| Date: | 21th January 14:00- |
| Speaker: | Yuki Tanaka |
| Title: | A new treatment of radiative cooling for MHD calculation of planetary atmosphere |
| Abstract: | Recently a great number of exoplanets have been found, and some of them are gaseous planets that have extremely small semi-major axis. These planets are so-called hot Jupiters, and numerous interesting physical properties and phenomena have been observed. Atmospheric escape is one of the phenomena of the hot Jupiters. We investigated effects of magnetic activities in the atmosphere of the hot Jupiters by using magnetohydrodynamic (MHD) simulations in a previous work, and found MHD waves have a lot of influence on the structure of the atmosphere, and they can drive a large amount of mass loss from the upper atmosphere (Tanaka et al. 2014). However, the MHD code we used was developed for the simulation of solar wind, and a treatment of radiative cooling is too simplified, especially for low temperature such as ~1000 K. In order to simulate more accurately, we improve the cooling method in the atmosphere. We apply a radiative cooling/heating method that used for calculations of envelope of AGB stars. We introduce the radiative cooling/heating method that used for AGB star, then show new results of the MHD simulation with new radiative cooling/heating method. |
| April | 4 | Break |
| 11 | Shu-ichiro Inutsuka | |
| 18 | Jennifer M. Stone | |
| 25 | Break (新学術領域研究会) | |
| May | 2 | Break (JpGU) |
| 9 | Torsten Stamer, Tomoya Miyake | |
| 16 | Hiroshi Kobayashi | |
| 23 | Break? (教授会 faculty meeting 14:30-) | |
| 30 | Break (OSSF2014) | |
| June | 6 | Kazunari Iwasaki |
| 13 | Break (憲章記念講演会 Charter Commemoration Symposium) | |
| 20 | Jun Nishizawa (教授会 faculty meeting 14:45-) | |
| 27 | Break (ASTRONUM 2014) | |
| July | 4 | Soonyoung Roh |
| 11 | Yuri Fujii | |
| 18 | Kei Koborinai (教授会 faculty meeting 14:45-) | |
| 25 | Ryota Okamoto | |
| Oct | 1 | Break (名古屋大学南半球研究センター研究会「惑星から大質量星の形成までを俯瞰する」) |
| 8 | Break (SPICA meeting) | |
| 15 | Hiroyuki Kurokawa | |
| 22 | Akihiro Tsutsumi | |
| 29, 30 | Interim Reports 中間発表会 | 31 | Masanobu Kunitomo (Tokyo-Tech), 16:00- |
| Nov | 5 | Break (The 2014 German-Japanese Exoplanet Conference) |
| 12 | Break (Star Formation Across Space and Time) | |
| 19 | Break (他大学で集中講義により犬塚さん出張) | |
| 26 | Break (Special Lecture by Dr. Yuri Aikawa) | |
| Dec | 3 | Takeru Suzuki |
| 10 | Interim Reports 中間発表会(4年生) | |
| 18 (Thursday) | Yusuke Tsukamoto | |
| 24 | Break (Rironkon Symposium) | |
| 31 | Break (New year's Eve) | |
| Jan | 7 | Yuri Fujii |
| 14 | 10:00- Soonyoung Roh | |
| 14 | 17:00- Kohei Inayoshi (Columbia University) | |
| 21 | Yuki Tanaka | |
| 28 | M2 students | |
| Feb | 4 | M2 students |
| 11 | Break (National Founding Day 建国記念の日) | |
| 18 | ||
| 25 | Break (Special Lecture by Dr. Masahiro Ikoma) | |
| Mar | 4 | |
| 11 | ||
| 18 | Break (天文学会 Annual Meeting of The Astronomical Society of Japan) | |
| 25 |
| Date: | 11th April 14:00- |
| Speaker: | Shu-ichiro Inutsuka |
| Title: | A Global Picture of Star Formation |
| Abstract: | I review the recent observations of the formation of stars with various masses, which apparently show very different environments. The general picture of star formation requires understanding of the formation of molecular clouds. Our recent efforts for understanding the cloud formation have shown that molecular cloud formation requires multiple external compressions on the long timescale on the order of 10Myr. I will discuss how this theoretical result leads to the general picture of star formation in the Galaxy, which describes star formation rates and the mass functions of clouds and stars. |
| Date: | 18th April 14:00- |
| Speaker: | Jennifer Stone |
| Title: | Stability of Shock Waves |
| Abstract: | I will review the corrugation instability of shock waves, in which a surface of discontinuity is unstable to periodic perturbations of its surface, and present a linear stability analysis for the hydrodynamic case. I will also briefly discuss extensions to MHD and some astrophysical applications. |
| Date: | 9th May 14:00- |
| Speaker1: | Torsten Stamer |
| Speaker2: | Tomoya Miyake |
| Title: | QSOの輻射力による種磁場の生成 |
| Abstract: | QSOからでる輻射によって生成される種磁場の原理を説明し、 それを２次元でモデル化して、そのモデルで生成した磁場の大きさ、 コヒーレンスレングスについて発表する。 |
| Date: | 16th May 14:00- |
| Speaker: | Hiroshi Kobayashi |
| Title: | Analytic formula for oligarchic growth |
| Abstract: | Planets are formed in a protoplanetary disk; dust accumulates into planetesimals, protoplanets are produced through the runaway growth of planetesimals, and planets are formed via the accretion of surrounding planetesimals in oligarchic growth. To discuss the masses and orbits of exoplanets, we need to understand the oligarchic growth of protoplanets. I will revisit this issue to introduce the theory of oligarchic growth. |
| Date: | 23th May 14:00- |
| Speaker: | Hiroshi Kobayashi |
| Title: | Method for numerically solving coagulation equation |
| Abstract: | I will introduce the method for numerical integration of coagulation equation in a wide mass range. I think that it will be a bit geeky. I recommend it only for guys who are strongly interested. |
| Date: | 6th June 14:00- |
| Speaker: | Kazunari Iwasaki |
| Title: | Gravitational Fragmentation of Shock Interstellar Gas Layers |
| Abstract: | IRecent observations reveal that stars form in filamentary molecular clouds. One of the promising mechanisms is shock compression of ISM. In our galaxy, shock compression is frequently induced by supernovae, expansions of HII regions, and cloud-cloud collisions, and so on. Shock compression forms sheet-like clouds that fragment into filamentary clouds by gravitational instability. In this talk, initially, I review gravitational fragmentation in shocked ISM for the case of non-magnetized medium. Finally, I show preliminary results for the case of magnetized medium. |
| Date: | 20th June 13:30- |
| Speaker: | Jun Nishizawa |
| Title: | 中小質量星の恒星風の運動エネルギー |
| Abstract: | 恒星風駆動には磁気流体力学の一種であるAlfven波が重要な役割を果たしていると考えられてい る。 光球表面での磁場や摂動速度の大きさを変えることにより注入するAlfven波のPoyntingフラックスを変化させ、 恒星風の運動エネルギーがどのように変化するか、磁気流体シミュレーションを用いて調べ た。 太陽型星と、それより小質量の恒星について調べた結果を発表する。 |
| Date: | 4th July 14:00- |
| Speaker: | Soonyoung Roh |
| Title: | Toward Understanding Particle Acceleration in Turbulent Interstellar Medium : Shear Field Acceleration |
| Abstract: | Supernova remnants (SNRs) are one of the most powerful astrophysical event and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report by Funk et al. (2013) has shown an unequivocal signature of pion-decay in the gamma-ray spectra of SNRs. This provides strong evidence that high energy protons are accelerated in SNRs. On the other hand, Fukui et al. (2012) showed that pion decay from protons dominates in emission from SNR RX J1713 based on the spatial correlation of gamma-rays and molecular line emission. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in a multiphase interstellar medium with molecular clouds (Inoue et al. 2012). In order to quantitatively describe the diffusion of high energy CRs from acceleration sites, we have performed test particle numerical simulations using a three-dimensional magnetohydrodynamics (MHD) simulation data provided by Inoue et al. (2012). I will introduce the shear field acceleration which is one of possible accelerations in the SNRs. |
| Date: | 11th July 14:00- |
| Speaker: | Yuri Fujii |
| Title: | Towards an understanding of the effect of MHD turbulence in circumplanetary disks |
| Abstract: | Gaseous disks are formed around gas giants during the formation phases of those planets. These disks are called circumplanetary disks. An understanding of them is crucial for understanding satellite formation, because satellites are thought to form in circumplanetary disks. In protoplanetary disks, gas can accrete onto the central star due to the turbulence caused by the magnetorotational inst ability (MRI). So, it has been assumed the MRI operates similarly in circumplanetary disks. In order to sustain the MRI, disks must satisfy two conditions. The first condition is that the Elsasser n umber must be larger than unity, and the second condition is that the magnetic pressure must be sufficiently smaller than the gas pressure. We have studied the viability of the MRI in circumplanetar y disks, and concluded that it cannot be sustained in circumplanetary disks. However, this conclusion strongly depends on the second condition. To confirm our findings, we conduct numerical simulati ons of the MRI in circumplanetary disks. Our ultimate goal is to calculate the dynamics including a consistent treatment of magnetic diffusivity, but in this talk we will only show the results of id eal-MHD simulations. |
| Date: | 18th July 13:30- |
| Speaker: | 小堀内 啓 |
| Title: | シアー流中の粒子加速 |
| Abstract: | 原始惑星系円盤のようなシアー流中において荷電粒子が加速する解が見つかっており、今回はそれについて発表する。 この現象は十分磁場が弱い場合にのみ起こり、例えばケプラー円盤中で加速されるには陽子の場合は10^-11G、0.1μmのダストの場合は 10^-1G以下でなければならない。 また、加速で陽子が1eVのエネルギーを得るには1AUほどの距離が必要である。 |
| Date: | 25th July 14:00- |
| Speaker: | 岡本 良太 |
| Title: | 銀河の渦状構造理論 |
| Abstract: | Schinnere et al. 2013による詳細な観測で、渦巻銀河であるM51銀河においてHⅡ領域のarmから伸びるspurの構造が発見された。 この構造は銀河の中心から 3-4kpcほどまで見られ、全てinterarmの下流の方向にのみ伸びている。 若い星団がarm上よりもspurに沿って存在することも指摘されてお り、spur構造の理解は星形を考える上でも重要である。 この構造の理解のためには、armの渦状構造の理解が重要であるので、渦状構造を説明する密度波理論について発表する。 |
| Date: | 15th October 14:00- |
| Speaker: | Hiroyuki Kurokawa |
| Title: | Interior Structure and Thermal Evolution of Giant Planets: Effects of Double-Diffusive Convection |
| Abstract: | Standard model of giant planets assumes that the interior of giant planets consists of a few, homogeneously mixed layers (e.g. rocky/icy core and H/He gaseous envelope) and that the interior has adiabatic temperature gradient induced by overturning convection. However, luminosity anonalies of solar-system giant planets and radius anomalies of extrasolar gaseous planets imply that the standard model of giant planets is not enough to describe their interior and evolution. Recently, layered convection separated by numerous interfaces has been proposed to explain these anomalies, instead of overturning convection. Such layered convection can be induced by compositional inhomogeneity as a result of double-diffusive instability. Inefficient heat transport of layered convection makes interior super-adiabat and can delay thermal evolution of giant planets. We developed an interior model considering layerd convection and tested whether layered convection develops in giant gaseous planets or not. In this talk, first I review the standard model of giant planets and the problems of anomalies. Second, I review recent understanding of double-diffusive convection. Finally, I show our results. |
| Date: | 22nd October 14:00- |
| Speaker: | 堤 昭裕 |
| Title: | 大振幅Alfven波の衝突による密度谷形成について |
| Abstract: | 太陽が太陽風によって自身の質量を外に放出していることはよく知られている。 一方、活動的な太陽型星風の質量放出率を見積もる数値シミュレーションの中で、星風に降着相が表れるものが見られた( Suzuki et al. 2013 )。 この太陽風の逆流現象は様々な要因が重なって引き起こされていると考えられるが、本発表では、その大きな要因の一つである密度谷の形成に注目 する。 基本的に、周囲と比較して物質の密度が薄い様な領域が存在すると、周囲の物質がその領域に流れ込むことにより密度を一様化させようとするのが 一般的である。 しかし一次元MHD流体シミュレーションにより、大振幅のAlfven波が衝突するような状況においては、非線形効果によって自発的な密度谷 形成が引き起こされることが判明した。 本発表ではこの数値シミュレーションの結果の紹介と共に、密度谷形成プロセスの考察について発表する。 |
| Date: | 31st October 16:00- |
| Speaker: | Masanobu Kunitomo (Tokyo-Tech) |
| Title: | Pre-main sequence evolution including mass accretion |
| Abstract: | Recent studies have revisited Pre-Main-Sequence (PMS) evolution and found that it can be dramatically different from the classical one if the mass accretion through the circumstellar disk onto the protostar is considered. Through the disk accretion, the smaller entropy of accreting materials results in the smaller luminosity and radius, and then the surface convective zone shrinks in the earlier age. This revision is important not only for stellar physics but also planet formation, such as the tidal interaction with a close-in planet. However, the previous studies investigated it under one setting. In this study, we investigate the PMS evolution under a variety of settings, particularly focusing on the surface convective zone mass. We found that the PMS evolution is affected by not the accretion history but the initial radius of the protostar and the heat transport efficiency of the accreting materials. There are two conditions that the surface convective zone shrinks within the disk lifetime (~6Myr): (1) the accreting materials inject extra energy into the star within 3% of its gravitational energy (namely the rest, 97%, should be radiated away before the stellar surface) and (2) the initial radius of the protostar must be below 3 Rsun. I would like to discuss the possibility of these conditions and the impact on the two problems: "luminosity spread" of the PMS stars and "solar anomaly of the surface composition". |
| Date: | 3rd December 14:00- |
| Speaker: | Takeru Suzuki |
| Title: | Magneto-turbulence -- an origin of noncircular motion in the Galactic center region |
| Abstract: | By performing a global magneto-hydrodynamical simulation for the Milky Way, we propose that magneto-turbulence naturally explains the observed noncircular motion of the gas in the Galactic center region, even though an axisymmetric potential is considered. A region with relatively small rotation frequency is formed in $0.3-1$ kpc to satisfy the global equilibrium including the gas pressure. The amplification of the magnetic field is suppressed there, because both magnetorotational instability and field-line stretching are ineffective. An inevitable consequence is the steep decline of the magnetic energy with increasing Galactocentric distance, and the associated magnetic pressure-gradient force drives radially outward flows in a stochastic manner. As a result, the simulated position-velocity diagram exhibits a time-dependent parallelogram-shape. While the toroidal magnetic field dominates in the disk by the radial differential rotation, both the poloidal and toroidal fields are comparably generated in the bulge because of the weak differential rotation. In the central region $< 200$ pc, nonaxisymmetric dense regions with mass $>10^7 M_{\odot}$ and hydrogen number density $> 2000$ cm$^{-3}$ transiently forms, which we discuss as a possible candidate of the central molecular zone. |
| Date: | 10th December 14:00- |
| Speaker: | Ungraduate Students |
| Title: | Interim Presentations |
| Abstract: |
| Date: | 18th December 10:30- |
| Speaker: | Yusuke Tsukamoto |
| Title: | Effects of magnetic field and radiation transfer for the formation and evolution of circumstellar disks |
| Abstract: | In this talk, we will discuss the simulation results of formation and evolution of circumstellar disks in collapsing molecular cloud core with smoothed particle hydrodynamics (SPH). In the first part of our talk, we will investigate the formation process of the circumstellar disk and its properties without sink particle using non-ideal radiation magneto-hydrodynamics simulations. In previous works (e.g., Li+ 07), it is suggested that formation of circumstellar disk around protostar in Class 0 phase is suppressed by strong magnetic braking. However, in the previous works, the important processes during protostar formation such as the formation of the first-core are neglected and numerical artifacts due to large sink cell may emphasize the effect of magnetic braking. Especially, it is expected that the magnetic flux is re-distributed by magnetic diffusions "in the first-core" in which the gas density is sufficiently high and the magnetic diffusivity become large. Using the simulation results of gravitationally collapsing cloud core with non-ideal radiation magneto-hydrodynamics simulations, we will investigate how much magnetic flux extracted from the fir st-core and how its angular momentum evolves and discuss the possibility of circumstellar disk formation in very early phase of protostar evolution. In the latter half of my talk, we will investigate the evolution of circumstellar disk using radiation hydrodynamics simulations (without magnetic field). Especially, we will focus on the temperature structure of circumstellar disk. Elucidating the physical mechanisms which determine the disk temperature structure is essential for the gravitational instability in the disk and its fragmentations. Using the results of radiation hydrodynamics simulations, we will show that the temperature structure of circumstellar disks is determined by non-local radiation transfer within the disk and local balance between the viscous heating and radiation cooling does not hold. We will also discuss the reliability of the disk fragmentation criterion based on the disk cooling timescale. |
| Date: | 7th January 14:00- |
| Speaker: | Yuri Fujii |
| Title: | Non-equilibrium Evolution of the Ionization State in Protoplanetary Disks |
| Abstract: | Gaseous disks are known to form around young star as by-products of star formation. Because planets are formed in them, these disks are called protoplanetary disks. Various observations suggest gas accretion toward the central star. Such mass accretion implies that the angular momentum of the gas is transported outward. The most promising mechanism of this is magnetohydrodynamical (MHD) turbulence driven by the magnetorotational instability (MRI). For the MRI to operate, the ionization degree must be sufficiently high. Because of its high numerical cost, the equilibrium solution is used to evaluate the ionization degree in many MHD simulations. In this study, we implement the ionization degree calculation scheme of Fujii et al. (2011) into the Athena MHD code and investigate the non-equilibrium evolution of the ionization state. When the disk has mostly dissipated and tiny dust grains have grown larger, we find that the ionization degree in the surface layer becomes smaller than the equilibrium value for the corresponding density. This is because a disk wind transports the low ionization degree gas in the lower layers to the surface layer. This effect may be important in the chemical evolution of protoplanetary disks. |
| Date: | 14th January 10:00- |
| Speaker: | Soonyoung Roh |
| Title: | Diffusion of Cosmic Rays from Supernova Remnants in a Multiphase Interstellar Medium |
| Abstract: | Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyze the diffusion of high-energy CRs from acceleration sites, we have performed test particle numerical simulations of CR protons using a three-dimensional magnetohydrodynamics (MHD) simulation of an interstellar medium swept-up by a blast wave. We analyze the diffusion of CRs in the length scale of order a few pc in our simulated SNR, and investigate the possibility of a superdiffusive process in CR propagation and acceleration in SNRs. We find the diffusion of CRs is accurately described by a Bohm diffusion ($\xi_\perp \propto t^{0.5}$) in multiphase medium. Our study indicates that we can use the Bohm diffusion coefficient in practical calculations of CR particle diffusion, a result that may support the interpretation of high-energy emission from SNRs. This can be attributed to the mechanism for creating background turbulence. |