation of sunspot-like large-scale magnetic structure in the convection zone surface self-consistently from a large-scale convective dynamo. Furthermore, the physical conditions for the dynamo excitation and surface magnetic structure formation are discussed, based on numerical modeling, parameter survey, and linear analysis, with emphasis placed on the impact of the strong stratification which should be inherent in the stellar interior. | References | Masada & Sano (2014a), PASJ Masada & Sano (2014b), ApJL Masada & Sano (2016), ApJL |
| Date/Room | July 6 (Wed.), 14:00- @ES606 |
| Speaker | Kensuke Kakiuchi |
| Title | 銀河系中心領域における磁気活動がもたらす星間現象の解明 |
| Abstract | 銀河系中心領域では、数 100kpc 以内の小さな領域に高温高密度の分子ガスが集積している。CO などの観測結果から、この領域における分子ガスは数 100km/s に及ぶ大きな視線速度の分散を持っており、銀河回転に沿った回転速度成分だけでは説明できないような複雑な速度構造をしていることが分かっている。この要因として、銀河系中心領域では、重力以外に磁場や衝撃波などの他の物理的要素が強く寄与する可能性が考察されている。特に、領域内の磁場の強さは局所的に 0.1-1mG 程度に達するという観測的示唆 (Morris et al. 1992) に基づけば、星間ガスにかかる磁気圧は、ガス圧 (∝温度×密度) と同程度に寄与することから、磁気活動は星間ガスの動力学に強い影響を与えていると推定される。これを理論的に検証するために、Suzuki et al. (2015) は、銀河系中心領域における大規模な磁気流体シミュレーションを行った。その結果、磁気乱流が動径方向の運動を励起し、位置-速度図に観測結果と合致するような特徴的な平行四辺形構造を再現することを明らかにしている。これに加えて、本研究では、同数値計算結果を用いて、未解析であった鉛直方向のガス運動、特にバルジ内部における磁場の鉛直構造に起因するガスの下降流について詳細な解析を行った。この下降流は上空から銀河面に向かうに従い加速するが、その領域を位置速度図に対応させたところ、観測されている CO 輝線の位置速度図に見られる高速度分散領域をよく説明することも分かった。見つかった下降流は複数存在しており、見込む角度により、位置-速度図に異なる分布を示すことが期待され、観測されている多様な構造を説明できる可能性がある。 |
| Date/Room | July 13 (Wed.), 14:00- @ES606 |
| Speaker | Yutaro Sato |
| Title | 微惑星の衝突破壊を考慮した巨大衝突ステージにおける原始惑星の軌道進化 |
| Abstract | 太陽系の地球型惑星は火星程度の大きさの原始惑星同士の衝突によって形成したと考えられており、惑星形成におけるこの段階は巨大天体衝突ステージと呼ばれる。このことは理論的な研究からだけでなく、地質学的な証拠からも支持されている。そこで、このステージで原始惑星が巨大衝突を起こして地球型惑星が形成する過程のN体シミュレーションを行った。その結果、確かに巨大衝突は起こり、地球型惑星が形成した。しかし巨大衝突が繰り返されるには、衝突を経験するたびに原始惑星の離心率が増加する必要があるので、最終的に形成した地球型惑星の離心率は現在の地球型惑星のものよりもはるかに大きくなってしまった。この地球型惑星の離心率を抑制する物理として力学的摩擦が有力であるため、本研究でも原始惑星の周りに小さな微惑星が多数存在しているような系を考える。この系では微惑星と原始惑星の力学的摩擦によって、地球型惑星の離心率は下がり、逆に微惑星の離心率が上がる。このとき、離心率の上がった微惑星はランダム速度が大きいため衝突して壊れる。その結果として、微惑星円盤の密度は下がる。この効果により力学的摩擦が効率的に効かないことが予想されるが、これまでの研究では微惑星の破壊を取り扱えず、この効果は調べられていない。そこで、本研究では従来のN体シミュレーションに統計的手法を加えることで微惑星の破壊を取り扱うことに成功した。このハイブリッドコードを用いて微惑星の破壊を考慮した場合と考慮していない場合での地球型惑星の離心率の減衰をシミュレーションすることで、地球型惑星形成における破壊現象の重要性を説明する。 |
| Date/Room | July 15 (Fri.), 14:00- @ES606 |
| Speaker | Kento Masuda (The University of Tokyo) |
| Title | Exploring the Architecture of Transiting Exoplanetary Systems with High-precision Photometry |
| Abstract | Launched by NASA in 2009, the Kepler space telescope has provided high-precision, continuous photometry data for thousands of transiting exoplanetary systems. Such precise light curves do not only yield precise planetary radii and orbital periods as the basic parameters obtained via the transit method, but also reveal the detailed system architecture that is otherwise difficult to obtain. In this talk, I focus on (1) the measurement of stellar obliquity, the angle between the planetary orbital axis and the spin axis of its host star, via precise modeling of subtle features in the light curve, and (2) the determination of physical and orbital properties of multi-planetary systems based on dynamical modeling of the orbital period variations of the member planets due to their mutual gravitational interaction. I will present the methodologies with applications to real systems and discuss how the architecture revealed by those analyses can be used to probe the dynamical evolution of exoplanetary systems. |
| Date/Room | Aug. 5 (Mon.), 14:00- @ES606 |
| Speaker | Zahed Wahhaj (ESO, Chile) |
| Title | Narrow circumstellar dust rings and their connection to planets. |
| Abstract | Debris disks are second generation dust disks around roughly 10 Myr or older stars, which have lost their original gas. They also have a total disk luminosity which is at least a 100 times fainter than their parent star. Since planets are very hard to detect around young systems, because of the scarcity of the systems and the low percentage yield of traditional planet detection techniques, we know very little about how planetary systems form. The plethora of observed debris disk features, like rings, spirals, warps hold the promise to reveal the configurations of hidden planetary systems in formation, through the physics of gravitational interactions between large bodies and dust. Here, I review the high resolution imaging observations of debris disks and pose questions about a particularly interesting and increasingly common type of debris disk: the narrow ring systems. Can such systems remain as narrow rings without the need for shepherding planets? If they can, what conditions are necessary for them to arise? What fraction of debris disks and at what ages should be such rings be narrow, according to model predictions? If narrow systems cannot remain narrow without planets, what sort of planets are necessary for their maintenance? |
| Date/Room | Sep. 9 (Fri.), 14:00- @ES606 |
| Speaker | Zahed Wahhaj (ESO, Chile) |
| Title | Constraints on planet formation from observations of young and old debris disks. |
| Abstract | Debris disks are second generation dust disks around roughly 10 Myr or older stars, which have lost their original gas. They also have a total disk luminosity which is at least a 100 times fainter than their parent star. The dust results from collisions between 1-100km size rocky bodies called planetesimals, the building blocks of planets. In this talk, we present preliminary constraints on planetesimal belt morphology at the beginning of the run-away growth of planets. The constraints are obtained from fitting photometry obtained from dynamical models (from Kobayashi et al. 2014), to spitzer photometry of debris disks from 10 Myr to a few Gyrs. In fact, we try to simulate an entire population of debris disks over different ages and obtain probabilities for disk model parameters, using Bayesian methods, MCMC and Metropolis Hastings. Thus, the talk will be interesting to anyone interested in Bayesian constraints from large population or histogram-type data. |
| Date/Room | Oct. 3 (Mon.), 14:00- @ES606 |
| Speaker | Andreas Schreiber (Max-Planck-Institute for Astronomy) |
| Title | All Planetesimals are born equal - Why the size distribution of Asteroids and Kuiper Belt objects is so similar. |
| Abstract | In situ planetesimal formation via gravitational collapse of massive particle clouds in PPDs is the most promising way around the drift barrier, also known as metre size barrier. In my project I study the circumstances under which planetesimals are thought to form by using hydro-simulations of the so called streaming instability. This instability is capable of both further concentrating dust on large scales once dust-to-gas density ratios around unity are achieved, but also has due to its dynamics a diffusive behaviour on small scales. We find this diffusion to be the process predetermining the planetesimal birth size. Moreover, we could derive from first principles a criterion, similar to the Jeans criterion for stars but for planetesimals, giving us a prediction on the initial planetesimal size. With typical PPD values we find ~100 km to be the initial planetesimal size independent of semi-major axis. We thus think to have found an explanation why observed asteroid and classical Kuiper-belt size distribution as well as Jupiter Trojans sizes and Pluto impact crater statistics have a kink exactly at this size. All simulations were done with the Pencil Code on the Hydra and JUQUEEN superclusters. |
| Date/Room | Oct. 6-7 @ES606 |
| Title | Interim Reports |
| Date/Room | Oct. 12 (Wed.), 14:00- @ES606 |
| Speaker | Doris Arzoumanian |
| Title | Observed properties of interstellar filaments |
| Abstract | The highly filamentary structure of the interstellar medium is now impressively revealed by Herschel and Planck images tracing the Galactic cold dust emission. Previous observations have shown that clouds are filamentary, however, only recently have interstellar filaments received special attention, thanks to the new observational results on their physical properties. The analysis of the column density profiles of the filaments derived from Herschel images indicates that they all share a common central width of 0.1pc, while they span a wide range in length, column density, mass per unit length. The results derived from observations tracing cold dust and gas emission, in total and polarised intensity, suggest that filaments can be divided into two families: On the one hand, low column density, unbound, and quiescent filaments mostly aligned with the local magnetic field orientation, and on the other hand, dense, self-gravitating filaments, mostly perpendicular to the local magnetic field orientation, and fragmented into star forming cores. I will present the properties of the filamentary structures derived from Herschel, Planck, and molecular line observations, and I will discuss the observational constraints on the formation and evolution of interstellar filaments and their key role in the star formation process. |
| Date/Room | Oct. 19 (Wed.), 14:00- @ES606 |
| Speaker | Torsten Stamer |
| Title | The formation of brown dwarfs: Numerical simulation of spherically symmetric collapse |
| Abstract | Brown dwarfs are substellar objects in the mass regime between giant planets and low-mass stars. Their formation is not well understood, since formation through gravitational collapse (like stars) requires extremely high cloud core densities, and formation through core accretion (like planets) is not believed to be efficient enough. Various scenarios have been proposed to explain the formation of brown dwarfs, among them the radial collapse and fragmentation of filaments, gravitational instabilities in protoplanetary discs, and formation through external compression due to turbulence. A number of observational factors need to be considered when trying to solve this puzzle, such as the initial mass function, the presence of a "brown dwarf desert" in binary systems, and the age of observed young stellar objects. In order to investigate the possibility of the different formation scenarios, numerical simulations are needed. The treatment of radiative transfer, in particular, is challenging and of great importance. I will present a new numerical method of radiation transfer in spherical symmetry, which is both fast and accurate in all regimes of optical thickness. I will also show the first results of collapse calculations using this method, which I used to investigate the turbulent compression scenario. |
| Date/Room | Nov. 1 (Tue.), 16:30- @ES606 |
| Speaker | Simon Portegies Zwart (Leiden Observatory) |
| Title | The Solar System Migration in Milky Way |
| Date/Room | Nov. 2 (Wed.), 14:00- @ES606 |
| Speaker | Tsuyoshi Inoue |
| Title | Effects of realistic ISM structure on the dynamics of supernova remnants |
| Date/Room | Nov. 8 (Tue.), 16:30- @ES606 |
| Speaker | Tomohiro Ono (Kyoto University) |
| Title | Physical Mechanism of the Rossby Wave Instability |
| Abstract | Recent observations at sub-mm wavelength with ALMA have revealed that some protoplanetary disks have strongly non-axisymmetric structure in dust distribution. In such protoplanetary disks, gas distribution is also weakly non-axisymmetric. It is often considered that the non-axisymmetric structure is formed by a gas large-scale vortex. The gas vortex captures large dust particles due to gas drag. Some vortex formation mechanisms in protoplanetary disks have been suggested. Among them, we focus on the Rossby wave instability (RWI), which is non-axisymmetric hydrodynamic instability in differentially rotating disks. The RWI is unstable and forms large-scale vortexes when the disk has a rapid radial variation in density (or pressure). We perform linear stability analyses of the RWI, and confirm that the RWI is one of the shear instabilities. In geophysics and plasma physics, the physical mechanism of the shear instability is interpreted by the interaction between two waves. In the case of the RWI, it is also explained by the interaction between two Rossby waves. I will show results of both linear stability analyses and numerical simulations of shear instability. The goal in my talk is to explain the physical mechanism of the RWI. |
| Date/Room | Nov. 9 (Wed.), 14:00- @ES606 |
| Speaker | Yutaro Sato |
| Title | 地球型惑星形成後期における微惑星の衝突・破壊の重要性 |
| Abstract | 太陽系の地球型惑星は火星程度の大きさの原始惑星同士の衝突によって形成されたと考えられており，惑星形成におけるこの段階は巨大天体衝突ステージと呼ばれる．このことは理論的な研究からだけでなく，地質学的な証拠からも支持されている．現在までに，このステージで原始惑星が巨大衝突を起こして地球型惑星が形成されるN体シミュレーションがいくつも行われている．その結果，確かに巨大衝突は起こり地球型惑星が形成された．しかし，巨大衝突が繰り返されるには，衝突を経験するたびに原始惑星の離心率が増加する必要があるので，最終的に形成した地球型惑星の離心率は現在の地球型惑星のものよりもはるかに大きくなってしまった (Chambers & Wetherill 1998, Chambers et al. 2001)．この地球型惑星の離心率を抑制する物理として力学的摩擦が有力である (O’brien et al. 2006, Morishima et al. 2010)．このため，本研究でも原始惑星の周りに小さな微惑星が多数存在しているような系を考える．この系では微惑星と原始惑星の力学的摩擦により，地球型惑星の離心率は下がる．一方で，微惑星の離心率は上がる．このとき，離心率の上がった微惑星はランダム速度が大きいため衝突して壊れる．その結果，微惑星円盤の密度は下がる．この効果により力学的摩擦が効率的に効かないことが予想されるが，これまでの研究では計算コストの問題から微惑星の破壊を取り扱えず，この効果は調べられていない．そこで，本研究では従来のN体シミュレーションに統計的手法を加えることで微惑星の衝突・破壊を取り扱うことにした (Morishima 2015, Kobayashi & Tanaka 2010)．このハイブリッド計算を用いて，微惑星の衝突・破壊が地球型惑星の離心率に与える影響を調べた．その結果，典型的なサイズの微惑星 (半径10km) では，地球型惑星の離心率を下げられないことが明らかになった．本研究ではさらに，力学的摩擦と衝突・破壊のタイムスケールを議論することにより，地球型惑星の離心率を下げられる微惑星のサイズに制限をつけることを目指す． |
| Date/Room | Nov. 10 (Thur.), 16:00- @ES640 |
| Speaker | Yansong Guo |
| Title | An origin of high velocity compact clouds: Dynamical signature of intermediate mass black holes |
| Abstract | Recently, new evidence of intermediate-mass black hole has been suggested by Oka et al. (2016). They observed molecular line emission from a high velocity compact molecular cloud (CO-0.40-0.22) in the central molecular zone (CMZ) of Milky Way Galaxy using radio telescope. The CO-0.40-0.22 cloud possesses extremely broad velocity width ~100km/s. They suggested that such a peculiar velocity structure is caused by gravitational scattering of a 10^5 solar mass black hole by comparing simple test particle orbits. Such an existence of an intermediate mass black hole may provide a support for the “bottom-up” formation scenario of massive black holes. However, their interpretation of the observation of large velocity dispersion neglected the effects of catastrophic heating, dissociation, and ionization by tidal compression and shock wave propagation during the gravitational scattering process. These effects may drastically change the gas state and the appearance of molecular line emissions. To analyze more realistic dynamics of gravitational scattering of a cold molecular cloud, we perform full 3D hydrodynamics simulations using a newly implemented high-resolution shock capturing numerical scheme called “Godunov smoothed particle hydrodynamics” (Inutsuka 2002) with non-equilibrium chemistry and radiative cooling/heating effects (Koyama & Inutsuka 2000). |
| Date/Room | Nov. 16 (Wed.), 16:15- @ES606 |
| Speaker | Kensuke Kakiuchi |
| Title | Magnetic Activity in The Galaxy Centre Region -Vertical structures and flows available from MHD simulation- |
| Abstract | 銀河系中心領域における分子雲は銀河回転に沿った回転速度成分だけでは説明できないような複雑な速度構造を内包していることが観測から分かっている．これらの速度構造形成の要因のひとつとして， Binney et al. (1991) で提案された銀河系内部の棒状ポテンシャル構造が粒子軌道に及ぼす影響が考えられる．しかし，最近の研究で棒状ポテンシャルのみでは複雑な分子雲構造を十分に説明できないことが分かっている (Rodriguez-Fernandez & Combes 2008)．そこで我々が注目しているのは，これまであまり議論されてこなかった磁場の影響である．銀河系中心近傍の磁場の強さは局所的に0.1-1mG (Morris et al. 1992)，全体的に少なくとも50μG 以上の強さがあると観測的に示唆されている (Crocker et al. 2010)．これは宇宙空間の平均的な磁場の強さの数μGを大きく上回り，十分に磁気活動が星間ガスの力学構造に影響を与えると考えられる． これを理論的に検証するために，Suzuki et al. (2015) は銀河系中心領域における磁気流体の3次元グローバル計算を行った．本研究ではこの数値計算結果を用いて，Suzuki et al. (2015)では未解析であった鉛直方向の運動，特に磁場の鉛直構造に沿って落下するガスの下降流について詳細な解析を行い，鉛直方向のガス運動が観測的な速度構造に与える影響について調べた．この下降流は上空から銀河面に向かうに従い重力によって加速落下するが，その下降流領域のみを抜き出して位置速度図に対応させたところ，観測されている CO 輝線の位置速度図にみられる高速度分散領域をよく説明することがわかった．本研究は，実際の分子雲中においても磁気活動によってガスの鉛直方向へ運動が励起され，複雑な速度構造の一因となりうることを示すものである．本講演では，さらに数値計算結果と滑り台落下モデルの比較を行い，磁気活動による速度構造への影響について議論する． |
| Date/Room | Jan. 24, 17:00- @ES640 |
| Speaker | Alvaro Hacar (Leiden Observatory) |
| Title | Filaments and clusters as complex fiber networks |
| Abstract | Recent observational results have revealed the fibers as the fundamental building blocks of molecular clouds in a wide range of environments. Complex networks of fibers are recognized in local filaments, compact clusters, and IRDCs. This primordial gas structures set the initial conditions for the formation of cores and stars within these regions. During my talk I will discuss the role of fibers in different star forming clouds like Taurus, Perseus, and Orion containing both isolated and clustered stars. During my talk I will discuss how these fibers can potentially unify our current description of the star formation process in clouds of increasing complexity. |
| Date/Room | Feb. 13(Mon.), 15:00- @ES606 |
| Speaker | Jungyeon Cho (Chungnam National University) |
| Title | Overview of Astrophysical Turbulence |
| Abstract | Many astrophysical fluids are in turbulent state. I will briefly introduce properties of magnetized turbulence. Astronomy deals with various length scales and different descriptions should be used for different scales. On large scales, astrophysical turbulence can be described in the framework of magnetohydrodynamics (MHD). In this regime, I will consider two extreme cases. First, I will consider a fluid threaded by a very weak mean magnetic field. In this case, growth of the magnetic field is of great importance. I will show that a weak magnetic fields can be amplified very efficiently by turbulent motions and discuss its astrophysical implications. Second, I will consider a fluid threaded by a very strong mean magnetic field. In the presence of a strong mean magnetic field, disturbances travel along the magnetic field line and collisions between opposite-traveling Alfven wave packets (or `eddies') are essential for development of turbulence. In this talk, I'll mostly discu ss Alfvenic turbulence. On the other hand, near and below the ion gyro-scales, we should include plasma effects. In this talk, I will discuss how we can treat such small-scale magnetized turbulence. |
| Date/Room | Mar. 1 (Wed.) 16:00- @ES606 |
| Speaker | Tristan Guillot (Observatoire de la Cote d’Azur) |
| Title | Forming the Solar System: The Role of Giant Planets |
| Abstract | The Sun and its system of planets formed from the same reservoir of gas and solids, a giant molecular cloud core that collapsed to form a central star and a protoplanetary disk. In spite of their tendency to drift rapidly in the disk, a fraction of solid grains (probably in the form of porous pebbles) managed to avoid being swallowed by the forming Sun and instead formed planetesimals and then planets. Some planetesimals grew rapidly enough, in a few Myr, to capture part of the circumstellar gas disk before its complete disappearance and become giant planets. These giant planets had a crucial role: after they formed, they prevented the arrival of grains and pebbles to the inner solar system. I will discuss how the compositions of giant planets is a crucial piece of the puzzle to understand planet formation and how their formation should have left an imprint on the composition of the Sun. |