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Colloquium のバックアップ(No.120)


DTA Colloquium 2018

DTA Colloquium(理論コロキウム)は原則として毎週火曜日の午後13:30から開催しています。
原則として英語で講演していただきますが、 講師・参加者が日本人だけの場合は日本語に切り替えてくださっても(英語のままでも)結構です。
台内・台外また分野を問わず広く発表者(台外の方には旅費・謝金あり)を募集しています。
お問い合わせは以下のコロキウム係までお願いします(_AT_を@に変更してください)。

  • 滝脇知也 takiwaki.tomoya_AT_nao.ac.jp
  • 荻原正博 masahiro.ogihara_AT_nao.ac.jp
  • 楠根 貴成 takayoshi.kusune_AT_nao.ac.jp
  • 高橋博之 takahashi_AT_cfca.jp
  • 朝比奈雄太 asahina_AT_cfca.jp
  • 佐々木宏和 hiro.sasaki_AT_nao.ac.jp

Schedule & History

FY2010 FY2011 FY2012 FY2013 FY2014 FY2015 FY2016 FY2017

DateSpeakerTitlePlace/TimeRemarksOrganizer
04/05all internal membersself-introductionRinko room, Main Building (East) / 13:30ThursdayTakiwaki
04/10Adriana Pohl (Max-Planck Institute of Astronomy, Heidelberg Germany)Revealing the evolution of planet-forming disks with polarization observationsLecture room / 13:30TuesdayOgihara
04/17Yoshiaki Kato (Riken)Radiation MHD Simulations of Waves and Vortices on the Sun and beyondLecture room / 13:30Takahashi
04/24Akihiro Suzuki (NAOJ)Multi-dimensional modeling of supernova ejecta with a central energy sourceRinko room, Main Building (East) / 13:30Ogihara
05/08Yoshiyuki Inoue (Riken)Coronal Magnetic Activity in a Nearby Active Supermassive Black HoleRinko room, Main Building (East) / 13:30Asahina
05/10Keiichi Maeda(Kyoto University)Progenitor Evolutions and Explosion Mechanisms of Type Ia SupernovaeRinko room, Main Building (East) / 15:00ThursdayAsahina
05/15Kyohei Kawaguchi (ICRR)Radiative-transfer simulation for the optical and near-infrared electromagnetic counterparts to GW170817Lecture room / 13:30Takahashi
05/22Kazumi Kashiyama (University of Tokyo)The repeating fast radio burst and the young neutron star modelRinko room, Main Building (East) / 13:30Ogihara
05/29ShingChi Leung (IPMU)Pulsation Pair-instability Supernova: Connection to massive black hole, circumstellar medium and collapsarLecture room / 13:30Sasaki
06/05Doris Arzoumanian (Nagoya University)Observed properties of nearby molecular filamentsLecture room / 13:30Kusune
06/12Teppei Minoda (Nagoya University)The effect of the primordial magnetic fields on the cosmic microwave background anisotropyLecture room / 13:30Kusune
06/26TBARinko room / 13:30
07/03Shota Notsu (Kyoto University)Possibility to locate the position of the H2O snowline in protoplanetary disks through spectroscopic observationsLecture room / 13:30Ogihara
07/10Riouhei Nakatani (University of Tokyo)Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence of UV and X-Ray PhotoevaporationLecture room / 13:30Kusune
07/24Shunsuke Ideguchi (NAOJ)Basics of Faraday Tomography Technique and Its Applications to Cosmic Magnetism StudyRinko room, Main Building (East) /13:30Sasaki
10/02Richard Teague (University of Michigan)Observing the Kinematics of Planet-Disk Interactions with ALMALecture room / 13:30Kusune
10/11Hector O. Silva (Montana State University)Illuminating the strong-field regime of gravityConference room, N6 3F / 14:00ThursdayKusune
10/11George Papas (Sapienza University of Rome)Testing the Kerr hypothesis with QNMs and ring downsConference room, N6 3F / 14:00ThursdayKusune
10/16Kazunari Iwasaki (Osaka University)The formation of molecular clouds by compression of atomic gasesLecture room /14:30Kusune
10/30Yen-Chen Pan (NAOJ)Understanding Type Ia Supernova with UV SpectroscopyLecture room /13:30Takiwaki
11/12A guest???? /13:30???i
11/20Yuri Fujii (Nagoya University)??? /13:30Ogihara
01/08Hiroshi Kobayashi (Nagoya University)??? /13:30Ogihara

Confirmed speakers

Abstract

4/10 Adriana Pohl (Max-Planck Institute of Astronomy, Heidelberg Germany) Revealing the evolution of planet-forming disks with polarization observations
Recent observational instruments like VLT/SPHERE and ALMA have reached an unprecedented level of resolution and sensitivity. Meanwhile, even the direct observation of substructures in planet-forming disks is within reach, by which the disk evolution can be traced. Features such as gaps, rings, spiral arms and clumps can be either associated with embedded, but yet unseen forming planets, or be related to other internal, physical disk processes. In this talk, I will compare theoretical predictions of dust evolution models and planet-disk interaction processes with current multi-wavelength observations of planet-forming disks. To this end, detailed radiative transfer calculations are presented, which are employed to model observational signatures in disks. An emphasis is placed on polarization diagnostics, which facilitates the detection of light scattered by dust grains in the disk. The latter is a crucial ingredient to constraining the size and composition of dust grains, which is necessary to understand the earliest stages of planet formation.
04/17 Yoshiaki Kato (Riken) Radiation MHD Simulations of Waves and Vortices on the Sun and beyond
One of the long-standing problems in solar physics is to understand a mechanism which maintains the solar atmosphere. The chromosphere, a layer between the photosphere and the corona, is a key to unveiling the mystery of the solar atmosphere. It is yet to be revealed entirely by observations because the chromosphere has complex structure and rapid variability. Therefore, radiation magnetohydrodynamic (RMHD) simulations play a major role for understanding such a complexity, which is difficult to interpret physical processes. I introduce my recent publications on the effect of MHD waves associated with an isolated magnetic flux concentration (or a flux tube), which is anchored in the photosphere and extended over the corona. This is a classical problem which is extensively discussed in many literatures and probably the best example to understanding MHD waves. While all studies so far relied on inflicting driving forces in the photosphere, only a self-consistent RMHD simulation of the solar atmospheric layers from the surface convection zone to the corona can resolve the realistic nature of MHD waves. First, I present the generation and propagation of mostly slow mode waves, driven by magneto-convective processes in the deep photosphere and beneath it. This is so-called magnetic pumping process which generates slow modes that propagate upward and develop into shock waves in the chromosphere. The magnetic pumping is a robust mechanism for generating shock waves in the vicinity of strong flux tube at the chromospheric height and therefore it’s most likely to sustain the chromosphere. Second, I present the identification of torsional waves in the chromosphere and the corona. Vortical flows in the upper convection zone and the photosphere force magnetic field structures to rotate and thus produce so-called solar “magnetic tornadoes”, which extend into the corona. Unlike slow modes, large portions of torsional modes can reach the corona without suffering significant dissipation and therefore it’s capable of sustaining the corona. Third, I present the detection of physical phenomena in the flux tube by magnetic pumping imprinted on the spectral lines. Thanks to the rapidly advancing solar observations over the past decades, we will have an unique opportunity to grasp the quantitative nature of MHD waves in the near future. It will enable us to extend our knowledge of plasma into those of the other astrophysical objects. Finally, I will briefly talk about the future perspective on my research in the next decades.
04/24 Akihiro Suzuki (NAOJ) Multi-dimensional modeling of supernova ejecta with a central energy source
Core-collapse supernova explosions are of fundamental importance in the universe. They are an outcome of massive star formation and evolution and at the same time affect their surrounding environments in various ways. This is the reason why many supernova researches and surveys have been intensively conducted. One of the remarkable successes of modern transient survey programs is the discovery of an extremely bright class of core-collapse supernovae, called superluminous supernovae. Because of their high brightness, we can detect high-z events, potentially making it possible to probe star-forming activity even in the high-z universe. However, the problem is that the energy source of their bright emission is still debated. A promising scenario for superluminous supernovae is the central engine scenario, in which the compact remnant (highly rotating neutron star, black hole accretion disk, or whatever) left in the supernova ejecta play a role in giving rise to bright thermal emission. However, there are many remaining problems, such as, how exactly the additional energy deposition is realized and how the supernova ejecta with a central engine evolve. I’m lately investigating the hydrodynamic evolution of supernova ejecta with such a central energy source by using multi-dimensional numerical simulations. In this talk, after a brief introduction of supernovae, I present results of my recent studies.
05/08 Yoshiyuki Inoue (Riken) Coronal Magnetic Activity in a Nearby Active Supermassive Black Hole
Black hole coronae are believed to be heated by their magnetic activity like the Sun. However, magnetic fields in the vicinity of active supermassive black holes have never been measured. Recently, we proposed a coronal radio synchrotron emission model for Seyfert galaxies. Here, we report the first detection of coronal radio synchrotron emission from a nearby Seyfert galaxy, which enables us to estimate the coronal magnetic field strength. We also found that coronae are composed of thermal and non-thermal electrons. Our results indicate that magnetic activity cannot sustain X-ray emitting coronae. Existence of non-thermal electrons in coronae implies that Seyfert galaxies may explain not only the cosmic X-ray background radiation but also the cosmic MeV gamma-ray background radiation.
05/10 Keiichi Maeda(Kyoto University) Progenitor Evolutions and Explosion Mechanisms of Type Ia Supernovae
I will provide a review on the current status of observational constraints on the progenitor systems and explosion mechanisms of type Ia Supernovae (SNe Ia). Recent development in the field is highlighted by accumulating observational discoveries of diversities found for SNe Ia. An idea is emerging that SNe Ia are perhaps not at all a uniform system as previously believed for many years, and thermonuclear explosions may lead to various outcomes which could correspond to various types of transients. In this talk, I will summarize different observational constraints placed for different sub-types of SNe Ia and related phenomena, and connect these findings to different types of progenitors and modes of explosions which have been theoretically predicted.
05/15 Kyohei Kawaguchi (ICRR) Radiative-transfer simulation for the optical and near-infrared electromagnetic counterparts to GW170817
Recent detection of gravitational waves from a binary-neutron star merger (GW170817) and the subsequent observations of electromagnetic counterparts provide a great opportunity to study the physics of compact binary mergers. The optical and near-infrared counterparts to GW170817 are found to be consistent with a kilonova/macronova scenario with red and blue components. However, in most of previous studies in which contribution from each ejecta component to the lightcurves is separately calculated and composited, the red component is too massive as dynamical ejecta and the blue component is too fast as post-merger ejecta. In this talk, I present our recent works performing 2-dimensional radiative-transfer simulations for a kilonova/macronova consistently taking the interplay of multiple ejecta components into account, and show that the lightcurves of optical and near-infrared counterparts can be reproduced naturally by a setup consistent with the prediction of the numerical-relativity simulations.
05/22 Kazumi Kashiyama (University of Tokyo) The repeating fast radio burst and the young neutron star model
Fast radio bursts (FRBs) are enigmatic radio transients with large dispersion measure. There are ~ 30 of FRBs discovered so far and only one of them, FRB121102, repeats. Recently, the host galaxy and persistent radio counterpart have identified for this repeating FRB. First, I will overview the observational results and the general implications. Then, I will focus on the young neutron star model for FRBs and discuss how one can test the model observationally. In particular, I will discuss the possible connection between FRBs and energetic supernovae.
06/05 Doris Arzoumanian (Nagoya Universiry) Observed properties of nearby molecular filaments
The highly filamentary structure of the interstellar medium is impressively revealed by the unprecedented quality and sky coverage of Herschel and Planck images tracing the Galactic cold dust emission. These observations provide the required data to describe in detail the properties of the filamentary structures observed in both quiescent clouds and in star forming regions, where the densest filaments appear to be the main sites of star formation. The omnipresence of filaments in observations as well as in numerical simulations suggests that the formation of filamentary structures is a natural product of the interplay between interstellar shock waves, gravity, and magnetic fields. The detailed description of their observed properties is important to improve our understanding of their formation and evolution process. I will present what we have learned about the properties of the filamentary structures derived from Herschel dust continuum and ground based single dish molecular line observations, and I will discuss the observational constraints on the formation and evolution of molecular filaments.
06/12 Teppei Minoda (Nagoya University) The effect of the primordial magnetic fields on the cosmic microwave background anisotropy
The magnetic fields are ubiquitous on the astronomical objects, from asteroids to clusters of galaxies. The origin of these cosmic magnetic fields is unknown, however, while the magnetogenesis in the early universe (such as inflation, the cosmic phase transition, the perturbation evolution, and so on) might be able to explain it. Magnetic fields generated by such cosmological mechanisms are called the primordial magnetic fields (PMFs), and some papers have pointed that the PMFs induce the matter density fluctuation due to their Lorentz force, and also affect the gas temperature distribution through a magnetic dissipation, so-called “ambipolar diffusion”. We consider these effects and calculate the time evolution of the baryon gas density and temperature before the formation of the first stars and galaxies. And we suggest a method to investigate such primordial gas structure with the cosmic microwave background (CMB) temperature anisotropy. In this talk, I will briefly introduce the PMFs and its general formalism, and review some basic concepts of the standard cosmology and CMB physics. Next, I will talk about the effect of the PMFs on the baryon gas history, and finally show the calculation method and the results of my recent work (Minoda et al., 2017, Phys. Rev. D, 96, 123525).
07/03 Shota Notsu (Kyoto University) Possibility to locate the position of the H2O snowline in protoplanetary disks through spectroscopic observations
Observationally locating the position of the H2O snowline in protoplanetary disks is important for understanding the planetesimal and planet formation processes, and the origin of water on Earth. The velocity profiles of emission lines from disks are usually affected by Doppler shift due to Keplerian rotation. Therefore, the line profiles are sensitive to the radial distribution of the line-emitting regions. In our studies (Notsu et al. 2016, ApJ, 827, 113; 2017, ApJ, 836, 118; 2018, ApJ, 855, 62), we calculated the chemical composition of the disks around a T Tauri star and a Herbig Ae star using chemical kinetics, and then the H2(16)O and H2(18)O line profiles. We found that lines with small Einstein A coefficients and relatively high upper state energies are dominated by emission from the hot midplane region inside the H2O snowline, and therefore through analyzing their profiles the position of the H2O snowline can be located. In addition, we found that H2(18)O lines trace deeper into the disk than H2(16)O lines since the number density of H2(18)O is low (Notsu et al. 2018). Thus these H2(18)O lines are potentially better probes of the position of the H2O snowline at the disk midplane, depending on the dust optical depth. Moreover, H2(18)O and para-H2(16)O lines with relatively lower upper state energies (~a few 100K) can also locate the position of the H2O snowline. There are several candidate water lines that trace the position of the H2O snowline in ALMA Bands 5−10. Finally, we have proposed the water line observations for a Herbig Ae disk HD163296 in ALMA Cycle 3, and partial data were delivered. We constrain the line emitting region (the location of the H2O snowline) and the dust properties from the observations.
07/10 Riouhei Nakatani (University of Tokyo) Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence of UV and X-Ray Photoevaporation
Protoplanetary disks are thought to have lifetimes of several million yr in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low-metallicity environment. We perform a suite of radiation hydrodynamics simulations of photoevaporating disks with varying the metallicity to study their long-term evolution and the metallicity dependence of mass-loss rates. Our simulations follow hydrodynamics, radiative transfer, and nonequilibrium chemistry in a self-consistent manner. Dust-grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-) emission. In the fiducial case with solar metallicity, including the X-ray effects does not significantly increase the photoevaporation rate when compared to the case with ultra-violet (UV) radiation only. At sub-solar metallicities in the range of Z ≳ 10^{-1.5} Zsun, the photoevaporation rate increases as metallicity decreases owing to the reduced opacity of the disk medium. The result is consistent with the observational trend that disk lifetimes are shorter in low metallicity environments. Contrastingly, the photoevaporation rate decreases at even lower metallicities of Z ≲ 10^{-1.5} Zsun, because dust-gas collisional cooling remains efficient compared to far UV photoelectric heating whose efficiency depends on metallicity. The net cooling in the interior of the disk suppresses the photoevaporation. However, adding X-ray radiation significantly increases the photoevaporation rate, especially at Z ~ 10^{-2} Zsun. Although the X-ray radiation itself does not drive strong photoevaporative flows, X-rays penetrate deep into the neutral region in the disk, increase the ionization degree there, and reduce positive charges of grains. Consequently, the effect of photoelectric heating by far UV radiation is strengthened by the X-rays and enhances the disk photoevaporation.
07/24 Shunsuke Ideguchi (NAOJ) Basics of Faraday Tomography Technique and Its Applications to Cosmic Magnetism Study
The synchrotron radiations from various astronomical objects and their Faraday rotation allow us to obtain the information about magnetic fields of the objects and of the media between them and us. The low-frequency, wide-band polarization observations made with the next-generation telescopes represented by Square Kilometer Array (SKA) and its precursors/pathfinders make it possible to use Faraday rotation to create a tomographic reconstruction of magnetized structures along the line of sight, a technique known as Faraday tomography. In this talk, I will introduce the basics of the technique and overview the cosmic magnetism studies using the technique so far.
10/02 Richard Teague (University of Michigan) Observing the Kinematics of Planet-Disk Interactions with ALMA
ALMA has undoubtedly revolutionised our understanding of planet formation. It has demonstrated that substructures in the both the gas and dust distributions are ubiquitous, indicative of on-going, unseen planet formation. However, our ability to recreate these substructures with a myriad of mechanisms and perturbers, ranging from multiple-planet systems to planet free ones, leaves us unable to distinguish between scenarios. I will present recent work in which we use the motions of the gas to provide a unique view of planet-disk interactions. I will show how we can measure velocities to a sub-percent level precision, allowing us to infer accurate pressure and density profiles for these disks. With these constraints to hand we are able beginning to be able to differentiate between these scenarios and understand the connections between the dust substructures we see, both at the midplane and in the disk atmosphere, and unseen planets.
10/11 Hector O. Silva (Montana State University) Illuminating the strong-field regime of gravity
Observation of the x-ray pulse profile emitted by hotspots on the surface of neutron stars offers a unique tool to measure the properties of these objects, including their masses and radii. The x-ray emission takes place at the star’s surface, where the gravitational field is strong, making these observations an incise probe into the spacetime curvature generated by these stars. In this presentation, I will discuss how general relativity plays a key role in the accurate modelling of pulse profiles and the prospects for testing Einstein's theory - and some of its contenders - using these observations.
10/11 George Pappas (Sapienza University of Rome) Testing the Kerr hypothesis with QNMs and ringdowns
The Kerr spacetime that describes all rotating black holes is one of the most important solutions of general relativity. The theoretical and astrophysical significance of this solution cannot be underestimated. For this reason it is of analogous importance to thoroughly test whether the objects that we have identified as the astrophysical incarnations of Kerr black holes are actually that or some alternative exotic compact object that simply mimics aspects of their behaviour. With the advent of gravitational wave astronomy, this is possible by observing the inspiral, merger, and ringdown of binary systems. This talk will discuss some ways that we can use to test for these impostors.
10/16 Kazunari Iwasaki (Osaka University) The formation of molecular clouds by compression of atomic gases
The formation of molecular clouds is one of the fundamental building blocks in star formation. In order to determine the initial condition of the star formation,it is crucial to reveal the formation and evolution processes of molecular clouds. We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamical simulations including non-equilibrium chemical reactions and heating/cooling processes. We consider super-Alfvenic head-on colliding flows of the atomic gas possessing the two-phase structure. We examine how the molecular cloud formation changes depending on the angle between the upstream flow and mean magnetic field. If the atomic gas is compressed almost along the mean magnetic field, the accretion of the highly inhomogeneous upstream atomic gas drives a super-Alfvenic velocity dispersion which decreases the mean density of the post-shock layer. Even a small obliqueness of the magnetic field weakens the post-shock turbulence. As a result, the post-shock layer becomes denser than that formed by a colliding flow almost aligned to the magnetic field. If the magnetic field is further inclined to the upstream flow, the shock-amplified magnetic pressure suppresses gas compression, leading to an extended post-shock layer. Our results, therefore, show that there is a critical angle which maximizes the mean density of the post-shock layers. Developing an analytic model and performing a parameter survey, we derive an analytic formula of the critical angle as a function of the mean density, collision speed, and field strength of the upstream atomic gas. We also found that the dependence of the post-shock layers on the angle causes a diversity of the physical properties of dense clumps.
10/30 Yen-Chen Pan (NAOJ) Understanding Type Ia Supernova with UV Spectroscopy
Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are useful tools for understanding progenitor systems and explosion physics. In particular, UV spectra of SNe Ia, which probe the outermost layers, are strongly affected by the progenitor metallicity. Theory suggests that SN Ia progenitor metallicity is correlated with its peak luminosity, but not its light-curve shape. This effect should lead to an increased Hubble scatter, reducing the precision with which we measure distances. If the mean progenitor metallicity changes with redshift, cosmological measurements could be biased. Models also indicate that changing progenitor metallicity will have little effect on the appearance of optical SN data, but significantly alter UV spectra. To address this problem, we reduced and published the largest UV spectroscopic sample of SNe Ia to date. With this sample, we confirm theoretical predictions that SN Ia UV spectra are strong metallicity indicators. Our findings show that UV spectra are promising tools to further our understanding of SN Ia while directly improving the utility of SN Ia for cosmology.