・・・・・今後の宇宙進化コロキウム/ OUTAP colloquium・・・・・
直近の宇宙進化コロキウム/ Next OUTAP colloquium
Date : 11/22 (Wed) 15:00~
Speaker : Prof. Masahiro Takada
Title : TBA
Date : 11/15 (Wed) 15:00~
Speaker : Dr. Kazuyuki Sugimura
Title : TBA
Date : 11/13 (Mon) 16:30~
Date : 11/8 (Wed) 15:00~
Speaker : Prof. Atsushi Nishizawa
Title : TBA
Date : 11/1 (Wed) 15:00~
Speaker : Dr. Shuta Tanaka
Title : Blocking Metal Accretion onto Population III Stars by Stellar Wind
Abstract: Recent studies of the formation of first stars (PopIII stars) show that low-mass PopIII stars could be formed via the fragmentation of the circumstellar disk around the primary proto-first-stars, although the initial mass function (IMF) of PopIII stars are considered to be top-heavy compared with the IMF of the present stars. Because low-mass PopIII stars of < 0.8 M could survive up until the present, the non-detection of them in our Galaxy has already put a stringent constraint on the IMF of PopIII. On the other hand, some claim that the lack of such stars stems from metal enrichment of their surfaces by the accretion of heavy elements from the interstellar medium (ISM). In this study, we investigated the effects of the stellar wind on metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind, even for neutrals. We found that the stellar wind and radiation block the metal accretion, if the wind has the similar power to that of the Sun. This demonstrates that low-mass PopIII stars remain pristine and will be found as metal-free stars and that further searches for them are valuable in constraining the IMF of PopIII stars.
Date : 10/18 (Wed) 15:00~
Speaker : Mr. Shoji Mori
Title : The Effect of Electron heating in Magnetorotational Instability on Protoplanetary Disks
Abstract: Turbulence in protoplanetary disks affects various stages of the planet formation. Turbulence is thought to be caused by magnetorotational instability(MRI). Since MRI largely depends on the ionization degree, to correctly know ionization degree is indispensable for predicting the turbulent region. We focus on dust adsorption of electrons by electron heating, which is a mechanism for changing ionization degree. This effect can decrease ionization degree because heated electrons by strong electric field are adsorbed to dust grains. In this talk, I will show MHD simulation results including the effect of the electron heating and discuss the impact on the dust growth.
Date : 7/26 (Wed) 16:00~
Speaker : Prof. Hiroshi Kobayashi
Title : Planet formation via collisions
Abstract: Planets are formed from dust grains in a protoplanetary disk composed of gas and solid (rock and ice). Solid is initially sub-micron sized dust grains. Collisional coagulation is believed to produce kilometer sized or larger planetesimals and then planets. For planetesimal formation via collisional growth of dust grains, the collisional growth of dust grains produce fluffy dust aggregates, and such high porosity aggregates overcome the radial-drift barrier for planetesimal formation. From planetesimals to planets, gravitational interaction between bodies is important. Once Mars-sized or larger protoplanets are formed, collisional velocities are accelerated due to planetary perturbation, and then collisional fragmentation becomes active. Protoplanets grow through collisions with surrounding planetesimals. Collisional fragmentation reduces the surface density of planetesimals, resulting in stalling protoplanet growth. The size of surrounding planetesimals depends on the strength of turbulence. Moderate strength turbulence results in 100km sized planetesimals that protoplanets accrete, which allow to produce massive cores to be gas giant planets via gas accretion. This planetesimal size agrees with the characteristic sizes of the main belt asteroids and Kuiper belt objects. On the other hand, the rapid formation timescale of Mars is estimated from Hf-W chronometry, which is explained by week turbulence in the terrestrial planet formation region.
Date : 7/19 (Wed) 14:00~
Speaker : Mr. Hajime Fukushima
Title : Upper stellar mass limit by radiative feedback at low-metallicities: metallicity and accretion rate dependence
Abstract: Massive star play dominant role in the formation and evolution of galaxies. They impact the mechanical feedback to the interstellar medium, such as the formation of HII regions, wind-driven bubbles, chemical evolution by ejecting heavy element. Detailed knowledge of massive star formation in young, and low-metallicity environment is essential in understanding the early structure formation. In massive star formation, radiative feedback is a key mechanism which regulate the stellar mass growth via accretion. In solar-metallicity, the radiation force on to the dust cocoon is dominant suppression mechanism of mass accretion. Also, in the primordial star formation, feedback is caused by an HII region created around the protostar. In this seminar, I will discuss the metallicity and accretion rate dependence of the protostar mass above which each feedback effect begin to affect the mass accretion flow. I will show the results of our numerical model of an accretion envelope around the protostar. Finally, I will discuss how the spectrum of massive star-forming cores changes with decreasing metallicity.
コロキウムの予定表はこちら/ Schedule of colloquium
・・・・・過去の宇宙進化コロキウム/ Previous OUTAP colloquium・・・・・
2017年度の宇宙進化コロキウム/OUTAP colloquium (2017)