| Date/Room | Mar. 28, 14:00- @ES606 |
| Speaker | Michael Kuffmeier (Niels Bohr Institute in Copenhagen, Denmark) |
| Title | Zooming in on star and protoplanetary disk formation |
| Abstract | Traditionally, protoplanetary disk dynamics have been modeled with a standard accretion disk approach, while the formation of young disks has been modeled by the collapse of a Bonnor-Ebert size gas sphere. Both approaches allow a detailed study of the influence of different physical parameters, but the initial conditions for the above models have to be assumed rather ad hoc. Taking this into account, we model the formation process around ten solar mass stars in the context of Giant Molecular Cloud (GMC) dynamics. Our starting point is a GMC of (40 pc)^3 in size and we solve the equations of ideal magnetohydrodynamics with a modified version of RAMSES. The efficient use of adaptive mesh-refinement allows us to refine the formation process of young protostars at AU scales during the first ~100 kyr of evolution. In this talk, I will focus on two major conclusions that can be drawn from this study. First, we find that protostellar accretion is a heterogeneous process for each individual protostar as well as the accretion history can differ significantly from star to star with significant effects on protoplanetary formation, and eventually planet formation. Second, we can study the distribution of Al-26 around the different stars during the period considered to be relevant for the formation of Calcium-Aluminum rich Inclusions (CAIs). This is interesting because cosmochemical measurements reveal that CAIs are the oldest solids in the solar system and probably formed within ~10 kyr shortly after protosolar formation. Under these constraints, the significant differences in Al-26/Al-27 between canonical CAIs and CAIs with fractionation and unidentified nuclear effects (FUN CAIs) of more than a factor of 10 have been explained by a supernova contamination during the early collapse phase. However, the environment around the stars in our simulations does not show such high early differences and we suggest instead that the different Al-26 abundances in CAIs are caused by thermal processing of dust grains. |
| April | 7 | Dimitris Stamatellos |
| 8 | Shu-ichiro Inutsuka | |
| 10 | Masanobu Kunitomo | |
| 15 | Hiroshi Kobayashi | |
| 20 | Kenji Kurosaki (Univ. of Tokyo) | |
| 30 | Jennifer Stone | May | 13 | Takeru Suzuki |
| 19 | Kenji Toma (Tohoku Univ.) | 20 | Torsten Stamer |
| 27 | Break (JpGU) | June | 3 | Break (DTA Symposium) |
| 10 | Shoji Mori | |
| 17 | Keisuke Sugiura | |
| 24 | Tomoya Miyake | July | 3 | Break (International Workshop "Disc Dynamics and Planet Formation") |
| 10 | Yuki Ohno | |
| 15 | Makoto Takamoto (Univ. of Tokyo) | |
| 17 | Yansong Guo (Rehearsal for Summer School for Young Astronomers in Japan) | |
| 22 | Kensuke Kakiuchi, Yutaro Sato, Kenta Nakashima (Rehearsal for Summer School) | September | 18 | Neal Turner (JPL/Caltech) |
| 28 | Oliver Gressel (Niels Bohr Institute) | October | 6 | Seminar on inflated Hot-Jupiters (speakers: A. Youdin, University of Arizona/H. Kurokawa, Tokyo Tech.) |
| 7-8 | Interim Reports 中間発表会 | |
| 21 | Shu-ichiro Inutsuka | |
| 28 | Shu-ichiro Inutsuka | November | 4 | Yuki Tanaka |
| 9 | Anders Johansen (Lund Observatory) | |
| 11 | Yuki Ohno | |
| 11 | Robert Wittenmyer (University of New South Wales) | |
| 18 | Break (Special Lecture by Prof. Munetake Momose) | |
| 25 | Break (Workshop on High Energy Astrophysics) | December | 2 | Shoji Mori |
| 9 | Keisuke Sugiura | |
| 16 | Masanobu Kunitomo | |
| 17 | Tomoya Miyake | |
| 18 | Wladimir Lyra (California State University, Northridge) | January | 7 | Interim Reports for Undergraduate Students |
| 13 | Yansong Guo | |
| 14 | Kensuke Kakiuchi | |
| 20 | Yutaro Sato | |
| 26 | Alexandre Lazarian (Univ. of Wisconsin) | |
| 27 | Torsten Stamer | February | 3 | Rehearsal for master defense |
| March | 23 | Philippe Andre (CEA, Saclay, France) |
| Date | April 7th, 16:00- |
| Speaker | Dimitris Stamatellos |
| Title | Planet migration and gap opening in self-gravitating protoplanetary discs |
| Abstract | Gravitational torques between the planet and its host disc result in orbital migration of the planet and affect its subsequent evolution. Studies of migration of planets formed by core accretion (i.e. late during the disc evolution) have identified two main types of planet migration (fast/slow), depending on the mass of the planet. The discovery of massive planets orbiting at large distances from the host star suggests that at least some giant planets may form early-on during the disc evolution via disc fragmentation. Such planets will interact with a relatively massive (self- gravitating) disc. I will discuss how Jovian planets interact with self-gravitating discs, what are the effects of such interactions (inward migration, gap opening, and planetary mass growth), and how these depend on the disc thermodynamics. I will present global hydrodynamic simulations of planet-disc interaction and planet migration, and discuss whether signatures of such processes could be observed. |
| Date | April 8th, 15:00- |
| Speaker | Shu-ichiro Inutsuka |
| Title | The Formation and Destruction of Molecular Clouds and Galactic Star Formation |
| Abstract | An overall picture of molecular cloud formation and destruction in the Galaxy is presented in this talk. Recent high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with cooling/heating and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of compressional events. Indeed, multiple compressions of ISM create both massive filamentary molecular clouds whose axes are perpendicular to the magnetic field lines and faint striations that are parallel to the field lines, which mimic general features of actual observations. Subsequent collapse and fragmentation of filamentary molecular clouds provide the initial conditions of protostellar collapse calculations that have been done extensively and successfully. This finding enables us to envision a scenario of molecular cloud formation as the interacting shells or bubbles in galactic scale. We estimate the ensemble-averaged growth rate of the individual molecular cloud, and predict the mass function of molecular clouds. This picture naturally explains the accelerated star formation over many million years that was previously reported by stellar age determination in nearby star forming regions. Recent claim of cloud-cloud collision as a mechanism of forming massive stars and star clusters can be naturally incorporated in this scenario, which explains why the massive stars formed in cloud-cloud collision follows the power-law slope of the mass function of molecular cloud cores repeatedly found in low-mass star forming regions. |
| Date | April 10th, 16:00- |
| Speaker | Masanobu Kunitomo |
| Title | Pre-main sequence evolution of low-mass stars with accretion |
| Abstract | Recently the multidimensional hydrodynamic simulations have revealed that the materials accrete onto a protostar through a circumstellar disk. This is different from the classical studies, which assumed the spherical accretion. Since the entropy of the materials in the disk accretion can be much lower than that of spherical accretion, the disk accretion has a potential to change the pre-main sequence (PMS) evolution significantly. Thus, we have studied the PMS evolution with disk accretion. We calculated PMS evolution using a 1D stellar evolution code with modeling the entropy of accretion materials. We found that the PMS evolution is highly affected by the accreting material’s entropy and the deuterium content. Moreover, we found that the internal structure evolution can be totally different from the classical model. With these results, we will discuss the implication for the observed luminosity spread of young stars and the peculiar solar surface composition. |
| Date | April 15th, 14:00- |
| Speaker | Hiroshi Kobayashi |
| Title | Turbulence and Planet Growth |
| Abstract | In the core accretion scenario, a massive solid core as large as 10 Earth masses induces rapid gas accretion to be gas giant planet such as Jupiter and Saturn. However, the collisional growth of planetesimals cannot produce massive cores because of the depletion of planetesimals caused by their collisional fragmentation and rapid radial drift of the fragments (Kobayashi et al. 2010, 2011). I will talk about the effect of turbulence in a protoplanetary disk. The density fluctuation caused by turbulence stirs planetesimals (Okuzumi and Ormel 2013). The turbulence delays the runaway growth of planetesimals, which is caused by gravitational focusing and dynamical friction. Solid cores of gas giants are formed via the runaway growth and further grow through the accretion of surrounding planetesimals. In a strong-turbulence disk, the runaway growth occurs late, planetesimals accreting onto the cores are large, and the cores can be as massive as 10 Earth masses, which may start rapid gas accretion to form gas giants. I will explain more detailed mechanisms of the planet formation in a turbulent protoplanetary disk. |
| Date | April 20th, 14:00- |
| Speaker | Kenji Kurosaki (Univ. of Tokyo) |
| Title | Threshold Masses and Radii for Survival of Photo-Evaporating Low-Mass Low-Density Planets |
| Abstract | The Kepler mission has enabled us to detect transiting planets down to the Earth in size. From measured radii and masses, we can infer planetary internal compositions. The mass-radius relationship suggested the existence of water-rich low-mass planets with short periods. Close-in low-mass planets experience loss of volatile material due to stellar X-ray and EUV (XUV) irradiation. In this study, we aim to obtain theoretical predictions for water-rich super-Earths and also sub-Earths that have undergone photo-evaporative mass loss. We have found that water-rich planets above a minimum mass can maintain water envelopes, regardless of initial water content from 0 wt% to 100 wt%. The threshold value decreases with the semi-major axis. Our results predict a habitat area for water-rich planets orbiting solar-type and M-type stars on a planetary mass/radius-period diagram. This helps us know the origin and planetary compositions of close-in low-mass planets from the Kepler and other planet surveys. |
| Date | April 30th, 16:00- |
| Speaker | Jennifer Stone |
| Title | Hydrodynamic Stability Properties of Phase Transition Layers in the Diffuse Interstellar Medium with Realistic Thermal Conduction |
| Abstract | We perform a linear stability analysis to calculate the complete hydrodynamic stability properties of ISM thermal fronts separating cold, dense gas from warm, rarified gas against corrugational perturbations. Previous investigations have employed simplifying assumptions in their treatment of thermal conductivity and assumed isobaric perturbations. In our analysis we retain all pressure perturbation terms and employ realistic, temperature-dependent thermal conductivity. We find that temperature-dependent thermal conduction leads to phase transition layers with steeper profiles, and the growth rate of the instability and range of unstable wave numbers are larger than for constant conductivity. Our complete analysis and proper treatment of thermal conduction will prove important in analyzing stability in dynamic, magnetized media, and will inform the implementation of thermal conduction parameters in numerical simulations. |
| Date | May 13th, 14:00- |
| Speaker | Takeru Suzuki |
| Title | Solar Wind / Sun-like Star Wind & Their Evolution |
| Abstract | We investigate how the mass loss by the solar wind depends on the solar activity levels, particularly focusing on the solar wind during extremely high activity.We perform forward‐type magnetohydrodynamical numerical experiments for Alfven wave‐driven solar winds with a wide range of the input Poynting flux from the photosphere.Increasing the magnetic field strength and the turbulent velocity at the solar photosphere from the current solar level, the mass loss rate rapidly increases at first owing to the suppression of the reflection of the Alfven waves. The surface materials are lifted up by the magnetic pressure associated with the Alfven waves, and the cool dense chromosphere is intermittently extended to 10 ‐ 20 % of the solar radius. The dense atmospheres enhance the radiative losses and eventually most of the input Poynting energy from the surface escapes by the radiation. As a result, there is no more sufficient energy remained for the kinetic energy of the wind; the solar wind saturates for the extreme activity level, as observed in Wood et al. The saturation level is positively correlated with the average magnetic field strength contributed from open flux tubes. If the field strength is a few times larger than the present level, the mass loss rate could be as high as 1000 times. |
| Date | May 19th, 16:15- |
| Speaker | Kenji Toma (Tohoku Univ.) |
| Title | Formation Mechanism of Black Hole Jets |
| Abstract | Collimated plasma outflows or jets with relativistic speeds are observed in active galactic nuclei, gamma-ray bursts, and Galactic microquasars. They are believed to be produced in the system of a black hole surrounded by its accretion flow, although no standard model or scenario has been established for driving and acceleration mechanisms of relativistic jets. This talk will focus on a current promising scenario of jet formation, which has been developed by recent numerical studies, and will also provide some analytical understandings. The topics in this talk will include the acceleration and collimation of relativistic ideal MHD wind and electromagnetic extraction of black hole rotational energy, i.e., Blandford-Znajek process. |
| Date | May 20th, 14:00- |
| Speaker | Torsten Stamer |
| Title | Protostellar Collapse and the Formation of Brown Dwarfs |
| Abstract | Like solar mass stars, brown dwarfs are believed to be formed when dense molecular cloud cores collapse under their own gravity. However, while for solar mass cores densities of 10^5 / cm^3 are sufficient, a cloud core with only a few dozen Jupiter masses requires densities on the order of 10^8 / cm^3 to trigger the collapse. As there is no good tracer molecule for these densities, the mass function of such cores is quite uncertain. I am working to develop a radiative hydrodynamics code to simulate the protostellar collapse of brown dwarf mass cores, in order to illuminate which initial conditions are necessary to explain observed brown dwarf formation. My talk will focus on the background, basic physics and initial conditions of such a simulation. |
| Date | June 10th, 14:00- |
| Speaker | Shoji Mori |
| Title | Electron Heating and Suppression of Magnetorotaional Turbulence in Protoplanetary Disks |
| Abstract | Magnetorotational instability (MRI) is a powerful mechanism of driving turbulence in regions of protoplanetary disks with a high ionization fraction. However, the ionization fraction is not necessarily constant in MRI-driven turbulence, because turbulence-induced electric fields heat up free electrons and thereby affect the ionization balance. In particular, in the presence of small dust grains, the ionization fraction decreases with increasing electric field strength (Okuzumi & Inutsuka, 2015), suggesting that MRI turbulence may not fully develop even outside the dead zone. We estimate how much this affects the saturation level of turbulence in protoplanetary disks. For a minimum-mass solar nebula with 1% of its mass consisting of 0.1um-sized dust grains, we find that the strength of MRI turbulence is suppressed by more than two orders of magnitude within 80 AU from the central star (Mori & Okuzumi, submitted; arxiv# 1505.04896). We will also discuss the accumulation of surface density by the suppression of MRI turbulence which may causes gravitational instability. |
| Date | June 17th, 14:00- |
| Speaker | Keisuke Sugiura |
| Title | An Extension of Godunov SPH : Application to Negative Pressure Media |
| Abstract | Planets are thought to be formed by a series of collisions between planetesimals. Thus, it is necessary to estimate the effect of growth and destruction via collisions in order to understand the origin of planets. However, since the typical size of planetesimals is approximately 1 km, we can not investigate the collision of planetesimals in laboratory experiments. Therefore, we need numerical simulations to evaluate the effect of collisions. Recently some researches have applied the Smoothed Particle Hydrodynamics (SPH) method into the elastic dynamics, and simulated collisions of planetesimals. The most common form of SPH is called standard SPH. However, recently it has been pointed out that the standard SPH method has a problem, which is the tensile instability. It causes a clustering of particles in the negative pressure region (Swegle et al. 1995). This negative pressure occurs in stretched elastic bodies. Although some previous studies tried to remove this instability (e.g., Monaghan 2000), they had some problems such as utilization of artificial pressure term. Moreover, their methods could not suppress the tensile instability completely. Therefore we need a more robust method to simulate the collisional process correctly. Another formulation of SPH is called the Godunov SPH method (Inutsuka 2002). It is a spatially second-order accurate method, whereas the standard SPH method does not have such a convergence property because of its rough approximation. We deal with the tensile instability using this Godunov SPH method. In this colloquim, we show the result of a linear stability analysis of the Godunov SPH method, and our new method to remove the tensile instability. We also show the result of a test calculation of the elastic dynamics, and demonstrate the effectiveness of this method. |
| Date | June 24th, 16:00- |
| Speaker | Tomoya Miyake |
| Title | Dust Dynamics in Protoplanetary Disk Winds Driven by Magneto-Rotational Turbulence |
| Abstract | One of the major unknowns in the evolution of protoplanetary disks is the dispersal mechanism at the final stage. The photoevaporating wind by ultraviolet and X-ray emissions from a central star has been thought to be a dominant process and extensively investigated so far. However, recently the disk wind driven by magneto-turbulence, which is the magneto-rotational instability (MRI) in a disk, has been proposed as a cooperating process or a powerful alternative in dispersing the gaseous component (Suzuki & Inutsuka 2009). In the MRI turbulence-driven disk winds, the dynamics of dust grains should be potentially interesting in the context of planetesimal formation but have not been studied yet. In this study we investigate the dynamics of dust grains with various sizes in protoplanetary disk winds driven by magnetorotational turbulence, by simulating the time evolution of the dust grain distribution in the vertical direction. We found that small dust grains, which are well coupled to the gas, are dragged upward with the upflowing gas, while large grains remain near the midplane of a disk. Intermediate-size grains float at several scale heights from the midplane in time-averated force balance between the downward gravity and the upward gas drag. Considering the dependence on the distance from the central star, smaller-size grains remain only in an outer region of the disk, while larger-size grains are distributed in a broader region. This implies that the dust depletion is expected to take place in small-to-large and inside-out manners. We also discuss the implication of our result to the observation of dusty material around young stellar objects. |
| Date | July 10th, 16:00- |
| Speaker | Yuki Ohno |
| Title | SPH法による対流の数値シミュレーション |
| Abstract | 近年、理論予測よりもはるかに大きな半径を持つ短周期巨大ガス惑星が検出されている。この異常膨張の原因に対していくつかの説が提示されているが、未だ解決はしていない。一つの可能性として、惑星内部での熱輸送効率が小さいことで半径の収縮を抑えることが考えられている。巨大ガス惑星は形成時に大量の集積熱を有しており、輻射冷却により準静的に収縮する。そのため、惑星内部でのエネルギー流束が小さければ集積熱を長期間保持することができ、半径が大きい時期が長く続くと考えられる。ここで、惑星内部でのエネルギー輸送はその構造に大きく依存する。一般に巨大ガス惑星は全対流しており、主に対流によりエネルギーが輸送されていると考えられている。対流現象は重力下で温度勾配が断熱温度勾配よりも急である場合に発生する（Schwarzschildの条件と呼ばれる）。しかし、組成（平均分子量）に勾配がある場合には、対流の安定性が変化することが知られている（Ledouxの条件と呼ばれる）。後者の条件で安定であるが前者の条件では不安定な領域では、熱と組成の拡散の影響を受けて、二重拡散対流と呼ばれる対流現象が起こり熱輸送効率が減少する。つまり、組成勾配によって内部構造が変化し、熱輸送効率も大きく影響を受ける。熱輸送効率が定量的にどの程度変化するかは数値シミュレーションにより検証する必要がある。そこで本研究では、温度と組成の勾配、およびそれらの拡散を考慮した流体計算を行い、エネルギー流束を定量的に評価することで、観測された巨大ガス惑星の半径異常の原因を解明することを目的としている。今回の発表では、熱伝導（熱の拡散）を考慮したSPH法を用いた2次元の数値シミュレーションの結果を紹介する。 |
| Date/Room | July 15th, 11:00- @ES606 |
| Speaker | Makoto Takamoto (Univ. of Tokyo) |
| Title | Relativistic Turbulent Reconnection in Poynting-Dominated Plasma and Applications to High-Energy Astrophysical Phenomena |
| Abstract | In this talk, we report on our recent findings of the enhancement of the magnetic reconnection rate by turbulent processes in a Poynting-dominated plasma. We simulate an evolution of a current sheet in driven turbulence using the relativistic resistive MHD approximation. Our numerical results show the turbulence greatly enhances the reconnection rate, and it becomes independent of the resistivity. We also discuss some effects from plasma compressibility, and how it affects the reconnection rate. |
| Date/Room | July 17th, 14:00- @ES606 |
| Speaker | Yansong Guo |
| Title | 大速度分散コンパクト分子雲の起源：大質量ブラックホールによる重力散乱過程についての理論的研究 |
| Date/Room | July 22nd, 13:30- @ES606 |
| Speakers | Kensuke Kakiuchi, Yutaro Sato, Kenta Nakashima |
| Title | 銀河系中心領域における磁気雲のダイナミクス 多体惑星系における軌道の長期安定性とその進化 分光観測によって発見された連星系 HD139461における「惑星」の存在について |
| Date/Room | Sep. 18th, 10:30- @ES606 |
| Speaker | Neal Turner (Jet Propulsion Laboratory, California Institute of Technology) |
| Title | Shadows round the campfire: what young stars' infrared variability reveals about protoplanetary disks |
| Abstract | Young stars' infrared emission shows several puzzling features: variability uncorrelated with visible-light changes, foreground extinction that recurs erratically on weekly timescales, and excesses over the stellar photosphere too large to explain with starlight absorbed and re-emitted by a hydrostatic protostellar disk. I will discuss how each of these features can be explained by a time-varying, magnetically-supported disk atmosphere like those found in MHD calculations of magneto-rotational turbulence. Through Monte Carlo radiative transfer calculations I will show that such an atmosphere yields variability amplitudes as observed. Since the starlight-absorbing surface lies higher than in hydrostatic models, a greater fraction of the stellar luminosity is reprocessed into the infrared, providing a natural explanation for the larger excesses. The atmosphere rises high enough to obscure the star in systems viewed near edge-on, and can explain the week-long extinction events, provided only that the dust in the outer parts of the disk has undergone some growth or settling, as likely required for planet formation to begin. |
| Date/Room | Oct. 7-8 @ES606 |
| Speaker | Graduate students |
| Title | Interim Reports |
| Date/Room | Oct. 21, 28, 14:00- @ES606 |
| Speaker | Shu-ichiro Inutsuka |
| Title | From Filaments to Cores, Protostars, and Protoplanetary Disks |
| Abstract | Essential physical processes in the formation of low-mass stars are reviewed. Recent progress in observation is remarkable and has improved our description of molecular cloud structures that include the characteristic distribution of the line-masses and widths of filamentary clouds. The core mass function is now being understood in terms of intrinsic property of molecular clouds, which provides a clue in understanding the stellar initial mass function. On the other hand, recent advances in the non-ideal radiation magnetohydrodynamical simulations have enabled our understanding of the driving of outflows/jets and the formation of protoplanetary disks in a self-consistent manner from molecular cloud cores. It emphasizes the importance of radiative heating/cooling and the effects of all three non-ideal effects of magnetohydrodynamics, and provides an improved description for the realistic initial conditions and environments for planet formation. We find that gaseous planetary-mass objects can be formed by gravitational instability in the regions that are de-coupled from the magnetic field and surrounded by the injection points of the magneto-hydrodynamical outflows during the formation phase of protoplanetary disks. Magnetic de-coupling enables massive disks to form and these disks are subject to gravitational instability in various radii. The study of the protoplanetary disk formation is now providing a diversity of their evolution and the resultant planet formation. |
| Date/Room | Nov. 4, 14:00- @ES606 |
| Speaker | Yuki Tanaka |
| Title | Impacts of the MHD waves on the atmospheric escape and the upper atmospheric structure |
| Abstract | Recent observations of exoplanets have been revealed a lot of interesting properties of their atmosphere. For example, atmospheric structures and compositions of exoplanets have been revealed through transit spectroscopic observations in the optical and near-infrared wavelength. Furthermore, structures of the upper atmosphere of hot Jupiters and a large amount of the escaping atmosphere from hot Jupiters have also been investigated through several transit observations in UV wavelength. Characteristics of the planetary atmospheres are believed to be related not only with their observational features but also their internal structures and formation mechanism, therefore research on the atmosphere of the exoplanets have become more popular. In our previous studies, we demonstrated magnetohydrodynamic (MHD) calculation to treat details of a mechanism of atmospheric escape and the structure of the upper atmosphere of gaseous planets, especially for the hot Jupiters. As a result, we showed that MHD waves heat up the upper atmosphere of the hot Jupiters, and they can drive a large amount of atmospheric escape from the upper atmosphere. We also carried out calculations that take into account the magnetic diffusion in the planetary atmosphere, and showed MHD waves have an important role even in the weakly-ionized atmosphere of the planets. It is important for MHD calculation in the planetary atmosphere to treat the heating by MHD waves and radiative cooling/heating. We used radiative cooling functions that are used for the solar and stellar atmosphere in the previous calculations, but more accurate calculations of the radiative transfer are needed to apply our model to cooler gaseous planets. Therefore we use the two-stream approximation to solve the radiative transfer and determine radiative cooling/heating rate for the MHD calculation. In this talk, first we introduce our results based on Tanaka et al. (2014) and Tanaka et al. (2015). Then we talk about results of our improvement on the radiative cooling/heating function. In addition, I will discuss recent calculation results for the evaporating hot Neptune GJ 436b. |
| Date/Room | Nov. 9, 16:00- @ES606 |
| Speaker | Anders Johansen (Lund University, Sweden) |
| Title | Formation of planetesimals and planets |
| Abstract | Planets form in protoplanetary discs around young stars. Observations of such discs reveal that dust grains grow to millimeter-to-centimeter-sized pebbles. Such pebbles can play important roles for planet formation. Pebbles are concentrated in the turbulent gas, and the overdense filaments collapse gravitationally to form planetesimals with a wide range of sizes. Those planetesimals continue to grow by accreting the remaining pebbles. The growth is so rapid that the cores of gas giants can form within a few million years, before the dissipation of the gaseous protoplanetary disc. I will present a coherent picture of planet formation, from dust grains to fully-formed planets, based on such pebble accretion models. |
| Date/Room | Nov. 11, 14:00- @ES606 |
| Speaker | Yuki Ohno |
| Title | ガス惑星内部における対流の直接数値シミュレーション解析 |
| Abstract | 近年、理論予測よりもはるかに大きな半径を持つ短周期巨大ガス惑星が検出されている。この異常膨張の原因に対していくつかの説が提示されているが、未だ解決はしていない。一つの可能性として、惑星内部での熱輸送効率が小さいことで半径の収縮を抑えることが考えられている。巨大ガス惑星は形成時に大量の集積熱を有しており、輻射冷却により準静的に収縮する。そのため、惑星内部でのエネルギー流束が小さければ集積熱を長期間保持することができ、半径が大きい時期が長く続くと考えられる。ここで、惑星内部でのエネルギー輸送はその構造に大きく依存する。一般に巨大ガス惑星は全対流しており、主に対流によりエネルギーが輸送されていると考えられている。対流現象は重力下で温度勾配が断熱温度勾配よりも急である場合に発生する（Schwarzschildの条件）。しかし、組成（平均分子量）に勾配がある場合には、対流の安定性が変化することが知られている（Ledouxの条件）。後者の条件で安定であるが前者の条件では不安定な領域では、熱と組成の拡散の影響を受けて、二重拡散対流と呼ばれる対流現象が起こり熱輸送効率が減少する。つまり、組成勾配によって内部構造が変化し、熱輸送効率も大きく影響を受ける。熱輸送効率が定量的にどの程度変化するかは数値シミュレーションにより検証する必要がある。本研究では、温度と組成の勾配、およびそれらの拡散を考慮した流体計算を行い、エネルギー流束を定量的に評価し、観測された巨大ガス惑星の半径異常の原因を解明することを目的としている。粒子を用いたラグランジュ的な流体数値計算手法の一つであるSPH法を用いて数値シミュレーションを行った。粒子法では拡散係数をゼロにすることで拡散の効果を完全に取り除いた計算をすることができ、拡散が結果に与える効果をはっきりと確認できるというメリットがある。今回の発表では、熱伝導（熱の拡散）と組成の勾配を考慮した2次元の数値シミュレーションの結果を紹介し議論する。 |
| Date/Room | Nov. 11, 16:30- @ES606 |
| Speaker | Robert Wittenmyer |
| Title | The great unveiling: a new golden age of exoplanetary science |
| Abstract | Less than a generation ago, we wondered, as we had for millions of years before, whether there were any other planetary systems at all. Now, we are privileged to be in the first generation of humans to know that many of the points of light dusting our night sky are host to orbiting worlds, some of which may be like our Earth. With that privilege comes the mighty task of our time, to unlock the secrets of the diversity of worlds. I will give an overview of the "discovery revolution" that brought us here. I will then describe the tremendous challenges and opportunities awaiting us in exoplanetary science over the next decade. The next revolution will be one of understanding - a "great unveiling" as we learn the detailed properties of the planetary systems in the Solar neighbourhood. |
| Date/Room | Dec. 2, 14:00- @ES606 |
| Speaker | Shoji Mori |
| Title | The Effects of Electron Heating on the Magnetorotational Instability in Protoplanetary Disks |
| Abstract | The magnetorotational instability drives vigorous turbulence in protoplanetary disks. Our previous work showed that electron heating in the magnetic turbulence causes an enhancement of the Ohmic resistivity, which in turn might quench the magnetic turbulence. To examine this possibility, we perform magnetohydrodynamical simulations in which the effect of electron heating on the Ohmic resistivity is mimicked by a simple analytic model. Our simulations confirm that electron heating suppresses magnetic turbulence. When the effect of electron heating is significant, turbulence completely dies away, leaving a steady laminar flow where the accretion stress is dominated by ordered magnetic fields. Based on the simulation results and the scaling relation between the Maxwell stress and current density, we obtain an analytic formula that successfully predicts the accretion stress in the presence of electron heating. |
| Date/Room | Dec. 9, 14:00- @ES606 |
| Speaker | Keisuke Sugiura |
| Title | 微惑星の高速衝突破壊現象の解明へ向けて：ゴドノフSPH法の弾性体力学への拡張と応用 |
| Abstract | 固体惑星や月などの衛星は小天体同士の衝突の結果形成されたと考えられている。従って惑星・衛星の起源を詳しく理解するにあたって、小天体同士の衝突の結果を詳細に理解することは極めて重要である。しかしながら小天体の典型的なサイズはkm以上であるため、室内実験で衝突を再現しその結果を観測することは不可能である。そのため数値計算の手法を用いて小天体衝突の詳細を調べることが必要である。 近年Smoothed Particle Hydrodynamics(SPH)法と呼ばれる流体力学の数値計算手法を弾性体力学に応用することで、天体衝突の数値計算が行われている。SPH法は粒子を用いたラグランジュ的な手法であり、天体衝突などの大変形問題に非常に適した手法である。一方で広く使われている標準SPH法にはいくつか問題点があることが指摘されており、一般の乱雑な粒子配置では空間1次精度以下である、強い衝撃波で流体粒子の突き抜けが起きてしまう人工粘性を利用している、引き伸ばされた固体を表す負の圧力の領域でtensile instabilityと呼ばれる数値不安定性が存在する、などの問題点が存在する。また多くの先行研究ではせん断応力などの固体が持つ効果を考慮せず流体の方程式を用いているため、その効果の影響も明らかになっておらず、数値計算手法の慎重な吟味が必要である。標準SPH法の問題点を解決するための手法として、ゴドノフSPH法と呼ばれる別のSPH法が提案されている。このゴドノフSPH法では空間2次精度の定式化がなされており、またリーマンソルバーと呼ばれる物理的な粘性を導入する手法を利用し人工粘性の利用を回避している。しかしながらゴドノフSPH法のtensileinstabilityへの安定性はよく理解されておらず、さらにゴドノフSPH法を弾性体力学に応用した研究は存在しない。そこで本研究では、線形安定性解析の手法を用いてゴドノフSPH法の安定性を調べ、ゴドノフSPH法を用いたtensile instabilityの自然な解決法を開発した。さらに、ゴドノフSPH法を弾性体力学に応用し、弾性体力学の高精度な数値計算を可能にした。また、開発した手法を用いて小天体の衝突計算を実行し、衝突の結果に対し固体の効果がどのような影響を及ぼすのか議論する。 |
| Date/Room | Dec. 16, 14:00- @ES606 |
| Speaker | Masanobu Kunitomo |
| Title | Dispersal of protoplanetary disks: Effects of photoevaporation and disk wind |
| Abstract | Since protoplanetary disks (PPDs) are the formation site of planets, their evolution and dispersal are crucial for understanding the distribution of planets. In the last decade the disk dispersal by viscous accretion onto the central star and irradiation from the central star (photoevaporation) has been focused. However, Suzuki et al. (2010) showed that the turbulence driven by magneto-rotational instability can generate vigorous disk wind and have an important role in disk evolution. In this talk I will show that the disk evolution including the three effects; viscous accretion, photoevaporation and disk wind. I will also discuss the origin of the short disk lifetime around intermediate-mass stars. |
| Date/Room | Dec. 17, 13:00- @ES640 |
| Speaker | Tomoya Miyake |
| Title | 原始惑星系円盤風中のダストの動力学と観測可能性 |
| Abstract | 原始惑星系円盤進化の上で、未だ理論的に解明されていないことの一つに円盤ガスの散逸機構があり、その候補として、中心星からのUV やX 線放射による光蒸発が幅広く研究されている。一方で、磁気回転不安定性(MRI) 起源の磁気乱流により駆動される円盤風もまた、光蒸発と同等以上に円盤散逸の効果があるとの指摘が、Suzuki & Inutsuka (2009) によりなされている。ガスの散逸はダストの運動に影響を与えるが、このような円盤風を考慮したダストの動力学をはこれまで調べられていない。そこで本研究では、磁気乱流駆動型の円盤風を考慮した原始惑星系円盤の、ダスト密度の鉛直方向分布の時間進化を簡単化した1 次元数値シミュレーションの手法で調べ、さまざまなサイズのダストの動力学について研究を行なった。その結果、摩擦力によりガスと強結合した小さなサイズのダストは、円盤風により円盤上空へと流れ出し、大きいダストは、円盤内部に留まることがわかった。さらに面白い事に、中間サイズのダストは、円盤内のガスとのカップリングが弱くなる、赤道面から数スケールハイトの円盤風の速度が音速点付近で浮き溜まることがわかった。また、中心星からの距離に対する依存性を考慮すると、比較的小さいダストは円盤外側の領域にのみ残り、大きいダストは円盤赤道面付近に留まるという結果を得たダストが原始惑星系円盤上空に持ち上げられていると思われる赤外線観測の結果を、本機構の観点から議論し，加えて、ALMA などによる今後の高空間分解能観測による検証方法についても議論する。本研究では更に、これらの結果を元に、円盤風を考慮したダストの円盤動径方向の大局的な面密度時間進化について調べた結果についても議論する。 |
| Date/Room | Dec. 18, 16:30- @ES606 |
| Speaker | Wladimir Lyra |
| Title | Planet Signatures in Transitional Disks |
| Abstract | Transitional disks are a class of circumstellar disks that lack a significant near-infrared excess, while showing steep slopes in mid-infrared and far-infrared excesses typical of classical T-Tauri disks. This “opacity hole” implies the absence of optically thick warm dust in the inner disk, with a dust wall generating the mid-IR emission, followed by cold dust in the outer disk. Considering also the age of these systems (1-10 Myr), there is strong evidence that these are objects caught in the evolutionary stage between gas-rich primordial and gas-poor debris disks, hence their name. Recently, high angular resolution imaging of the outer regions of transitional disks have become available, showing a plethora of puzzling asymmetries that beg for explanation. The presence of planets is a particularly attractive interpretation for explaining these asymmetries, since they generally match the range of structures predicted by hydrodynamical models of planet-disk interactions. Examples of these structures are the spiral arms that have been seen in images obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and with the Atacama Large Millimeter Array (ALMA). Yet, comparing the model predictions of spirals launched by planet-disk interaction to the observed features has shown far from perfect agreement. This may be due to the strong approximations used for the predictions; for example, spiral arm fitting typically uses results that are based on low-mass planets in an isothermal gas. I will show how, by relaxing this approximation, spiral wakes of massive protoplanets produce significant shock heating that can trigger buoyant instabilities. The spirals that form in disks heated by these effects are less tightly wound, better fitting the spiral structures observed in transition disks. |
| Date/Room | Jan. 13, 14:30- @ES606 |
| Speaker | YanSong Guo |
| Title | 大速度分散コンパクト分子雲の起源：大質量ブラックホールによる重力散乱過程についての理論的研究 |
| Abstract | 銀河系中心の分子雲領域 (CMZ) では星間物質が一般に高密度であり広い速度幅を有している。その中でさらに広い速度幅とコンパクトな空間構造を持っている「高速度コンパクト雲 (HVCC)」と呼ばれる雲が多数発見されている。最近、慶応大学・岡朋治らの観測グループにより、Sgr C 領域において爆発起源に特徴的な膨張シェル構造を持たない HVCC が観測された。その HVCC は非常に広い速度幅 (Delta v>100km/s) を持ち、かつ低温 (~10K) であることが明らかになった。このような温度が低くかつ構造も非常に乏しい HVCC の大きな速度分散の生成機構については大質量コンパクトな構造による重力散乱が考えられている。そこで、本研究では衝撃波を正確に捉えるゴドノフ SPH 法 (Inutsuka. 2002) を駆使し数値模擬観測を行うことにより、観測データを再現することを目指している。 |
| Date/Room | Jan. 14, 14:00- @ES606 |
| Speaker | Kensuke Kakiuchi [PDF] |
| Title | 磁気流体計算による銀河中心領域の高速度分散領域形成メカニズムの解明 |
| Abstract | 銀河系中心領域における星間物質のダイナミクスを解明することは、銀河の進化や運動学を理解する上で非常に重要である。中心領域内の星間物質の分布が幾つかの特徴的な構造を内包していることは観測から明らかにされているが、それぞれの構造形成のメカニズムについては未解明なものが多い。また、星間空間の平均的な磁場の強さが μG 程度であるのに対し、銀河系中心付近の磁場は mG 程度であるという複数の観測的示唆がある。近年では、観測された銀河系中心方向のガスのループ構造 (Fukui et al. 2006) が、磁気浮力に起因するパーカー不安定性によって説明が可能であるとの報告もなされている。これらの結果は、磁気活動が銀河系中心の動力学に大きな影響を与えていることを示している。我々は銀河系バルジの磁気流体シミュレーションを行い、磁気活動がガスの運動に与える影響を調べた。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が分かった (Suzuki et al. 2015)。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 | Jan. 20, 14:00- @ES606 |
| Speaker | Yutaro Sato |
| Title | 微惑星の破壊を取り入れた巨大天体衝突ステージにおける原始惑星の軌道計算コードの開発 |
| 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、これ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される過程の 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、地球型惑星が形成された。しかし、形成された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。本研究において、原始惑星同士の衝突のみを扱ってきたことが原因だと考えられる。実際の宇宙にはもっと様々なサイズの粒子が存在しているからである。そこで、原始惑星の他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、力学的摩擦によってランダム運動の運動エネルギーはどの惑星も等しくなる。さらに、運動エネルギーは質量と速度の二乗の積に比例しているので、質量の大きな原始惑星はランダム速度が小さくなる。ランダム速度を小さくするには軌道傾斜角と離心率が小さくならなければならない。このようなプロセスで、形成された地球型惑星の離心率を下げられるはずである。その一方で、N 体計算の計算量は粒子数の二乗に比例するので、多くの粒子を使って長時間の計算をすることができないことが問題視されていた。しかし、N 体計算と、統計的計算を融合した新たな惑星集積コードが開発され、その問題は解決に向かっている (Morishima 2015)。さらに、Morisima (2015) では形成後の地球型惑星の離心率の抑制に成功した。しかしながら、Morishima (2015) では、微惑星の合体しか取り扱っておらず、破壊の影響が考えられていない。地球型惑星形成後期では微惑星のランダム速度が大きいので微惑星同士の衝突は合体ではなく破壊を引き起こす。本研究では、微惑星の破壊を考慮しても、形成後の地球型惑星の離心率が抑制されるのかを明らかにしたい。そこで、本発表では微惑星の破壊を考慮した計算コードの開発に向けて行った種々のテスト計算を説明する予定である。 |
| Date/Room | Jan. 26, 14:30- @ES606 |
| Speaker | Alexandre Lazarian (Univ. of Wisconsin) |
| Title | Physics of Turbulence and Acceleration and Propagation of Cosmic Rays |
| Abstract | I shall review the activities of my group in the area of cosmic ray (CR) propagation and acceleration. The achieved advances are related to better understanding of properties of magnetic turbulence and magnetic reconnection. I shall show why super diffusion is natural in MHD settings and how it changes the properties of propagation and acceleration of CRs. I shall also show that sub diffusion is a very unlikely phenomenon in astrophysical settings. I shall show how dynamo in a precursor of shocks can increase the efficiency of CR acceleration and discuss how turbulent magnetic reconnection can accelerate particles in different environments. |
| Date/Room | Jan. 27, 14:00- @ES606 |
| Speaker | Torsten Stamer |
| Title | RHD Simulation of Brown Dwarf Formation |
| Abstract | Brown Dwarfs are substellar objects occupying the mass range between giant planets and dwarf stars. Officially, deuterium burning is used to distinguish between Brown Dwarfs and giant planets: Objects heavy enough to fuse deuterium (>~13 Jupiter masses) are classified as Brown Dwarfs, objects below that limit are called giant planets. However, this definition is being challenged by recent observations which argue for a distinction based on formation mechanisms instead. According to this definition, Brown Dwarfs, like stars, are formed from the gravitational collapse of a molecular cloud core, while giant planets are created through core-accretion in a protoplanetary disk. In the context of this controversy, I am developing a spherically-symmetric radiative hydrodynamics simulation of the protostellar collapse scenario, which I intend to apply to Brown Dwarfs: Can such objects be formed through protostellar collapse? What parameters are required for this? What are the initial properties of the objects created in this way and how do they compare to actual observations of young (sub)stellar objects? For the simulation, accurate treatment of radiation transport is essential. Since the initially optically thin gas becomes optically thick in the course of the collapse, simple approximations like radiative diffusion are unsatisfactory. In my talk, I will explain the basics of a new method of radiative transfer calculation for spherically symmetric systems. I will also present the results of test calculations which show the method to be highly accurate in optically thin, thick and intermediate systems, so that it can be used throughout the whole collapse simulation. |
| Date/Room | Mar. 22, 15:00- @ES606 |
| Speaker | Philippe Andre (CEA, Saclay, France) |
| Title | The Herschel view of star formation: Overview and prospects |
| Abstract | Herschel imaging surveys of Galactic molecular clouds have emphasized the quasi-universality of the filamentary structure of the cold interstellar medium and the role of filaments in the star formation process. I will present a summary of the Herschel main results and will discuss on-going and future follow-up work on this topic. This will include initial results obtained with the new ArTéMiS submillimeter continuum camera on the APEX telescope at 350 microns. |