Date : 5/31 (Wed) 15:00~
Speaker : Prof. Isaac Shlosman
Title : WHAT MAKES THE FAMILY OF BARRED GALAXIES SO RICH?
Abstract: Galaxy evolution is governed by mergers and cold accretion from cosmological filaments. But disk galaxies are embedded in dark matter halos and a large fraction of them host stellar bars. But is the disk aware of the dark matter beyond its gravitational pull? How do galactic disks "talk" to the dark matter and what is the role of bars in this process? Can this interaction be a sign of an internal evolution in galaxies? For example, can it shape the Hubble Fork Diagram? In my talk I will present our latest results on the effect dark matter has on the disk evolution and will discuss the reverse effect of the disk on the dark matter halo.
Date : 5/26 (Fri) 10:00~
Speaker : Dr. Chang-Goo Kim (Princeton University)
Title : Supernova as a regulator of galactic star formation rates and winds
Abstract: Supernova (SN) explosions inject a prodigious amount of energy into the interstellar medium (ISM). This powerful feedback implies that SNe are the dominant regulator of star formation in galaxies. Also, SNe may be a major driver of galactic winds at least in dwarf galaxies. In this talk, I’m going to revisit the evolution of radiative SN remnants in the warm and cold ISM. I then discuss the roles of SN feedback in regulating galactic star formation rates and driving galactic winds with simple theoretical arguments and state-of-the-art numerical simulations, called TIGRESS (Three-phase ISM in Galaxies Resolving Evolution with Star formation and Supernova feedback).
Date : 5/24 (Wed) 15:00~
Speaker : Prof. Hideki Asada (Hirosaki University)
Title : Gravitomagnetic bending angle of light by rotating objects
Abstract: One of the outstanding effects of the theory of general relativity is the gravitational deflection of light by mass. The angular momentum also contributes to the light deflection. This deflection, which can be related with a vector part of the gravitational field, may be called the gravitomagnetic deflection of light (because of its electromagnetic analogy), though the gravitomagnetic deflection has not been observed. We discuss a possible method of more precisely calculating the bending angle of light in stationary, axisymmetric and asymptotically flat spacetimes. For this purpose, we consider the light rays on the equatorial plane in the axisymmetric spacetime. We introduce a spatial metric to define the bending angle of light in a finite-distance situation. We show that the proposed bending angle of light is coordinate-invariant by using the Gauss-Bonnet theorem in differential geometry. The non-vanishing geodesic curvature of the photon orbit with the spatial metric is caused in gravitomagnetism, even though the light ray in the four-dimensional spacetime follows the null geodesic. Finally, we consider Kerr spacetime as an example in order to examine how the bending angle of light is computed by the present method. We discuss the finite-distance corrections to the gravitomagnetic deflection angle for two cases (1) the Sun and (2) Sgr A?. The effects are unlikely to be observed with present technology.
Date : 5/17 (Wed) 15:00~
Speaker : Prof. Kazuyuki Omukai (Tohoku University)
Title : Discovery of SMBHs in the universe with age less than 1Gyrs renewed our interest in supermassive stars (>10^5Msun) as possible progenitors of the seed BHs. Among possible formation scenarios, so-called the direct collapse scenario attracts most attention these days. Here I discuss necessary conditions for their formation, i.e., protostellar collapse without fragmentation, accretion growth to the supermassive range, and the final collapse to BHs.
Date : 5/15 (Mon) 13:30~
Speaker : Prof. Jonathan Tan (University of Florida)
Title : A Light in the Dark - Massive Star Birth Through Cosmic Time
Abstract: Massive stars have played a dominant role in shaping our universe since its earliest times, but there is still no consensus on the mechanism by which they form. I review the physics important for massive star formation and the intimate connection this process has with star cluster formation. I then focus on a particular theoretical model, Turbulent Core Accretion, which assumes the initial conditions are massive, turbulent, magnetized cores of gas and dust that are reasonably close to virial equilibrium. Our group has been exploring this scenario via analytic models and numerical simulations of the physics and chemistry of the interstellar medium. Crucially, these models can now be tested in detail with ALMA and I present the latest results from multiple projects that are zooming in to massive star birth in the darkest shadows of giant molecular clouds. Extension of this work has the potential to also determine how the full stellar initial mass function is established across different Galactic environments. I then switch to the protostellar accretion phase and the emergence of feedback processes, presenting both theoretical predictions and observational tests. Finally, I discuss an application of massive star formation theory to the early universe: how massive were the first stars and could they have been the progenitors of supermassive black holes?
Date : 5/15 (Mon) 10:00~
Speaker : Dr. Lee Spitler (Macquarie University)
Title : Introduction to Macquarie, and the Frontier Research in Astronomy
Date : 4/26 (Wed) 15:00~
Speaker : Mr. Sunmyon Chon (The University of Tokyo)
Title : The Super Massive Star formation in the early Universe
Abstract: Many luminous QSOs have been observed in the early universe. This implies that the massive BHs are already exist at z > 6. The so-called direct-collapse scenario postulates the formation of supermassive stars (SMSs) exceeding 10^5 M_sun to provide massive seeds for such massive BHs. This SMS formation takes place under the extreme condition. That is, they are formed in the primordial clouds which are located at the close vicinity of the massive luminous galaxy. However, SMS formation is mainly studied in the isolated cloud, so far. To see whether the SMS will be formed in the Universe, we performed a cosmological simulation considering the evolution of nearby galaxies. In this seminar, I will show the result of the SMS formation obtained by the cosmological hydrodynamical simulation. Here, we mainly focus on the cloud evolution until the proto-star is formed at the center. We found ~60 candidate clouds but only 2 clouds are going to collapse. This is because the nearby massive galaxy disrupts the cloud collapse by the tidal field. Thus the SMS formation is more difficult than it was though. I will also discuss the required condition for the SMS formation.
Date : 4/19 (Wed) 15:00~
Speaker : Dr. Dimitris Stamatellos (University of Central Lancashire)
Title : The theory of the formation of brown dwarfs and low-mass stars
Abstract: More than half of all stars (including brown dwarfs) have masses below 0.2 Msun. The formation mechanism of these objects is uncertain. I will review the four main theories for the formation of low-mass objects: turbulent fragmentation, ejection of protostellar embryos, disc fragmentation, and photo-erosion of prestellar cores. I will discuss the observational predictions of these models regarding the low-mass initial mass function, the brown dwarf desert, and the binary statistics of low-mass stars and brown dwarfs. I will further discuss whether observations may be used to distinguish between different formation mechanisms, and give a few examples of systems that strongly favour a specific formation scenario. Finally, I will argue that it is likely that all mechanisms may play a role in the formation of brown dwarfs and low-mass stars.
Date : 4/12 (Wed) 15:00~
Speaker : Dr. Yuri Fujii (Niels Bohr Institute/Nagaya University)
Title : Formation of Resonant Moons in Weakly Accreting Circumplanetary Disks
Abstract: During the formation phase of gas giants, circumplanetary gaseous disk form around the planets. Circumplanetary disks are important not only for mass supply to gas giants but also for formation of regular satellites. Because of the comparatively small size-scale of the sub-disk, quick magnetic diffusion prevents the magnetorotational instability (MRI) from being well-developed at ionization levels that would allow MRI in the parent protoplanetary disk. In the absence of significant angular momentum transport, continuous mass supply from the parental protoplanetary disk leads to the formation of a massive circumplanetary disk. We have developed an evolutionary model for this scenario and have estimated the orbital evolution of satellites within the disk. In a certain temperature range, we find that inward migration of a satellite can be stopped by a disk structure due to the opacity transitions. We also find that the second and third migrating satellites can be captured in mean motion resonances. In this way, a compact system in Laplace resonance, which are similar to inner three bodies of Galilean satellites, can be formed in our disk models.
Date : 4/12 (Wed) 12:15~
Speaker : Mr. Masato Kobayashi (Nagoya University)
Title : Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks
Abstract: We formulate the time-evolution equation for giant molecular cloud (GMC) mass functions in galactic disks. The computed time-evolutions show that the cloud-cloud collision effect is only limited in the massive end of mass function, and that almost 100 percent of the gas dispersed by stellar feed back is transformed into a newer generation GMCs in inter-arm regions whereas only about 40 percent in arm regions. Our results suggest that measuring the power-law slope of GMC mass functions can be a powerful method to constrain GMC formation and dispersal timescales and gas resurrection processes in various galactic environments.
コロキウムの予定表はこちら/ Schedule of colloquium