Date : 11/22 (Wed) 15:00~
Speaker : Prof. Masahiro Takada
Title : Subaru HSC constraints on primordial black holes and SuMIRe project
Abstract: We used the Subaru HSC data of Andromeda galaxy (M31) to search for microlensing effect on stars in M31 by primordial black holes (PBHs) that might exist in an intervening space in the Milky Way and M31 halo regions. We extensively used the image difference technique to search for time-variable star candidates, but found only one possible candidate compared to ~1000 expected events if PBHs make up all the dark matter. In turn we were able to obtain the tightest upper bound on the abundance of PBHs at lunar mass scales. I will present these results. If time is allowed, I will also introduce how the ongoing and planned wide-area imaging and spectroscopic surveys with Subaru, which we call the SuMiRe projects, are powerful to advance our understanding of the dark Universe.
Date : 11/15 (Wed) 15:00~
Speaker : Dr. Kazuyuki Sugimura
Title : Accretion onto seed BHs: the impacts of anisotropic radiation and gas angular momentum
Abstract: Black hole (BH) growth by gas accretion is supposed to play a crucial role in the formation of supermassive BHs. I will present the results of our simulations of accretion onto seed BHs under radiation feedback and discuss the impacts of anisotropic radiation and gas angular momentum.
Date : 11/13 (Mon) 16:30~
Speaker : Prof. Nozomu Kawakatsu (National Institute of Technology, Kure College)
Title : Obscuring fraction of active galactic nuclei implied from supernova and AGN feedbacks
Abstract: We examine the formation of obscuring circumnuclear disks (CNDs) by considering the supernovae (SNe) from nuclear starburst and the effect of anisotropic radiative pressure. In this work, supposing that the mass accretion onto a central SMBH by the SN driven turbulence and , we explore how the structure of CNDs depend on the black hole mass, the AGN luminosity and the gas surface density of CNDs. We found that the obscuring fraction (f_obs) peaks at L_AGN=0.1L_Edd (Eddington luminosity), and its maximal value is 0.6 for smaller SMBH mass (e.g., M_BH <10^8M_sun). This is because SN feedbacks enhance both L_AGN and f_obs through the SN-driven accretion for smaller L_AGN, while beyond the critical luminosity the dusty molecular gas in CNDs could be blown away by the intense radiation pressure from AGNs. On the other hands, for larger SMBH mass (e.g., M_BH> 108M_sun), f_obs is always low (< 0.1 ) and independent of L_AGN since the scale height of CNDs can be mainly controlled by maximal star formation efficiency in CNDs. These results indicate that f_obs is small for both low and high luminosities for the Seyfert-type AGNs, while f_obs is independent of L_AGN for the QSO-type AGNs. Finally, we discuss whether the super-Eddington accretion is possible at the CND scale (~10pc).
Date : 11/8 (Wed) 15:00~
Speaker : Prof. Atsushi Nishizawa
Title : Baryon Acoustic Oscillations and Redshift Space Distortion with variou tracers
Abstract: Acoustic oscillatory features of the photon-baryon coupled fluid in the early universe can be seen both in CMB and galaxy power spectra. The precise measurement of the location and amplitude of CMB by WMAP and Planck enabled us to know the typical scale of the oscillation, a sound horizon of the photon-baryon fluid at the recombination. As the sound horizon scale is frozen at the recombination, we can use the scale as a standard ruler in the lower redshift universe to know the relation between redshift and angular diameter distance, which potentially has a capability to constrain the cosmological model. In this seminar, we will introduce the basics of Baryon Acoustic Oscillations and Redshift Space Distortion which is always along with us as long as we observe the distance to the tracers in redshift space. We also briefly discuss on the possiblity to measure the BAO and RSD with 21cm line associated to the neutral hydrogen which will be extensively observed in the future experiments like SKA.
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.
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