Ta-Lab Colloquium

Next Talk

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Schedule for 2025

April	10	Shu-ichiro Inutsuka (Nagoya U.)
	17	Hiroshi Kobayashi (Nagoya U.)
	24	Kanta Kitajima (Nagoya U.)
May	8	Tomotaka Nishikawa (Nagoya U.)
	15	Yuji Miko (Nagoya U.)
	22	Kota Kobayashi (Nagoya U.)
	29	Nanda Kumar (Universidade do Porto)
June	12	Hayato Shimizu (Nagoya U.)
	19	Shusuke Utsumi (Nagoya U.)
	26	Ryo Sawada (ICRR, U.Tokyo)
July	17	Rehearsal of presentations for the summer school by M1 students
		(Akihito Asai, Kanaho Imata, Taisei Shioya, Yuki Tamaki, and Kosei Nozaki)
Octorber	2	Riona Yamada (Nagoya U.)
	9	Izumi Seno (Nagoya U.)
	16	Kenshin Onogawa (Nagoya U.)
	23	Tomotaka Nishikawa (Nagoya U.)
	30	Ryushi Miyayama (Nagoya U.)
TBD		Yuji Miko (Nagoya U.)
		Kota Kobayashi (Nagoya U.)
		Hayato Shimizu (Nagoya U.)
		Shusuke Utsumi (Nagoya U.)

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Previous Talks

Date/Room	April 10, 14:00 @ES606
Speaker	Shu-ichiro Inutsuka (Nagoya U.)
Title	Star Formation and Evolution of Milky Way Galaxy
Abstract	I will briefly summarize the recent progress in our understanding of ISM dynamics and Star Formation and try to envision the future development of the research in our group. The latter includes the evolution of disk galaxies, halo-disk connection, star cluster formation with many binaries, and planet formation. But the choice of actual subjects explained in the talk may depend on the number of questions and available time of three hours!

Date/Room	April 17, 14:00 @ES606 & zoom
Speaker	Hiroshi Kobayashi (Nagoya U.)
Title	Theory of Planet Formation
Abstract	The origin of gas giant planets, such as Jupiter and Saturn, is discussed along the core-accretion scenario, in which gas giants are formed via rapid gas accretion onto massive solid cores. However, the formation of massive cores includes a lot of difficulties. I review the difficulty of core formation via planetesimal or pebble accretion. I show the possible scenario for core formation based on dust-to-planet simulations. The collisional outcome is important for planet formation. I drive a nice outcome model based on collisional simulations, which may allow us to understand collisional outcomes in various scales from dust to planets.

Date/Room	April 24, 14:00 @ES606
Speaker	Kanta Kitajima (Nagoya U.)
Title	Particle-Based Analysis of Relativistic Jets
Abstract	In this presentation, we will analyze a simulation of a stationary high-temperature gas accelerating to a relativistic velocity. The Special Relativistic Godunov Smoothed Particle Hydrodynamics (SRGSPH) [Kitajima+ 2025, submitted] will be employed to model the fluid as a collection of discrete particles, known as SPH particles, and to describe the fluid's motion through the interaction of each SPH particle with its environment. By leveraging the advantages of the SPH method, we can investigate fluids in vacuum regions, which have traditionally been challenging to address with standard numerical methods. This study takes advantage of the unique aspects of the SPH method to simulate a jet from a high-temperature source into a vacuum and analyze its acceleration mechanism. The findings from this analysis provide valuable insights into the driving mechanisms of relativistic jets, including those observed in active galactic nuclei and gamma-ray bursts.

Date/Room	May 8, 14:00 @ES606 & zoom
Speaker	Tomotaka Nishikawa (Nagoya U.)
Title	Observational prediction of gamma-ray emission from knee-energy cosmic rays accelerated by core-collapse supernovae
Abstract	Galactic cosmic rays (CRs) are commonly thought to undergo acceleration through diffusive shock acceleration (DSA) mechanism within supernova remnants (SNRs). Recent observations of SNRs with ages ~10^2 - 10^3 yrs indicate that the maximum energy of cosmic rays does not reach ~PeV level. Recently, Inoue et al. 2021 demonstrated through kinetic-MHD simulations that cosmic rays gain high energy up to 3 PeV when a blast wave shock propagates through a dense circumstellar medium (CSM) within tens of days after the explosion. In their model, the dense CSM is assumed to be created by a stellar wind of a red supergiant (RSG) with a mass-loss-rate of 10^-3 M_Sun yr^-1, which is supported by recent observations of supernovae. To prove PeV accelerations, observations of 100 TeV gamma-rays, which are generated by PeV CRs via neutral pion decay, can be effective. However, these hadronic gamma-rays from a very young SNR can be hardly attenuated by interactions with soft photons from the supernova photosphere and cosmic background radiations. Previous studies argued that it is very hard to detect these gamma-rays if we assume a CSM formed by conventional RSG wind with ~10^-5 M_Sun yr^-1. In this study, using the kinetic-MHD simulations data by Inoue et al. 2021, we calculate the gamma-ray flux emitted from a blast wave shock propagating in the dense CSM by considering the environmental attenuations. We find that we can expect considerably larger gamma-ray flux than that reported in the previous studies, if we assume the modern mass-loss-rate wind of ~10^-3 M_Sun yr^-1. We predict that the Cherenkov Telescope Array can detect 100 TeV gamma-rays even by 50 hours integration if a type II SN happens in nearby galaxies within 4.8 Mpc. Based on the observed star formation rates, we can expect such an event once per 10 years.

Date/Room	May 15, 14:00 @ES606
Speaker	Yuji Miko (Nagoya U.)
Title	隕石の惑星大気進入による気流・熱状態への影響のモデリング
Abstract	若い地球型惑星には頻繁に隕石が衝突し、その大気の進化には衝突過程が大きな影響を及ぼす。隕石衝突前の惑星大気は、地表からの熱により、地表付近の対流圏と上層の成層圏が存在する。しかし、隕石衝突により高温・高圧領域が発生、膨張することで、大気の流れや熱状態が大きく乱される。この流れは非常に複雑であり、流体シミュレーションにより調べる必要がある。 本研究ではAthena++を用いて、大気に突入する隕石が及ぼす影響について3次元流体シミュレーションを行った。大きな隕石の場合、地表に衝突し、エネルギーを解放することで爆発現象のような膨張が発生する。一方、小さな隕石の場合、大気中で破壊され、上空でエネルギー解放が起こる。また、隕石の軌道上に生じる煙突状の希薄領域も重要である。そこで、エネルギー解放領域と煙突状の希薄領域に分けて隕石の影響のモデル化を行った。そして、背景大気の初期条件として、下層に対流圏、上層に成層圏が存在している大気を用意し、以上のモデリングのもとシミュレーションを行った。その結果、エネルギー解放による膨張波が希薄領域で早く伝わることで、爆発領域付近の下層大気がはるか上層にまで運ばれることを確認した。 次に、本研究のシミュレーション結果を実際の隕石落下事例と比較した。2013年2月15日に小天体Chelyabinskが衝突したイベントは多くの観測がなされており、衝突体の大きさ、密度、進入角度など多くのデータがある。これらのデータと我々のモデルをもとに数値計算を行い、地表での圧力を求めた。本研究のシミュレーションの結果と実際の被害から推測される圧力分布を比較した結果、実際の被害分布を再現可能であり、モデルの妥当性が確かめられた。また、大気中での隕石の破壊に伴うエネルギー解放領域の形状に制限を与えることもできた。

Date/Room	May 22, 14:00 @ES606
Speaker	Kota Kobayashi (Nagoya U.)
Title	大質量星による輻射フィードバック過程の多次元シミュレーション
Abstract	大質量星によって形成するHII領域は、星周物質の構造に大きな影響を与え、分子雲破壊を引き起こすことでさらなる星形成活動を阻害する役割を持っていると考えられている。先行研究では球対称1次元における数値流体シミュレーションによりHII領域の膨張則の近似解が与えられた。一方、3 次元構造を考慮したHII領域の膨張過程に関する研究は十分に行われていない。HII領域の伝搬には周辺ガスの密度構造や複数の恒星からの輻射が大きな影響を与えるため、多次元的なガスおよび恒星の分布を考慮することが重要である。 　本研究では、流体計算コードのAthena++を用いて大質量星形成によるフィードバックを考慮した3次元輻射流体シミュレーションの開発および計算を行った。テスト計算においては、球対称１次元の高精度な数値計算結果および解析解と比較して、良い一致を示すことが確認された。次に、複数の大質量星が存在する現実的な環境における場合の計算を実施した。その結果、HII領域同士の衝突箇所では高密度の中性領域が形成され電離効率が減少していることが確認できた。

Date/Room	May 29, 14:00 @ES606
Speaker	Nanda Kumar (Universidade do Porto)
Title	Star Formation in Hub-Filament Systems: A Unifying View
Abstract	Most stars in our galaxy form within clusters. Young stellar clusters (YSCs) exhibit well-defined characteristics, including radius, mass segregation, and initial mass function. While low-mass stars take roughly ten times longer to form than their high-mass counterparts, both populations emerge together before radiative feedback from massive stars can disrupt the parent cloud. In this talk, I will present a unified framework for star formation in hub-filament systems (HFS), offering a coherent explanation for these observed properties. I will explore the mechanisms driving the accumulation of dense gas and dust in the hub — the primary birthplace of massive stars — and discuss the evolving dynamics of magnetized HFS, including new findings on the subject. Finally, I will conclude by outlining key unresolved questions in HFS star formation.

Date/Room	June 12, 14:00 @ES606
Speaker	Hayato Shimizu (Nagoya U.)
Title	系外惑星の重力によるダストの非対称空間分布
Abstract	近年、直接撮像による系外惑星の検出が進展しており、これまでは木星のような巨大惑星の検出がされていたが、将来的には赤外線での地球型惑星の検出が期待される。一方、太陽系でも黄道光として知られるように、惑星周りにはダストが分布しており、ダスト熱輻射により惑星からの放射が埋もれてしまうか、逆に、ダストによる特徴的な構造が惑星の存在を示唆するといった可能性がある。したがって、惑星の直接撮像による検出を行う上では、惑星周囲のダスト分布、すなわちデブリ円盤の存在や構造を正確に考慮することが重要となる。 本研究では、太陽系の小惑星帯（2-3 au）を模擬した微惑星帯で定常的に生成されるダストが、ポインティング・ロバートソン効果（P-R効果）により内側に移動し、軌道半径1 auの地球質量惑星の周りに形成されるダスト分布を調べ、ダストの輻射フラックスを評価する。そのために、まず、中心星の輻射圧およびP-R効果を考慮し、中心星・惑星・ダストの3体からなる3次元軌道運動を数値計算により求めた。その結果をもとに、惑星周囲のダスト分布を求めた。計算領域を半径方向と方位角方向に分割し、各領域におけるダストの滞在時間を集計することで、円盤内のダストの数密度分布を得た。 次に、中心星光に対するデブリ円盤の赤外輻射フラックス比についても評価を行った。P-R効果によって主に落下してくるサイズのダストが、典型的なデブリ円盤のダストとなるため、ダストサイズを微惑星帯での衝突時間と落下時間から見積もった。この一連の調査の結果、明るいデブリ円盤では主に小さなダストが中心星へ落下するため、構造は滑らかで惑星検出に不利になるのに対し、暗いデブリ円盤では大きなダストが惑星周囲にまで落下して、特徴的な構造が形成されるため、惑星検出に有利になることが分かった。

Date/Room	June 19, 14:00 @ES606
Speaker	Shusuke Utsumi (Nagoya University)
Title	An Extension of Smoothed Particle Hydrodynamics to Elastic Dynamics Utilizing Spring Force Model
Abstract	Elastic dynamics plays a fundamental role in understanding the mechanical behavior of solid bodies, with applications ranging from planetary collisions to engineered structures. Numerical methods, especially particle-based approaches, have become indispensable tools for simulating such phenomena. In this study, we develop a novel formulation of Smoothed Particle Hydrodynamics (SPH) extended to linear isotropic elasticity by introducing a spring force model. Conventional SPH-based elastic solvers typically rely on the time evolution of stress tensors, which not only increases computational cost but also leads to violations of angular momentum conservation due to non-central inter-particle forces. Our spring-based approach resolves these issues by expressing the deviatoric stress through central forces, thereby eliminating the need for stress evolution equations and ensuring strict conservation of angular momentum. This simplification dramatically reduces computational overhead while preserving physical accuracy. The proposed method is particularly effective in simulating dynamic elastic events such as impacts and deformations. Through numerical experiments, we demonstrate the robustness and efficiency of our approach.

Date/Room	June 26, 13:00 @ES606
Speaker	Ryo Sawada (ICRR, The University of Tokyo)
Title	Cosmic-Ray Bath in a Past Supernova Gives Birth to Earth-Like Planets
Abstract	A key question in astronomy is how ubiquitous Earth-like rocky planets are. The formation of terrestrial planets in our solar system was strongly influenced by the radioactive decay heat of short-lived radionuclides (SLRs), particularly 26Al, likely delivered from nearby supernovae. However, current models struggle to reproduce both the relative and absolute abundance of SLRs without destroying the protosolar disk. We propose the ‘immersion’ mechanism, where cosmic-ray nucleosynthesis in a supernova shockwave reproduces estimated SLR abundances at a supernova distance (∼1 pc), preserving the disk. We estimate that solar-mass stars in star clusters typically experience at least one such supernova within 1 pc, supporting the feasibility of this scenario. This suggests solar-system-like SLR abundances and terrestrial planet formation are more common than previously thought.

	Rehearsal of presentations for the summer school by M1 students
Date/Room	July, 17th 14:00- @ES606
Speaker	Akihito Asai (Nagoya University)
Title	原始惑星系円盤進化を考慮した巨大惑星形成の理論的研究
Speaker	Kanoho Imata (Nagoya University)
Title	高速度分子雲と銀河系円盤の相互作用の理論的研究
Speaker	Taisei Shioya (Nagoya University)
Title	固体天体の衝突シミュレーションを用いた惑星形成段階における微惑星の衝突特性の理論的研究
Speaker	Yuki Tamaki (Nagoya University)
Title	銀河中心アーク構造解明に向けて：シンクロトロン冷却不安定性による縞状化
Speaker	Kosei Nozaki (Nagoya University)
Title	粒子法によるブラックホール周りの流体計算法の開発に向けて

Date/Room	Octorber 2nd, 14:00 @ES606
Speaker	Riona Yamada (Nagoya U.)
Title	Primordial Planetesimal Collisions with Dust Aggregate Equation of State
Abstract	Understanding planetesimal collisions is important for elucidating the fundamental physical mechanisms involved in planetary formation. Prior work has typically modeled planetesimals as monolithic rocky bodies with elastic material properties (e.g., Sugiura et al., 2018), an approach that has successfully reproduced the morphology of many observed asteroids. However, growing planetesimals are unlikely to have experienced thermal metamorphism. Therefore, modeling them as monolithic rocks may fail to capture their actual internal structure and mechanical behavior. Instead, it is necessary to assume a structure more similar to loosely bound dust aggregates, which better reflects their primordial, unprocessed nature. In this study, we implement the equation of state for dust aggregates proposed by Tatsuuma et al. (2019) into a Smoothed Particle Hydrodynamics (SPH) code to simulate planetesimal collisions. Each SPH particle is treated as a volume element of a dust aggregate. This approach allows us to reproduce the bulk stress response of dust aggregates without resolving individual dust particle interactions. Using this method, we carry out a systematic study of planetesimal collisions through numerical simulations. We employ the Friend-of-Friend (FoF) algorithm to identify collision fragments and quantitatively evaluate their masses. By varying parameters such as impact velocity, impact angle, and mass ratio, we investigate the dependence of collision outcomes on these parameters and the associated impact energy. These simulations provide new insights into how the internal structure of primordial planetesimals influences collisional outcomes, offering a more realistic framework for understanding the early stages of planetary accretion. Collisions among planetesimals govern the early stages of planetary accretion. Previous studies typically modeled planetesimals as monolithic rocky bodies with elastic strength. However, primordial planetesimals likely grew from submicron dust grains without thermal processing, and their bulk behavior is better represented by porous dust aggregates. We implement the dust aggregate equation of state proposed by Tatsuuma et al. (2019) into a Smoothed Particle Hydrodynamics (SPH) code. Each SPH particle represents a volume element of dust aggregates, characterized by compressive and tensile stresses. The method reproduces self-gravitating equilibrium structures consistent with the Lane–Emden solution, confirming the reliability of the model. We then perform systematic simulations of head-on and oblique collisions between planetesimals. Impact velocity, impact angle, and projectile-to-target mass ratio are varied. Collision outcomes are evaluated by the mass of the largest remnant normalized by the projectile and target masses. The simulations show transitions between merging, partial accretion, hit-and-run, and disruption. We find that both impact angle and mass ratio strongly influence these transitions. Near head-on collisions lead to accretion at low velocities but to disruption at higher velocities. Oblique collisions reduce the range of accretion but suppress disruption. Increasing mass asymmetry makes complete merging less likely, while requiring higher velocities for disruption. The inclusion of tensile strength reduces mass loss and increases the stability of remnants. These results indicate that the collisional outcome of primordial planetesimals is controlled not only by impact energy but also by internal aggregate physics. Furthermore, the accretion threshold appears to be linked to energy dissipation through compression, suggesting a path toward an analytic description of merging conditions.

• TA Colloquium from 1st April (2024) to 31st March (2025)
• TA Colloquiums to 31st March (2022)

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