TA-Lab Colloquium

Next Talk

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Schedule for FY2026

April	10	Shu-ichiro Inutsuka
	16	Hiroshi Kobayashi
	23	Hidehiro Kaneda (Prof., Uir Lab., Nagoya Univ.)
May	7	Tomotaka Nishikawa
	21	Izumi Seno
	28	Jihye Hwang (Kyushu University)
		Nguyen Chau Giang (UST/KASI)
June	4	Riona Yamada
	25	Kanaho Imata
July	2	Akihito Asai
	9	Yuki Tamaki
	13	Presentation rehearsal for the summer school by M1 students
	16	Taisei Shioya
	23	Kosei Nozaki

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Previous Talks

Date/Room	10 April at 14:00 in ES606
Speaker	Prof. Shu-ichiro Inutsuka (TA-Lab., Nagoya Univ.)
Title	Star Formation and The Evolution of The Galaxy
Abstract	Click to expand/collapse I will briefly summarize the recent progress in our understanding of ISM dynamics and Star Formation and explain the promising directions of our future research. Those include the formation of hub-filament systems, star cluster formation with binaries, and the evolution of disk galaxies with halo-disk connection. The actual contents may depend on the questions from the audience.




Date/Room	16 April at 14:00 in ES606
Speaker	Assoc. Prof. Hiroshi Kobayashi (TA-Lab., Nagoya Univ.)
Title	Formation of Planetary Systems
Abstract	Click to expand/collapse Planets are formed in protoplanetary disks. I briefly introduce the theory of planet formation. Especially for gas giant planets, the core accretion scenario is somehow consistent with the interior structures of Jupiter and Saturn. However, core formation is impossible not only via the classical theory of planetesimal accretion but also via the recent model of pebble accretion. The total collisional evolution from dust to planets works for the core formation (Kobayashi & Tanaka 2021, 2023). This fancy theory should be confirmed through observation data of protoplanetary/debris disks and analysis of isotope anomaly of meteorites. To tackle the mission, the reconstruction of collisional model is required. I will introduce the state-of-art collisional outcome model (Kobayashi et al. 2026).




Date/Room	23 April at 14:00 in ES606
Speaker	Prof. Hidehiro Kaneda (Uir-Lab., Nagoya Univ.)
Title	Organic matter in space: Polycyclic Aromatic Hydrocarbons (PAHs)
Abstract	Click to expand/collapse Planets are formed in protoplanetary disks. I briefly introduce the theory of planet formation. Especially I will give a lecture on the fundamentals of PAHs for the members of Ta Lab. In the near- and mid-infrared wavelength range, there are many important spectral bands of dust particles such as silicates, carbonaceous grains, and ices. Among them, emission features due to polycyclic aromatic hydrocarbons (PAHs) are ubiquitously observed in the interstellar space including not only photo-dissociation regions, which are widely distributed around star-forming regions in a galaxy, but also regions of rather quiescent or even harsh interstellar environments. Their spectral inter-band ratios are known to significantly vary from galaxy to galaxy or from region to region within a galaxy, mainly depending their ionization states and/or size distributions which reflect the interstellar conditions. The profiles of the PAH features, such as peak positions and widths, also vary, providing us with information on the properties of the interstellar medium in a galaxy. In my lecture, I briefly review the observational history of PAHs since their discovery and explain how to interpret their spectral features based on our observational results obtained with Spitzer, AKARI, and JWST.




Date/Room	7 May at 14:00 in ES606
Speaker	Tomotaka Nishikawa (TA-Lab., Nagoya Univ.)
Title	超新星爆発直後に加速されたPeV宇宙線のガンマ線による観測可能性
Abstract	Click to expand/collapse 銀河宇宙線は，超新星残骸（SNR）における拡散衝撃加速機構によって加速されると広く考えられている．しかし，年齢が数百年程度のSNRに対する近年の観測は，加速された宇宙線の最大エネルギーが∼PeVに達していないことを示唆している．これに対し，Inoue et al. (2021) は，爆発後数十日以内に高密度な星周物質（CSM）中を衝撃波が伝播する場合，宇宙線がPeVまで加速され得ることをkinetic-MHDシミュレーションによって示した．このような高密度CSMの存在は，近年の超新星観測によっても支持されている．∼PeVまでの加速を観測的に確認するには，宇宙線による中性パイ中間子崩壊で生成されるガンマ線の検出が重要な手段となる．しかし，非常に若いSNRから放射されるガンマ線は，超新星の光球からの軟光子や宇宙背景放射との相互作用により減衰する可能性を考慮せねばならない．先行研究の観測から推定された質量損失率を持つ赤色超巨星（RSG）の星風を起源とするCSMの場合，これらのガンマ線をCherenkov Telescope Array Observatory (CTA)で検出することは困難であると先行研究は示唆している(Cristofari et al. 2020)．本研究では，Inoue et al. (2021) のkinetic-MHD シミュレーションの結果を用いて，減衰効果を考慮したガンマ線フラックスを算出した．その結果，近年観測されているRSG風の密度を仮定した場合，予想されるガンマ線フラックスは従来の推定値を大きく上回ることが明らかになった．4Mpc程度の近傍銀河で重力崩壊型超新星が発生した場合，CTAによる50時間程度以内の観測でPeV宇宙線由来のガンマ線が検出可能であると予測される．近傍銀河の星形成率に基づけば，このような事象は約13年に1度の頻度で発生すると期待される．




Date/Room	21 May at 14:00 in ES606
Speaker	Izumi Seno (TA-Lab., Nagoya Univ.)
Title	Precipitation from the Galactic Halo As a Fuel for Star Formation in the Galactic Disk
Abstract	Click to expand/collapse To sustain the Galactic star formation rate over ~10 Gyr, continuous gas accretion from the halo is required. High-Velocity Clouds (HVCs), which is ~10^4 K HI gas condensing from the ~10^6 K hot halo plasma via thermal instability, are considered a key source of this accreting gas. Determining the exact mass accretion rate of these falling HI clouds is crucial for understanding the evolution of the Galaxy. However, an uncertainty of an order of magnitude remains from both observational and theoretical perspectives, partly because previous models overestimated the physical sizes of these clouds. In this presentation, we conduct 2D hydrodynamical simulations to investigate the underlying physics that determines this accretion rate and to quantify the resulting gas accretion. Our simulations, which incorporate the Galactic gravitational potential, radiative cooling, and thermal conduction, reveal that these warm HI clouds form via approximately isochoric cooling followed by compression from the surrounding medium. During this process, the accreting clouds effectively dissipates compressional heating through radiative cooling, maintaining its thermal state and naturally explaining the observed compact sizes. Furthermore, we find a saturated mass accretion rate of ~ 6 solar masses per year onto the Galactic disk. This confirms that halo thermal instability provides a sufficient and steady gas supply to sustain long-term star formation. Finally, we will discuss the steady-state conditions of the Milky Way, a new sound-wave-driven formation scenario for HVC complexes, and connections to future radio and X-ray observations.




Date/Room	28 May at 14:00 in ES606
Speaker	Jihye Hwang (Kyushu Univ.)
Title	Spatial Distribution of Magnetic Field Strengths in Star-forming Regions
Abstract	Click to expand/collapse Magnetic fields play an important role in regulating star formation by preventing the rapid gravitational collapse of molecular clouds. In previous studies, magnetic field strengths have typically been estimated as averaged values over entire star-forming regions. Instead, we propose a method to estimate the spatial distribution of magnetic field strengths using an application of the Davis–Chandrasekhar–Fermi (DCF) method. We apply this method to four star-forming regions—OMC-1, Mon R2, G28.34, and G35.20—and derive their magnetic field strength distributions. Our results consistently show that the importance of magnetic fields decreases with increasing density. In high-density clumps, magnetic fields are not sufficient to support regions against gravity, while they remain important for supporting more diffuse, large-scale structures.
Speaker	Nguyen Chau Giang (UST/KASI)
Title	Synthetic Modeling of Polarized Dust emission - a tool to connect Star formation and Grain alignment theory to Observations.
Abstract	Click to expand/collapse Polarized dust emission from magnetically aligned dust grains is widely used to trace the plane-of-sky magnetic field (B) and probe dust physics in star-forming regions. However, dust grains are usually expected to be poorly aligned with B in these dense environments, but how badly they behave, and how significantly they affect dust polarization, are not well quantified. Synthetic observations of dust polarization with the radiative transfer code POLArized RadIative Simulator (POLARIS) provide a valuable opportunity to connect theory of star formations, dust properties and grain alignment theory, to observations of dust emission polarization toward star-forming regions. In this talk, I will present our development of POLARIS, which allows us to accurately model the alignment dynamics of magnetized dust grains inside highly gas-randomized environments, and discuss about its applications to understanding the physics behind the ALMA dust polarization observed in Class 0/I Young Stellar Objects.




Date/Room	4 June at 14:00 in ES606
Speaker	Riona Yamada (TA-Lab., Nagoya Univ.)
Title	High-Velocity Merging of Dust-Aggregate Planetesimals Driven by Mechanical Energy Dissipation
Abstract	Click to expand/collapse Collisional properties of planetesimals in the meter-to-kilometer size range are important for determining the pathway to planet formation. For lithified rocky planetesimals, collisional growth is inhibited by disruptive impacts (e.g., Benz and Asphaug 1999). However, primordial planetesimals are likely to be composed of dust aggregates, loosely packed collections of primordial dust grains, rather than being lithified planetesimals. The mechanical properties of dust aggregates differ from those of lithified rock: they exhibit asymmetric stress responses under tension and compression, and this behavior can be described by a macroscopic equation of state. We therefore incorporate this equation of state into a Smoothed Particle Hydrodynamics (SPH) framework, where each SPH particle represents a dust-aggregate unit, enabling self-gravitating kilometer-scale collision simulations without resolving individual dust grains. We perform systematic two-body collision simulations with a target radius of 30 km, varying impact velocity, impact angle, and impactor-to-target mass ratio. We find that primordial planetesimals merge at significantly higher impact velocities than lithified planetesimals. At low-to-moderate impact velocities up to about twice the mutual escape velocity, the energy dissipation by compaction and tension dominates. At high impact velocities, shock-wave dissipation contributes approximately half of the total energy dissipation. We also find that smaller impactors can merge with the




Date/Room	25 June at 14:00 in ES606
Speaker	Kanaho Imata (TA-Lab., Nagoya Univ.)
Title	高速度雲と銀河系円盤ガスの相互作用に関するシミュレーション
Abstract	Click to expand/collapse 本研究の目的は、高速雲（HVC）と円盤ガスとの相互作用を通じて、天の川銀河の進化を解明することである。 観測結果によると、天の川銀河の円盤では約100億年にわたり星形成が定常的に続いており、その星形成率は年間数太陽質量程度と推定されている。 しかし、円盤内に存在するガスの総質量はわずか約10億太陽質量であるため、円盤ガスのみを用いてこれほど長期間にわたり星形成を維持することは困難である。 したがって、円盤外からのガスの供給が必要となる。 そのようなガス供給源の有力な候補として挙げられるのがHVCsであり、これらは主に、銀河の回転に対して視線速度成分が著しくずれた中性水素ガスから構成されている。 近年の研究により、HVCが円盤から数十キロパーセクの高度で「雲」へと凝縮し、その後円盤へと「降り注ぐ」という見方が支持されてきている。 しかし、HVCが実際に円盤の近傍に到達した際に生じる流体力学的相互作用については、まだ完全には解明されていない。 もしHVCが高温のハロー内で蒸発することなく生き残り、円盤に到達した場合、円盤ガスとの相互作用を詳細に研究することは、銀河円盤の進化を理解する上で極めて重要である。 本研究では、計算流体力学シミュレーションを用いて、形成されたHVCと円盤ガスとの相互作用を調査する。 我々は、ガス円盤とハローガスを輻射加熱と冷却を考慮してモデル化し、HVCの質量と速度をパラメータ化することで、HVCの運命と円盤の形態的進化を調査する。 さらに、熱的不安定性によって駆動されるHVC形成過程を吟味し、これまでの研究では不確実性が大きかったHVCのサイズや質量といった物理的特徴を観測から制限する方法を提案することを目指す。



• Colloquiums in FY2025
• Colloquiums before FY2025

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