Ta-Lab Colloquium

(LAST UPDATE on 1-April-2021)

Next Talk

Date/Room June 9, 14:00 ES606, zoom
Speaker Kensuke Kakiuchi

Schedule for 2020

April 8 Shu-ichiro Inutsuka
15 Tsuyoshi Inoue
22 Hiroshi Kobayashi
May 6 Kenji Kurosaki
13 Jiro Shimoda
20 Kazuyuki Sugimura
27 Gabriel Rigon
June 10 Kensuke Kakiuchi
17 Kisetsu Tsuge
24 Yoshiaki Misugi
July 1 Tatsuya Okamura
15 Kanta Kitajima
29 Ryo Higuchi
August 5 Rehearsal of presentations for the summer school by M1 students

Previous Talks

Date/Room April 8, 14:00 zoom
Speaker Shu-ichiro Inutsuka
Title The Future of Star Formation Studies
Abstract I summarize our current understanding of star formation processes in the disk region of our Milky Way Galaxy with the extended context of the Galaxy evolution and planet formation. The important concepts in the talk are phase transition, gravitational instability, radiative cooling/heating, angular momenta, transonic turbulent fluctuations, feedback from massive star formation, bubbles and filaments, and hub-filament systems. I will explain possible developments in various directions, such as star formation in the Galactic Center, disk-halo connection, star formation in the first 4 billion years of the Galaxy.
Date/Room April 15, 14:00 zoom
Speaker Tsuyoshi Inoue
Title PeV Cosmic Ray acceleration in the supernova post breakout expansion phase
Abstract Origin of cosmic rays (CRs) is still not known. In this work we argue that PeV cosmic rays can be accelerated during the early phase of a supernova blast wave expansion in dense red supergiant winds. We solve in spherical geometry a system combining a diffusive-convection equation which treats CR dynamics coupled to magnetohydrodynamics to follow gas dynamics. The fast shock expanding in a dense ionized wind is able to trigger the fast non-resonant streaming instability over day timescales. We investigate the maximum energy CRs can reach in this configuration accounting for pp losses. Multi-PeV energies can be reached if the progenitor mass loss rates are of the order of, or larger than, 10-3 solar masses/year. It has been recently invoked that prior to the explosion hydrogen rich massive stars can produce enhanced mass loss rates. These enhanced rates would then favor the production of a Pevatron phase in early times after the shock breakout. We discuss observational tests to probe our model using future radio and gamma-ray facilities.
Date/Room April 22, 14:00 zoom
Speaker Hiroshi Kobayashi
Title Collisional Evolution from Dust to Planets
Abstract Planets are formed via collisional evolution in a protoplanetary disk. The early evolution produces growth fronts in the disks, which are observed as ring-like structures (Ohashi et al. 2021). The later collisional evolution may forms debris disks (Kobayashi et al. 2014, 2019; Genda et al. 2015). The evidence of collisions is found in minor bodies in the solar system (Kobayashi & Tanaka 2010; Sugiura et al. 2018, 2019, 2020). However, the total evolution from dust to planets are not understood well, because the rapid radial drift of meter-sized bodies or planetary bodies disturbs the simple collisional growth. We tackle this issue via “Dust-to-Planet” Simulation (DTPS), which implements the collisional evolution and radial drift of bodies. As a result of DTPS, the solid bodies accumulates in 10AU due to the radial drift of meter-sized bodies. The enhancement of the solid surface density induces the formation of a solid core of gas giants around 7AU in 0.2 Myrs prior to the type I planet migration. The early giant planet formation may explain the water-ice distribution in the solar system and the massive exoplanet formation.
Date/Room May 6, 15:00 ES606, zoom
Speaker Kenji Kurosaki
Title Toward the understanding of the atmospheric escape regimes induced by the giant impact
Abstract Recent observation reveals that many kinds of exoplanets whose masses are Earth to Neptune-mass while those radii are more extensive than Earth-radius. Those planets possess a significant atmosphere whose mass fractions are from 1 % to 30 %, which means a diversity of the atmospheric mass fraction. Such diversities are caused by the diversities of the formation processes of exoplanets. Especially in the late stage of the formation process, planets experience giant impact events that cause atmospheric escape. I perform the smoothed particle hydrodynamic simulation to reveal the impact-induced atmospheric escape. I find that the kinetic energy of escaped atmospheric mass is simply proportional to the kinetic energy of the giant impact. I demonstrate the relationship between the kinetic energy of the escaped mass and the escaped atmospheric mass fraction. I find two regimes that determine the atmospheric escape: the momentum-driven regime and the other is the energy-driven regime. Combined the relationships among the kinetic impact energy, kinetic escape energy, and the escaped atmospheric mass, I reveal an analytic expression for the atmospheric escape as a function of the impact energy. This study will be helpful to predict the atmospheric loss in the late stage of the planet formation, which will have a significant impact on the origin of the diversity of the atmospheric mass.
Date/Room May 13, 15:00 ES606, zoom
Speaker Jiro Shimoda
Title Study of Galactic Wind to Understand the Star Formation History
Abstract Star formation rate of the Milky Way, ~ 1 M_sun/yr, indicates depletion of all gas in the Galactic disk within a time of ~ 1 Gyr. Therefore, to understanding the star formation history that maintains during ~ 8 Gyr with almost constant rate, we must understand gas replenishment mechanisms. Recent observations of metal absorption lines (e.g. MgII, OVI, etc...) around external galaxies suggest that the circum galactic medium (CGM) is huge mass reservoir with a mass of 10^9 - 10^12 M_sun. Since these absorption lines are ubiquitously observed around the host galaxy with a distance more than ~100 kpc, we can naturally expect the galactic wind as a metal transfer mechanism. Once the wind is really driven, the metal polluted gas cools significantly by the radiative line cooling, and eventually falls to the host galaxy. Hence, to understanding the star formation history in terms of the gas replenishment, we study conditions that the transferred metals make the gas sufficiently cool. In this seminar, we will report our preliminary results.
Date/Room May 20, 15:00 ES606, zoom
Speaker Kazuyuki Sugimura
Title Formation of the First-star Binaries
Abstract The first stars in the Universe, born at the redshift z ∼ 20−30, bring an end to the dark ages of the Universe. While the first stars are thought to be born as massive stars, we know little about their multiplicity or related properties. In this talk, I will present the results of our recent radiation hydrodynamics simulations of cosmological first star formation, in which we follow gas accretion considering the ionization/dissociation feedback from multiple protostars with a newly developed adaptive mesh refinement (AMR) code with the adaptive ray tracing method, SFUMATO-RT. As a result of the simulations, we found that the first stars form as massive binaries.
Date/Room May 27, 15:00 ES606, zoom
Speaker Gabriel Rigon
Title From Rayleigh-Taylor to Turbulence - Ab experimental insight -
Abstract The development of laser technology, during the second half of the XXs century, lead to the development of many new research fields, one of which makes use of the most extreme conditions producible with a laser, namely the high energy density (HED) field. After many years of development, the technology and techniques were applied to studies relevant to astrophysics. Thus the research field of laboratory astrophysics appeared in 1996. This young research field aims, despite its numerous constraints, to produce additional information for an astrophysical usage. Instabilities are a recurring research subject in HED. Two of the most studied are the Rayleigh-Taylor (RT) and the Richtmyer-Meshkov instabilities. They arises in many situations from inertial confinement fusion, where they interfere with ignition (this is the main reason for their study), to astrophysics, for instance in supernova. The present study will focus on the evolution of the RTI from its initial state to its final turbulent regime. Such study is a first for the experimental HED domain and it opens possibilities for future application.

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