| Date: | 12th Apr. |
| Speaker: | Takahiro Nagayama (Uir lab, Nagoya Univ.) |
| Title: | Recent results of IRSF, a 1.4m NIR telescope in South Africa |
| Abstract: | The InfraRed Survey Facility (IRSF) is our facility for near-infrared (NIR) observation with a 1.4m telescope and a JHKs camera SIRIUS located at South African Astronomical Observatory. IRSF has several unique capabilities, 1) measurement of linear and circular polarization, 2) line observation with multi-bandpass narrow band filters, 3) accurate photometry suitable for ex-planet transit events, as well as normal JHKs simultaneous imaging photometry. I present some remarkable results of IRSF, and am happy to discuss them with theoretical astronomers. |
| Date: | 19th Apr. |
| Speaker: | Satoko Sorahana (TA lab, Nagoya Univ.) |
| Title: | New Aspect of Brown Dwarf Atmospheres Revealed with AKARI |
| Abstract: | We present the behavior of molecular absorption bands in 2.5-5.0 µm spectra obtained by AKARI, a Japanese infrared astronomical satellite. This wavelength range is very important because it includes major molecular bands, such as H2O at 2.8 µm, CH4 at 3.3 µm, CO2 at 4.2 µm and CO at 4.6 µm. The strengths of these molecular bands provide crucial information for better understanding of the chemical and physical status of brown dwarf atmospheres. The Infrared Camera (IRC) on-board AKARI is capable of yielding low-resolution (R~120) spectra devoid of any degradation by telluric interference. Sixteen good spectra of possibly single brown dwarfs in a wide range of spectral types from L to T were obtained by AKARI. We investigate the appearance of CH4, CO2, and CO absorption bands in the brown dwarf spectra relative to their spectral types. We find that the CH4 band appears in the spectra of dwarfs later than L5 and the CO band is seen in the spectra of all spectral types. CO2 is detected in the spectra of late-L and T type dwarfs. We attempt to interpret these results with the Unified Cloudy Model. We suggest a possible variation in the C and O elemental abundances with respect to the solar values based on the analysis of the CO2 band. We derive the radii of the 16 AKARI objects, and for the first time observationally assess a theoretical prediction of how radius changes as a function of effective temperature. |
| Date: | 26th Apr. |
| Speaker: | Kenji Toma (Osaka Univ.) |
| Title: | Efficient Acceleration of Relativistic Magnetohydrodynamic Jets |
| Abstract: | Relativistic jets in active galactic nuclei, galactic microquasars, and gamma-ray bursts are widely considered to be magnetohydrodynamically driven by black hole accretion systems, although conversion mechanism from Poynting into particle kinetic energy flux is still open. Recent detailed numerical and analytical studies of global structures of steady, axisymmetric magnetohydrodynamic (MHD) flows with specific boundary conditions have not reproduced as rapid an energy conversion as required by observations. In order to find more suitable boundary conditions, we focus on the flow along a poloidal magnetic field line just inside the external boundary, without treating transfield force balance in detail. We find some examples of the poloidal field structure and corresponding external pressure profile for an efficient and rapid energy conversion as required by observations, and that the rapid acceleration requires a rapid decrease of the external pressure above the accretion disk. We also clarify the differences between the fast magnetosonic point of the MHD flow and the sonic point of de Laval nozzle. |
| Date: | 10th May. |
| Speaker: | Yuri Fujii (TA lab, Nagoya Univ.) |
| Title: | Spectral Energy Distribution of T Tauri Stars with Passive Circumstellar Disks |
| Abstract: | Nowadays, thanks to ALMA and many other telescopes, more and more detailed observations of protoplanetary disks are available. In order to understand these results, the comparison with the theoretical calculations of radiative transfer is required. I n particular, radiative transfer determines the thermal structure of the disks which in turn controls their dynamical evolution. In this talk, I will review the passive disk model from the classical paper by Chang & Goldreich (1997) and discuss the spectral energy distribution of a T Tauri star with a disk. |
| Date: | 17th May. |
| Speaker: | Shu-ichiro Inutsuka (TA lab, Nagoya Univ.) |
| Title: | The Formation & Evolution of Protoplanetary Disks: From Massive Disks to ALMA Banana |
| Abstract: | First I review the formation of a protostar and a protoplanetary disk from a molecular cloud core. Then I briefly describe a current picture of the long-term evolution of protoplanetary disks. After I show recent observation of remarkable structures in protoplanetary disks, I will explain how those structures can be created. Future directions of theoretical research are also discussed. |
| Date: | 31st May. |
| Speaker: | Kazuya Takahashi (Yamada lab, Waseda Univ.) |
| Title: | On the existence and non-uniqueness of solutions of Riemann problems in ideal magnetohydrodynamics |
| Abstract: | Ideal Magnetohydrodynamics (MHD) is frequently used as a means to study the astrophysical phenomena in magnetic fields. However, there still remain unresolved issues on the existence and non-uniqueness of the solutions of ideal MHD, which are highly associated with the so-called intermediate shocks, across which the transverse magnetic field is reversed by exactly 180 degree. Although the intermediate shocks are commonly observed as stable waves in numerical simulations, they are concluded to be non-evolutionary in the theory of ideal MHD. Therefore there have been many arguments on the reason why the intermediate-shock-solutions are realized in simulations instead of the solutions without intermediate shocks. Recently, we have developed an exact Riemann solver that can find all the solutions for a given initial condition and investigated the existence and non-uniqueness of the solutions that contain intermediate shocks. With the new solver, we found uncountably many solutions that include an intermediate shock for the Brio & Wu problem and its neighborhood. Furthermore, we have discovered an initial condition that does not have any solution without intermediate shocks. The former result proposed a new problem: why other solutions are not realized in numerical simulations? This is an inevitable question for discussing the stability of the shocks in MHD. The second finding is the first counter-example for the classical conjecture that there is always a unique solution that includes only evolutionary waves in ideal MHD Riemann problems. We will discuss the above problems and our recent results. |
| Date: | 14th June. |
| Speaker: | Minako Noguchi (TA lab.) |
| Title: | フィラメント状分子雲におけるコア質量関数 |
| Abstract: | 天体の質量分布を表す質量関数(Mass Function)は、天体の形成機構を知る上で大きな手がかりとなり得る。分子雲の質量関数(dN/dM)がM^-1.4~-2.0に比例しているのに対し、星の初期質量関数(IMF)はM^-2.5~-2.7に比例している。また、観測によって、分子雲コアの質量関数とIMFが似ているということが報告されている。これより、分子雲中で分子雲コアが形成される機構が星の質量分布を支配していることが示され、分子雲コアの質量関数を理論的に求める事が重要となる。また、最近の観測より、星はフィラメント構造を持つ分子雲の中で形成されることがわかってきた。Inutsuka(2001)は、フィラメント状分子雲の重力的分裂によって形成される分子雲コアの質量関数をPress-Schechter理論を用いて求め、観測結果と良く一致する質量関数であることを示した。しかし、実際の分子雲は動的に収縮しているため、これを踏まえた現実的なコア質量関数が必要となる。本研究では、詳細な加熱冷却過程を考慮した1zoneモデルを用いて、フィラメント状分子雲の収縮過程を広いパラメータ範囲（金属量、初期温度、線密度）で調べた。今回の発表では、本研究をコア質量関数に応用するために基本となるInutsuka(2001)を紹介し、静水圧平衡状態のフィラメント状分子雲のコア質量関数を求める。 |
| Date: | 28 June. |
| Speaker: | Yasumasa Teranishi (TA lab.) |
| Title: | 活動的な太陽型星風における降着相の出現 |
| Abstract: | 恒星風は大きく分けて、大質量星や漸近赤色巨星など光度の大きい星の輻射圧駆動型恒星風と、 中小質量星の磁気流体波動、乱流などを介した擾乱駆動型恒星風の2種類のものがある。太陽程度 の質量の恒星の恒星風駆動機構とされる後者は、表面対流層の乱流の持つエネルギーが磁気流体 過程など何らかの方法で外層に輸送され恒星風が駆動されると考えられているが、複雑な非線形過 程が絡むため、解析的に解くことは困難で数値シミュレーションによるアプローチが必要になる。 活動的な太陽型星風の1次元MHDシミュレーションの中で、表面からエネルギーを注入し続けている にもかかわらず、星風が降着に転じるという現象がいくつか見られた。そのうち1つを解析した結果、 この現象の原因は、磁気圧の大きな変化もしくは熱不安定によって生じる、密度揺らぎによるAlfven 波の反射であることが分かった。反射され恒星表面に向かうAlfven波のエネルギーが恒星風の運動 エネルギーに輸送され、恒星風の降着が引き起こされていた。この星風の降着相の出現は、恒星風 による質量放出率に影響を与えることが考えられ、星風が降着すること自体が質量放出率を減らす という効果の他にも様々な影響を与える可能性がある。 この結果を受けて今後は、活動度が高く、降着相を伴う星風が多く存在すると推測される、M型星 の数値シミュレーションに取り組むことを考えている。 |
| Date: | 5 July. |
| Speaker: | Takeru Suzuki (TA lab.) |
| Title: | MHD Simulations of Global Accretion Disks with Vertical Magnetic Fields |
| Abstract: | We report results of three dimensional mangetohydrodynamical (MHD) simulations of global accretion disks threaded by weak vertical magnetic fields. We particularly focus on effects of different temperature profiles on MHD turbulence in the disks. In the cases with a spatially constant temperature profile, giving a constant rotation frequency along with the vertical direction, general properties of the turbulence excited by magnetorotational instability (MRI) are quantitatively similar to those observed in local shearing box simulations. On the other hand, if the temperature depends on the radial location, the equilibrium rotation profile inevitably shows the differential rotation in the vertical direction in addition to the usual radial direction. The winding of the magnetic field lines by the vertical differential rotation affects properties of the amplified magnetic fields: (i) the coherent wound magnetic fields contribute to the Maxwell stress even though the numerical resolution is insufficient and small-scale MRI turbulence is not captured. (ii)the saturated magnetic energy does not depend on the numerical resolution, while the Maxwell stress shows a positive dependence on the resolution, similarly to that in local simulations. Our global simulations give somewhat larger density fluctuation, δρ/ρ = 0.1−0.2, near the midplane than the values obtained in previous local shearing box simulations and global simulations without net vertical magnetic field. The velocity fluctuations, dominated by the radial component, are also ≈ 0.1 − 0.2 of the local sound speed. The azimuthal power spectra of the magnetic fields show shallow slopes, m0 − m−1, where m is an azimuthal mode number, because the MRI injects the energy from small scales. On the other, the power spectra of the velocities and density show steeper slopes, m−1 −m−2. We observe the disk winds that are driven by the Poynting flux associated with the MHD turbulence, with the slightly smaller mass fluxes than that obtained in our local simulations. The Poynting flux originating from magnetic tension is injected from the regions above a scale height towards both the midplane and the surfaces. Related to these injection regions of the magnetic tension, acoustic-like waves are directed to the midplane from the surface regions. The mass accretion mainly occurs near the surfaces and the gas slowly moves outward near the midplane. The difference of the velocities at the midplane and the surfaces might cause large-scale meridional circulations. Applying to protoplanetary disks, these waves and circulation possibly play an essential role in the dynamics of solid particles. |
| Date: | 23 July. |
| Speaker: | Tetsuya Kudoh (TA lab.) |
| Title: | ハイブリッドシナリオにおける惑星の軌道進化 |
| Abstract: | 惑星形成において、円盤について生い立ちを含めて中心星形成の段 階から考え、核集積と重力不安定の２つを組み合わせたハイブリッドシナリオは、 重力不安定でできたガス惑星が中心星近くと円盤途中に残っており、 その後は核集積シナリオのように惑星が形成されるというシナリオである。 そこでは形成される地球型惑星は円盤との相互作用で中心星に向けて落ちていき、 中心星に近い内側ガス惑星と相互作用し、自身の落下の代わりにガス惑星を中心星に落とす。 しかしここでは、現在の太陽系と比較すると残された地球型惑星の軌道位置が中心星に近いという問題が残る。 この問題に向けて、円盤の進化とともに地球型惑星の軌道位置が伴って変化すること、 複数の惑星の軌道を同時に見ていく。 そのため、その準備段階として円盤の構造について計算する。 |
| Date: | 23 July. |
| Speaker: | Kouki Kanou (TA lab.) |
| Title: | 惑星形成における天体の衝突・破壊の数値シミュレーションについて |
| Abstract: | 現在の理論では、原始惑星系円盤内において微惑星同士が衝突・合 体することにより原始惑星が形成され、原始惑星が10倍の地球質量程度まで成長すると、周囲のガスを集積させることによって木星型惑星(巨大ガス惑星)が形成されると考えられている。ガス集積を起こせるような大きな原始惑星ができる過程では、天体の衝突は合体だけでなく破壊ももたらし、天体の破壊強度によって原始惑星の成長できる大きさの限界が決められる。しかし、破壊を記述するモデルは未完成である。惑星形成に適用するための破壊モデルを完成させるため個別要素法を用いて数値実験を行う。今回は、個別要素法について説明する。 |
| Date: | 26 July. |
| Speaker: | Kouichi Hirotani (TIARA, Taiwan) |
| Title: | High-energy emission from rotation-powered pulsars |
| Abstract: | Recent years have seen a significant progress in our understanding of the high-energy emission from pulsar magnetospheres. The Fermi Gamma-ray Space Telescope, launched 5 years ago, increased the number of detected pulsars from 6 to more than 120, in the GeV energy range. The VERITAS and MAGIC experiments have detected pulsed signals from the Crab pulsar up to 400 GeV. The light curves and the spectra obtained from the Crab and other rotation-powered pulsars, suggest that the gamma-ray pulsars have high-altitude emission zones, which avoid super-exponential cutoff due to magnetic pair production. I thus examine the outer-magnetospheric accelerator model, solving the plasma motion and emission self-consistently with the radiation and electro-magnetic fields. I demonstrate that their gamma-ray luminosity is naturally proportional to the square root of the spin-down luminosity, which has been a long-standing issue in high-energy pulsar physics. It is also revealed that the Crab pulsar’s very-high-energy pulsed emission is due to the synchrotron-self-Compton scatterings taking place in the higher altitudes close to the so-called ‘light cylinder’. |
| Date: | 4 Oct. 14:00- |
| Speaker: | Kazunarai Iwasaki |
| Title: | physical properties of self-gravitating filaments |
| Abstract: | Recent observations revealed stars form in filamentary molecular clouds. I will talk about equilibrium states and gravitational collapse processes of filaments with and without magnetic fields. |
| Date: | 9 Oct. 16:00- |
| Speaker: | Tetsu Hiroyuki (Nakamoto Lab, Tokyo Tech) |
| Title: | フィラメント状分子雲の重力収縮とその分裂 |
| Abstract: | 星形成の現場である分子雲は、しばしばフィラ メント様の構造を持つ。この構造に沿って星なしコアや原始星が埋まっているという観測(e.g. Andre et al. 2010)からも示唆されるように、フィラメント構造内での星形成は、星形成の主要なモードのひとつとして有力である。 本研究では、フィラメント構造における星形成という大きな括りのなかで、【フィラメントの収縮から静水圧平衡ガス塊の形成に至るまで】に注 目している。このプロセスは大きく、1) 等温のまま細く重力収縮していく段階と、2) 等温が崩れることで分裂モードが顕在化し、静水圧平衡の分裂片が形成される段階と、に分けられる。これら２つの段階は、それぞれ別個には調べられてきた （e.g., Masunaga & Inutsuka 1999; Inutsuka & Miyama 1992, 1997）。しかし 1) から 2) への転換、つまり「いつ」分裂に至り「どのような質量の」分裂片が形成されるのかについては、詳しく調べられてはいない。そこで本研究ではZEUS- 2D（Stone et al. 1992）をカスタマイズした２次元輻射流体計算を実行し、これを調べた。 その結果、フィラメントがいつ分裂し、どのような質量の分裂片を作るのかを、さまざまな初期条件ごとに得ることができた。また、フィラメン ト分裂を経ていずれ形成されるはずの恒星や矮星の質量に、下限値を設けることができた。 |
| Date: | 11 Oct. 16:00- |
| Speaker: | Hiroshi Kobayashi |
| Title: | Small Planetesimals in a Massive Disk Formed Mars |
| Abstract: | Mars is likely to be a planetary embryo formed through collisions with planetesimals, which can explain its small mass and rapid formation timescale obtained from 182Hf-182W chronometry. In the classical theory of planet formation, the final embryo mass is determined only by the solid surface density. However, embryos can stir surrounding planetesimals, leading to destructive collisions and fragmentation. Radial drift of small fragments reduces the solid surface density. On the other hand, embryo growth is accelerated by fragment accretion. Since collisional fragmentation efficiency depends on the initial size of planetesimals, the final embryo mass and its growth time are determined by the initial planetesimal size and disk surface density. We have investigated the effect of these two parameters on the mass of Mars and the predicted radiogenic excess of 182W in the martian mantle. We conclude that Mars is likely to have formed in a massive disk of about 0.1 solar mass from planetesimals smaller than 10 km in radius. Such small planetesimal size cannot explain core accretion of Jupiter, suggesting that there may have been a heliocentric gradient in planetesimal size in the solar nebula. |
| Date: | 1 Nov. 13:30- |
| Speaker: | Takuma Matsumoto |
| Title: | Cooling in the Solar Chromosphere |
| Abstract: | Since the optical depth is slightly less than unity, radiative cooling processes in the solar chromosphere are complicated. In this seminar, I will introduce treatments of the cooling processes in the classical solar atmospheric model and the recent radiative MHD simulations. |
| Date: | 8 Nov. 15:30- |
| Speaker: | Ken-ichi Nishikawa (Huntsville, AL) |
| Title: | Radiation from accelerated particles in relativistic jets with shocks, shear-flow, and reconnection |
| Abstract: | We investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma. Strong magnetic fields generated in the trailing shock contribute to the electron’s transverse deflection and acceleration. Kinetic Kelvin-Helmholtz instability (KKHI) is also responsible to create strong DC magnetic fields. The velocity shears in core-sheath jets create strong magnetic field perpendicular to the jet. We examine how the Lorentz factors of jets affect the growth rates of KKHI. We have calculated, self-consistently, the radiation from electrons accelerated in these turbulent magnetic fields in the shocks. We found that the synthetic spectra depend on the bulk Lorentz factor of the jet, its temperature and strength of the generated magnetic fields. We will investigate synthetic spectra from accelerated electrons in strong magnetic fields generated by KKHI. The calculated properties of the emerging radiation provide our understanding of the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. |
| Date: | 15 Nov. 14:00- |
| Speaker: | Sanemichi "Zanetti" Takahashi |
| Title: | On Ring Structure Formation in Protoplanetary Disks by Two Fluid Secular Gravitational Instability |
| Abstract: | Recent high-angular-resolution direct imaging techniques have found remarkable structures in protoplanetary disks, such as rings and spirals. These structures may provide clues to understand an evolution of protoplanetary disks and formation processes of planets. Therefore, it is important to investigate the mechanism to form those structures. One candidate of the formation mechanism for rings is secular gravitational instability (SGI). The SGI is the gravitational instability of dust due to the friction between gas and dust. We have performed the liner stability analysis of axisymmetric SGI taking into account back reactions of drag force on gas. The liner analysis has suggested that the ring structure can be formed in 100,000 years by SGI and its width is about few AU. The SGI may play an important role in core formation of gas giants located at radius ∼100 AU or origin of debris disks if the instability can accumulate the dust. To compere the resultant structure of the instability with the observation, we have to investigate non-axisymmetric modes by linear analysis and non-linear effect of the instability by numerical simulation. |
| Date: | 21 Nov. 14:00- |
| Speaker: | Shigeo Kimura (Osaka University) |
| Title: | The Effect of High-Energy Particles on Accretion Flows |
| Abstract: |
| Date: | 6 Dec. 13:00- |
| Speaker: | Masahiro Ogihara |
| Title: | Our recent studies of planet/satellite formation |
| Abstract: | I will introduce several topics that I have recently studied since I came to Nagoya, which include (1) crowding-out of giants by dwarfs, (2) new model for formation of terrestrial planets in the Solar system, and (3) compositional evolution of the Galilean satellites. |
| Date: | 24 Dec. 15:00- |
| Speaker: | Yukihiko Hasegawa |
| Title: | 原始惑星系円盤におけるダストの成長と沈殿およびshear不安定の可能性 |
| Abstract: | 原始惑星系円盤内では、円盤の赤道面へのダストの沈殿によって Kelvin-Helmholtz不安定 (KHI) が起こり、ダスト層の重力 不安定 (GI) の発生が妨げられる。この円盤でのKHIに関する 先行研究では、ダストの成長は考慮されていなかった。 ダストは成長することによって沈殿速度が変化するため、 ダストの成長はダストの沈殿に影響する。そこで、我々は ダストの成長がKHI発生の可能性に与える影響について着目した。 本研究では、原始惑星系円盤でのKHIに関して、先行研究では 考慮されていなかったダストの成長を考慮した場合における KHI発生の可能性を計算した。計算の際には、ダストが 成長しつつ沈殿する様子を反映したダスト密度分布を用いた。 その結果、ダスト成長を考慮するとダスト量が大きくても KHIがGIよりも前に起こる可能性があることが示唆された。 この結果は、ダスト成長を考慮しない場合には円盤の ダスト量が大きくなるとGIがKHIよりも前に起こる可能性が あることとは異なる結果となった。この違いの物理的な原因は、 ダストが受けるガス抵抗がダスト成長によってEpstein則から Stokes則に変化することであることが示唆された。 |
| Date: | 17 Jan. 14:00- |
| Speaker: | Yusuke Tsukamoto |
| title: | structure of the self-gravitating disk and its fragmentation |
| abstract: | In this colloquium, I will talk about recent my study about the structure and the fragmentation of the circumstellar disk and the evolution of the fragments with 3D radiation hydrodynamics simulations. At first, I derive the power law of the physical quantities using steady viscous accretion disk model with several energy balance equations. Then, I apply it for the simulation result and show the simulation result is well described by the steady model. Next, I will investigate the evolution of the fragments and show that the fragments is resemble to the first-core and its central entropy can not become smaller than that of the first core expected from the 1D spherical calculation even though the fragments have much chance to reduce the central entropy by radiation cooling. Finally, I will compare our results and previous works and show that the disk fragmentation is only possible by strong mass accretion from the envelope and the fragmentation by the radiation cooling is impossible around the low mass star. |
| Date: | 30 Jan. 14:00- |
| Speaker: | Hiroyuki Kurokawa |
| title: | Composition and Origin of Super-Earths |
| abstract: | Recent observations have revealed a new class of exoplanets, called "Super-Earths" which have ~1-20 Earth masses. Super-Earths are quite common as they are orbiting about 30% of solar-type stars (Howard et al., 2012), whereas their compositions and origins are poorly constrained. At first, I review recent observations to characterize these Super-Earths and to understand their origins. Next I show the results of my numerical calculation of their evolution to investigate how atmospheric escape affects their compositions. I focus on the dependence on host-stellar types to distinguish the effects of formation processes and later evolution on their compositions. Finally I provide some constraints on their compositions and origins by comparing my results with observations. |
| Date: | 18 Feb. 14:00- |
| Speaker: | Yuki Tanaka |
| title: | Albedo and Reflection Spectra of Extrasolar Giant Planets |
| abstract: | Characteristics of exoplanet atmosphere can be understood by spectra of exoplanet. From transmission spectra and reflection spectra, we can get information on exoplanet atmospheres for example atmospheric structures, atmospheric compositions, and albedo. However, albedo and spectra of planets depend on many processes such as chemical composition, condensation of species, radiative transfer, scattering, and so on. In this talk, I will review the paper on the calculation of albedo and reflection spectra of giant planets by Sudarsky, Burrows and Pinto (2000). They classified extrasolar giant planets into five classes depending on the effective temperatures, and calculated albedo and refraction spectra. In consequence, albedos and reflection spectra of extrasolar giant planets strongly depend on the presence of clouds and its height. They also refer to the effects of non-equilibrium species, condensation fraction of condensates, and particle size distribution of cloud and haze. |
| Date: | 25 Feb. 14:00- |
| Speaker: | Andrej M. Sobolev (Ural Federal University) |
| title: | Lecture on Masers and Turbulence |
| abstract: | I will briefly describe observational data on the images and spectra for masers of different types, make a brief review of using masers as a tools to study characteristics of interstellar turbulence, consider how the maser images and spectra are formed in the turbulent medium, compare results of the modelling with the observational data, and briefly consider how the turbulence can affect accuracy of the parallax measurements. The lecture will be based on my papers with W.D. Watson (1998ApJ...498..763S and 2003ApJ...590..333S), some unpublished results, and review of results by the other observers and theorists. |
| Date: | 26 Feb. 14:00- |
| Speaker: | Yuto Teraki (OU-TAP) |
| title: | RADIATION FROM RELATIVISTIC ELECTRONS IN A TURBULENT ELECTROMAGNETIC FIELD |
| abstract: | We present the results of the first principle numerical calculations for the radiation spectrum from relativistic electrons moving in a turbulent electromagnetic field. We investigate the cases for which we cannot use anyIapproximated radiation formula. We first show the radiation spectra from electrons moving in a static and small scale magnetic turbulence. The radiation is characterized by the strength parameter a = ω st / ktypc =e σ/ mc2ktyp, where ωst is the cyclotron frequency defined by the spatial averaged magneticfield strength =< B2 >1/2, and k-1 is the typical scale of the magnetic turbulence. We investigate the radiation spectra for previously unexplored parameter range of 1 < a < γ where γ is the Loerentz factor of the radiating electron. The obtained spectrum shows a novel shape which has not been seen before. We secondly show the radiation from electrons in a Langmuir (electrostatic) turbulence. Since the Langmuir waves oscillate rapidly with plasma frequency ωp, we take it into account. We perform calculations of radiation spectra for various values of "a" defined by using σ=< E2 >1/2 and of oscillation parameter b = ωp/ktypc, and we obtain a chart of spectral signatures in a-b plane. We lastly discuss the applications to high energy astrophysical objects. |
| Date: | 28 Feb. 11:00- |
| Speaker: | Soonyoung Roh |
| title: | [Review] High energy cosmic-ray diffusion in Molecular clouds |
| abstract: | I would like to review the paper of M.Fatuzzo et al. 2010. The propagation of high-energy cosmic rays (CRs) through giant molecular clouds constitutes a fundamental process in astronomy and astrophysics. The diffusion of CRs through these magnetically turbulent environments is often studied through the use of energy-dependent diffusion coefficients, although these are not always well motivated theoretically. Now, however, it is feasible to perform detailed numerical simulations of the diffusion process computationally. While the general problem depends upon both the field structure and particle energy, the analysis may be greatly simplified by dimensionless analysis. That is, for a specified purely turbulent field, the analysis depends almost exclusively on a parameter singlethe ratio of the maximum wavelength of the turbulent field cells to the particle gyration radius. For turbulent magnetic fluctuations superimposed over an underlying uniform magnetic field, particle diffusion depends on a second dimensionless parameter that characterizes the ratio of the turbulent to uniform magnetic field energy densities. We consider both of these possibilities and parametrize our results to provide simple quantitative expressions that suitably characterize the diffusion process within molecular cloud environments. Doing so, we find that the simple scaling laws often invoked by the high-energy astrophysics community to model CR diffusion through such regions appear to be fairly robust for the case of a uniform magnetic field with a strong turbulent component, but are only valid up to |
| Date: | 4 Mar. 14:00- |
| Speaker: | Wladimir Lyra (Caltech) |
| title: | Gas in debris disks: a new way to produce patterns? |
| abstract: | "Debris disks" around young stars (analogues of the Kuiper Belt in our Solar System) show a variety of non-trivial structures attributed to planetary perturbations and used to constrain the properties of the planets. However, these analyses have largely ignored the fact that some debris disks are found to contain small quantities of gas, a component that all such disks should contain at some level. Several debris disks have been measured with a dust-to-gas ratio around unity at which the effect of hydrodynamics on the structure of the disk cannot be ignored. In this talk I will present linear and nonlinear modelling that shows that dust-gas interactions can produce some of the key patterns attributed to planets. We find a robust photoelectric instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk. The conclusion that such disks contain planets is not necessarily required to explain these systems. |
| Date: | 7 Mar. 14:00- |
| Speaker: | Akihiro Tsutsumi |
| title: | Nonlinear evolution of Alfven wave in density cavity structure |
| abstract: | Wave transport in plasma (e.g. Alfven wave) is a universal phenomenon for astrophysical fluid which is effected by electromagnetic force. Presence of density structure in plasma causes these wave reflection , and prevents smooth transport for one direction. However, it is known that if the density structure is like square cavity form, Alfven wave is trapped in the structure and wave reflection does not occur. It seems that this wave trapping is an ordinary case and concerns with physical phenomenon, because density valley usual exist in plasma. For example, it is pointed out that Alfven wave energy is dissipated at low-density area which is located in surface of the sun, and this mechanism is relate to coronal heating. We can understand Alfven wave transport analytically if it is linear phase. Therefore , we can get no reflection condition (resonance condition) by linear analysis. However, the situation shifts nonlinear phase as wave transport is continue. In this phase, density can not remain its cavity structure and resonance condition may change volantarily. I carry out numerical simulation focus on the shift linear phase to nonlinear phase. As s result , linear phase resonance condition and density structure may affect nonlinear evolution. I introduce the result. |
| Date: | 14 Mar. 14:00- |
| Speaker: | Satoko Sorahana |
| title: | A Signature of Chromospheric Activity in Brown Dwarfs Revealed by 2.5-5.0 um AKARI Spectra |
| abstract: | Brown dwarfs play an important role as a bridge between stars and planets. The physical and chemical structures of brown dwarf atmospheres are complicated and cannot be understood with a simple extension of stellar atmospheres. No previous brown dwarf atmosphere models have considered chromospheric activity. However, in our current study, we find a signature of chromospheric activity in brown dwarfs in near-infrared spectra between 2.5 and 5.0 um. Furthermore, we find that the observations of three L dwarfs can be matched by our new brown dwarf atmosphere model that includes chromospheric activity. Our result suggests that chromospheric activity is essential to understand the near-infrared spectra of L dwarfs. |
| Date: | 28 Mar. 14:00- |
| Speaker: | Kohei Inayoshi (Kyoto University) |
| title: | 巨大ブラックホール形成過程(仮題) |
| abstract: |