Ta-Lab Colloquium

(LAST UPDATE on 24-June-2022)

Next Talk

Date/Room June 30, 14:00 ES606, zoom
Speaker Ryushi Miyayama

Schedule for 2022

April 8 Shu-ichiro Inutsuka
14 Hiroshi Kobayashi
21 Kenji Kurosaki
28 Jiro Shimoda
May 12 Gabriel Rigon
16 Kensuke Kakiuchi
30 Valdivia Valeska
June 2 Daisei Abe
9 Ryunosuke Maeda
16 Masato Kobayashi
21 Satoru Yamaguchi
23 Yoshiaki Misugi

Previous Talks

Date/Room April 8, 14:00 zoom
Speaker Shu-ichiro Inutsuka
Title Star formation in The Galactic Disk: The Evolution of Filament Paradigms
Abstract I will summarize our overall understanding of star formation process in The Galactic Disk. Key concepts are the initial mass function, angular momenta of astrophysical objects, star formation rate, star formation efficiency, dense gas mass fraction in molecular clouds, and the widths of the dense filamentary molecular clouds. I also explain some remaining issues in the context of the bubble-filament Paradigm and hub-filament paradigm.
Date/Room April 14, 14:00 zoom
Speaker Hiroshi Kobayashi
Title Collisional Evolution from Dust to Planets in protoplanetary disks
Abstract I will talk about planet formation via collisional evolution of dust grains in protoplanetary disks. Gas giant planets such as Jupiter and Saturn are formed via the rapid gas accretion of solid cores each with about ten Earth masses. The formation of such massive cores via planetesimal accretion is slower than the rapid orbital decay by type I migration. To understand the formation we need to treat the whole mass range from dust to planets. I will introduce the result of dust-to-planet simulations. Collisional growth of dust is inferred in the resoloved multi wavelength observations of protoplanetary disks. I will also introduce the collisional outcome model of dust grains and the size distribution of dust grains.
Date/Room April 21, 14:00 zoom
Speaker Kenji Kurosaki
Title Giant impact on Earth-like planet with a massive atmosphere
Abstract Recent observation revealed several exoplanets with masses ranging from Earth to Neptune and radii more extensive than that of the Earth. These planets possess a unique atmosphere with mass fractions ranging from 1 \% to 30 \%, reflecting diversity in the atmospheric mass fractions. Such diversities are supposed to be caused by the differences in the formation processes of exoplanets. Especially in the later stage of their formation, planets experience giant impact events causing the atmospheric escape. We perform smoothed particle hydrodynamic simulations to reveal the impact-induced atmospheric escape for young super-Earths with 10 % to 30 % of the atmospheric mass fraction. We find that the kinetic energy of the escaped atmospheric mass is almost proportional to the sum of the kinetic impact energy and self-gravitational energy released from the merged core. We evaluate the relationship between the kinetic energy of the escaped mass and the escaped atmospheric mass fraction. The present study provides strong constraints on the formation scenarios of observed rocky planets. Since a giant impact event removes the primordial atmosphere efficiently, observed rocky planets with atmospheres must have formed before the protoplanetary disk dispersal. The atmospheric erosion can result in atmospheric pollution via the core material vapor. The present study of giant impact events emphasizes a significant connection between the origin and observation of the planet.
Date/Room April 28, 14:00 zoom
Speaker Jiro Shimoda
Title Novel Approach to Cosmic-Ray Accelerating Collisionless Shock: Plasma Diagnostics of Supernova Remnant in XRISM Era
Abstract Recent studies on the dynamics of interstellar medium related to our galaxy's star formation history state that cosmic rays play a key role to maintain star formation for a long time (e.g., Shimoda & Inutsuka 2022, ApJ 926, 14). The cosmic rays are energetic, charged particles that move through the interstellar medium at nearly the speed of light. Despite their importance in astrophysics, our understanding of the cosmic rays is far from sufficient: What objects provide the cosmic rays? What is the mechanism to produce the cosmic rays? How do the cosmic rays behave in the interstellar medium? and so on. Collisionless shocks of supernova remnants are thought as the most likely candidates of cosmic-ray origin. The collisionless shock is formed by interactions between particles and electromagnetic disturbances unlike the usual shock in hydrodynamics. The formation process of collisionless shocks is considered to be closely related to the production of the cosmic rays, however, there is no firm theory predicting the cosmic-ray amount in the collisionless shock. In this seminar, we will introduce a novel theoretical model predicting the cosmic-ray amount and how the model is examined by observations.
Date/Room May 12, 14:00 zoom
Speaker Gabriel Rigon
Title Supernova remnant expansion in a turbulent interstellar medium
Abstract Supernovae (SN) and their remnants play an important role in the dynamic of our universe. They inject energy and heavy elements into the interstellar medium and accelerate cosmic rays. In particular, supernova remnants (SNR) are thought to be the main source of galactic cosmic ray acceleration. This theory is backed up by the sheer energy of an SN explosion, the observation of x-ray synchrotron emissions from the SNR, as well as some potential acceleration mechanisms (1st and 2nd order Fermi acceleration). But a question then remains, which part of the SNR accelerates the cosmic rays and does it have an impact on the SNR dynamic. This question was tentatively answered some 17 years ago (Warren 2005) following the detailed observation of Tycho. A blast wave too close to the contact discontinuity to be explained by 1D simulation was observed. Thus the cosmic rays are accelerated at this shock front. But does this conclusion still hold in 3D simulations with a turbulent interstellar medium? This question is the starting point of my research in the Ta-lab, which I will introduce in this seminar.
Date/Room May 16, 14:00 zoom
Speaker Kensuke Kakiuchi
Title MHD simulations in the Galactic center region: Influence of heating and cooling effects on magnetic activity
Abstract The Galactic central region is the key of galaxy evolution in the galaxy, and has a complex and enigmatic region. Based on observations, the strength of the magnetic field within the central few hundred parsecs of the Galaxy is stronger than in the Galactic disk region, and its magnetic energy is comparable to or even surpasses the thermal and kinetic energy of the interstellar gas. Therefore, it is essential to elucidate the role of the magnetic field to understand the behavior of the interstellar gas in the Galactic center region. In this talk, we present the results of numerical simulations in which we treat the interstellar gas in the Galactic center region as a magnetohydrodynamic fluid, and newly take into account radiative heating and cooling for the interstellar gas in the effects of thermal evolution. We found the formation of a mid-latitude low-plasma beta zone (dominated by magnetic field pressure), which would not have appeared without radiative heating and cooling.
Date/Room May 30, 14:00 zoom
Speaker Valdivia Valeska
Title Towards the Solution of an 80 year old mystery: The origin of CH+ in diffuse clouds
Abstract For years, the observations of atomic and molecular species along with theoretical models have been providing us with a wealth of information on the physico-chemical conditions of a wide variety of astrophysical environments. Nevertheless, the accuracy of the interpretation of these observations relies on how well the models incorporate the relevant physics and any mismatch between theoretical predictions and observations might indicate that an important piece of the astrochemical puzzle is missing. For instance, the large abundances of CH+ in the diffuse interstellar medium (ISM) are a long-standing issue of our understanding of the thermodynamical and chemical states of the gas, since classical or simplified approaches are not able to account for the observed abundances. Molecular clouds are known to be magnetized and to display a turbulent and complex structure where warm and cold phases are interwoven. The turbulent motions within molecular clouds transport molecules, and the presence of magnetic fields induces a relative velocity between neutrals and ions known as the ion-neutral drift (vd). These effects all together can influence the chemical evolution of the clouds. In this talk I will present this longstanding riddle that has been challenging astrochemical models for years and review the previous attempts to account for the large abundances of CH+ observed in the ISM. Then I will introduce the current developments of a model that incorporates the multiphase nature of the ISM as well as the out-of-equilibrium molecular evolution of the gas that might shed some light on this 80 years old mystery.
Date/Room Jun 2, 14:00 zoom
Speaker Daisei Abe
Title Topic 1 星形成フィラメント進化過程解明に向けたMHDシミュレーション Topic 2 統計的なフィラメント質量進化の理論的研究: フィラメント破壊と生存条件
Abstract Topic 1: 星は分子雲中の高密度領域で形成されるが、その高密度領域がフィラメント状であることや (e.g., Andr ́e et al. 2010) 、分子雲を通過する衝撃波がそのフィラメントの形成を誘発することがわかった (e.g., Inoue & Fukui 2013; Abe et al. 2021)。フィラメントの形成・進化過程は星形成の初期条件を決定するため重要である。特にフィラメントの幅は星形成開始条件や星の質量を決めうる重要な量である。観測結果からフィラメントの幅はその線密度によらず普遍的に0.1pcであることがわかっている(Arzoumanian et al. 2019)。ところが、理論的にはフィラメントの幅は高密度なものほど小さいはずであり、観測結果と矛盾する。本研究ではAthena++ (Stone+ 2020)を用いたMHDシミュレーションによってフィラメントの幅の普遍性を説明することを目指す。フィラメント境界は"MHDスローショック"となっている可能性が高い。スローショックの波面は不安定であり、フィラメント内に乱流を駆動しフィラメントを重力収縮から支えるための運動エネルギー供給が期待される。フィラメントのスケールではプラズマと中性流体の間のドリフトによる磁場の拡散現象である両極性拡散が起こる。両極性拡散は小スケールにおいてスローショック不安定性を抑制するため、フィラメントでどのくらい強くスローショック不安定性が起こるか調べる必要がある。そこでまず両極性拡散入りのスローショック不安定性の物理を理解するために二次元MHDシミュレーションをおこなった。その結果、両極性拡散のスケールの4倍でスローショック不安定性が抑制されることがわかった。不安定性の非線形発展や今後の展望についても議論する。 Topic 2: Abe et al. (2021) でのシミュレーションでは、衝撃波-分子雲相互作用が長時間継続する問題設定となっていたが、現実はある大きさの分子雲へ衝撃波が通過するため、衝撃波圧縮の継続時間に限りがある。 よって、現実的な分子雲の進化と星形成過程を解明するには、衝撃波の継続時間をパラメータとしたシミュレーションをすることで、どのくらいの衝撃波継続時間で形成されたフィラメントがどうなるのかや星形成が起こるかどうかを調べる必要がある。結果として、短い衝撃波圧縮時間だと形成されたフィラメント全てが破壊され、星形成が起こらないことがわかった。長い衝撃波圧縮時間の場合だと、圧縮層が膨張しても超臨界フィラメントは生き残り星形成が誘発されることがわかった。
Date/Room Jun 9, 14:00 zoom
Speaker Ryunosuke Maeda
Title Massive star cluster formation by fast HI gas collision with stellar feedback
Abstract Young massive clusters (YMCs) are dense aggregates of young stars, which are essential to galaxy evolution, owing to their ultraviolet radiation, stellar winds, and supernovae. YMCs typically have M∼10^4 M_sun and R∼1 pc, indicating that many stars are located in a small region. The formation of YMC precursor clouds may be difficult because a very compact massive cloud should be formed before stellar feedback blows off the cloud. Recent observational studies suggest that YMCs can be formed as a consequence of the fast HI gas collision (∼100 km/s). Maeda et al. (2021) show that massive gravitationally bound gas clumps with M>10^4 M_sun and L∼4 pc are formed in the shock compressed region induced by the fast HI gas collision, which massive gas clumps can evolve into YMCs. However, Maeda et al. (2021) did not take into account feedback effects from the formed stars. This is because the escape velocity of the formed gas clamp is larger than the sound speed of the HII region (indicating gravity dominant), and it was thought that the feedback could not dissipate the gas clamp. But before the clumps become dense enough, the feedback effect from the stars may become important. In this study, we performed a simulation of the formation of massive stellar clusters by HI gas collisions, taking into account the photoionization effect from stars. The results show that the formation of massive gas clumps, which are precursors of massive star clusters, is possible even when the photoionization effect from stars is taken into account.
Date/Room Jun 16, 14:00 zoom
Speaker Masato Kobayashi
Title Formation and evolution of molecular clouds across a galactic disk
Abstract Molecular clouds are the progenitor of stars and their evolution is an important driver of galaxy evolution. The key questions are, (1) what the lifecycle of molecular clouds is and (2) how that lifecycle determines the star formation efficiency and the cosmic star formation history of the Universe. To answer these questions, I will mainly focus on the HI-to-H2 transition in this colloquium. I will first briefly describe the overall picture of molecular cloud formation and evolution across a galactic disk from a statistical point of view, mostly in terms of the mass function of molecular clouds. Based on this understanding, I will show MHD simulations of molecular cloud formation and evolution on a 10 Myr scale. This reveals that the coupling between the thermal state and the turbulence is important of the emergence of log-normal density probability distribution function in molecular clouds, which determines the onset of star formation but is not fully understood in previous isothermal theories. I will also mention how this detailed understanding can be implemented as a sub-grid star-formation modeling in galaxy evolution simulations. Lastly, I will list the future prospects expected in simulations and observations.
Date/Room Jun 21, 14:30 zoom
Speaker Satoru Yamaguchi
Title 教師なし機械学種を用いた分子雲ガスデータの解析
Date/Room Jun 23, 14:00 zoom
Speaker Yoshiaki Misugi
Title Evolution of the Angular Momentum of Star Forming Cores in Magnetized Filamentary Molecular Clouds
Abstract Molecular cloud cores are the dense region in molecular clouds, which are the birth place of stars and planets. Therefore, revealing the physical properties of molecular cloud cores is important to understand the initial condition of star formation. Magnetic field and angular momentum of molecular cloud core play an essential role in disk formation and angular momentum transfer, respectively. Recent observational results derived from the analysis of Herschel data revealed that most cores are formed in filamentary structures. However, the effect of magnetic field on angular momentum transfer during core formation stage in the filament is still unclear. We perform the three dimensional simulations using Smoothed Particle Magnetohydrodynamics method to study the evolution of magnetic field and angular momentum of molecular cloud cores formed through filament fragmentation process. In this talk, I will report the recent progress of our research.

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