天文学教室談話会(2019年度)

last-update:2019/07/16

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  第1679回 2019年4月16日(火) 16:00-17:00 (language: English)

Asteroseismology of fast-rotating stars : the contribution of two-dimensional calculations
Giovanni Mirouh (University of Surrey, United Kingdom)
Massive stars directly and indirectly drive the chemical enrichment of the interstellar medium. These stars usually exhibit rapid differential rotation, and are known pulsators. Because of rotation, oscillations in these stars are hard to interpret and modes cannot be readily identified, preventing us from using well-established asteroseismology techniques. In this talk, I will present recent advances in the description of oscillations in such stars. After a general introduction, I will describe low-frequency oscillations in presence of differential rotation: this rotation affects their propagation and is even responsible for the excitation of some modes. I will then discuss higher-frequency oscillations, that can be split into different categories. Using a convolutional neural network, it was possible to classify these modes automatically and derive a scaling law linking regular patterns in the spectra to stellar properties. Finally, I will suggest future work needed for realistic seismic modelling of fast-rotating stars.

  第1680回 2019年4月18日(木) 15:00-16:00 (language: English)

A Novel Modeling of Magneto-Rotating Stellar Evolution
髙橋 亘 / Koh Takahashi (Albert-Einstein-Institut, Potsdam, Germany)
About 10% of massive and intermediate-mass main-sequence stars posses strong surface magnetic fields, and the magnetic massive stars may be progenitors of strongly magnetized neutron stars known as magnetors. However, the evolution of magnetic fields in stellar interiors remains a big open question for the stellar evolution theory. We are developing a new stellar evolution code which is capable to follow a long-timescale evolution of stellar magnetism. Because of the far different timescales between the MHD and the evolutionary times, high degree of simplification in the modeling is inevitable. First, we assume that the configuration of the stellar magnetic field can be approximated by axially symmetric toroidal + poloidal components, which explicitly has only a radial dependence. Then the evolution of the two component magnetic field is described by the mean-field dynamo equation. The new formalism self-consistently includes the effects of omega-dynamo, which results from large scale shear in the rotation flow, magnetic dissipation, and angular momentum transfer due to magnetic stress. We will present our preliminary results and discuss how the model can be verified by observations.

  第1681回 2019年4月23日(火) 16:00-17:00 (language: English)

The Formation of Rings and Gaps in Magnetized Wind-Launching Disks
Scott Suriano (University of Tokyo)
Radial substructures in circumstellar disks are now routinely detected by state-of-the-art observational facilities. There is also growing evidence that large-scale magnetic fields threading disks are responsible for launching disk winds and driving accretion. We investigate how rings and gaps form in magnetized disks through non-ideal MHD simulations. In axisymmetric 2D simulations including either Ohmic resistivity or ambipolar diffusion (AD), prominent features form in the disk surface density with a strong radial variation of the poloidal magnetic flux relative to the mass. Regions with low mass-to-flux ratios accrete quickly and lead to the development of gaps, whereas regions with higher mass-to-flux ratios accrete more slowly, allowing matter to accumulate and form dense rings. Specifically, in the AD-dominated disks, the radial variation of the magnetic flux is set by the reconnection of a highly elongated poloidal magnetic field across a thin midplane current sheet, through which fast laminar accretion occurs. We extend the simulations of AD-dominated disks to 3D and find that rings and gaps still develop naturally from the same basic mechanism that was identified in 2D. The rings and gaps remain stable in 3D for a few thousand orbital periods at the inner edge of the simulated disks, making them attractive sites for trapping large grains that would otherwise be lost to rapid inward radial migration.

  第1682回 2019年5月7日(火) 16:00-17:00 (language: English)

WR DustERS: a JWST-ERS program to resolve the nature of dust in Wolf-Rayet binary winds
Ryan M. Lau (ISAS/JAXA)
Thermal infrared (IR) emission from dust is a key probe of the evolution and death of short-lived, massive stars. Dust itself is a key component of the interstellar medium; however, the dominant channels of dust production throughout cosmic time are uncertain. In this talk, I will discuss our JWST Director’s Discretionary Early Release Science (DD ERS) program, where we will investigate the formation mechanism and chemical composition of dust formed in the colliding winds of a Carbon-rich Wolf-Rayet (WC) binary. With dust production rates ranging from 10-8 – 10-6 M/yr, such massive stellar binaries may have a significant influence on the dust abundance of galaxies in both the local and early Universe. Dust abundances, composition and formation pathways in the hostile and luminous environment around WC+OB binaries are, however, uncertain due to observational challenges in achieving both high spatial resolution and sensitivity in the mid-IR. Our planned JWST/MIRI+MRS and NIRISS+AMI observations of the archetypal periodic dust forming Wolf-Rayet binary system WR140 will address these uncertainties and also demonstrate the utility of these observing modes for IR bright targets with faint extended emission.

  第1683回 2019年5月14日(火) 16:00-17:00 (language: English)

Inefficient Magnetic Accretion Heating in Protoplanetary Disks
森 昇志 (天文学教室) / Shoji Mori (Department of Astronomy)
The structures of the inner region of protoplanetary disks are essential for understanding the formation process of terrestrial planets. The gas temperature in the region is thought to be determined by accretion heating, which is conventionally attributed to turbulent dissipation. However, recent studies have suggested that the inner disk (a few AU) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation.
We perform local stratified MHD simulations including nonideal MHD effects and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that most of the accretion energy is used to drive disk winds, with the remaining for Joule heating that typically occurs at several scale heights above the midplane, making the midplane temperature much lower than that with the conventional viscous heating model. Our results suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating. We also discuss the evolution of the water snow line based on our results and the formation process of the Earth.

  第1684回 2019年5月21日(火) 16:00-17:00 (language: English)

The strongest magnetic fields in sunspots
岡本 丈典 (国立天文台) / Joten Okamoto (National Astronomical Observatory of Japan)
Sunspots are concentrations of magnetic fields on the solar surface. Then, where is the strongest field in each sunspot ? It is generally located in an umbra, but sometimes stronger fields are found outside umbrae, such as a penumbra and a light bridge. The formation mechanism of such strong fields outside umbrae is still puzzling. Now we have numerous high-quality datasets taken with the Hinode/Spectro-Polarimeter over 10 years, which motivate us to address this question via a statistical analysis of strongest fields in sunspots. Hence, we complied a ranking list of active regions by their largest field strengths and investigated conditions for appearance or formation of strong magnetic fields. In this seminar, we will introduce a sunspot with a field strength of 6250 G as a case study, and then discuss the key features to produce strong fields in a statistical sample.

  第1685回 2019年5月28日(火) 16:00-17:00 (language: Japanese)

銀河系中心領域に発見された中間質量ブラックホールの痕跡
竹川 俊也(国立天文台)
大部分の銀河はその中心に超大質量ブラックホール(SMBH)を有すると考えられているが、その形成・成長過程は未だよくわかっていない。これらSMBHは、中間質量ブラックホール(IMBH)を”種”として、質量降着および合体を繰り返すことで成長してきた可能性がある。しかし、これまでIMBH候補天体の発見は、矮小銀河および球状星団の中心核や超高輝度X線源など複数報告例があるものの、いずれの結果に対しても異を唱える報告があり、IMBHの存在は未だ確定的ではない。我々は銀河系中心領域において、空間的にコンパクトかつ異常に大きな速度幅を持つ高速度コンパクト雲(HVCC)と呼ばれる特異分子雲を多数発見している。そしてその一部は、重いIMBHと分子ガスとの重力相互作用の結果生じたものであるという可能性が近年浮上してきた。2018年、我々は銀河系中心核近傍に発見された2つのHVCCについて、ALMAによる詳細な分子輝線観測を行なった。その結果、これらHVCCは明瞭な軌道運動を示す分子ガス流から成ることがわかった。ガス流の軌道解析および同方向に他波長対応天体が見当たらないことから、そこには非活動的な重いIMBHが潜んでいる可能性が高い。本セミナーでは、HVCCに関する最近の研究成果を紹介し、HVCCが銀河系内に多数潜む暗いブラックホールを探し出す手がかりになる可能性について議論する。

  第1686回 2019年5月29日(水) 15:00-16:00 (language: English)

First M87 Event Horizon Telescope Results: The Shadow of the Supermassive Black Hole
水野 陽介 / Yosuke Mizuno (Goethe University Frankfurt)
The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. These images show a prominent ring with a diameter of ~40 micro-arcsecond, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. We also briefly discuss the possibility of the alternatives to a black hole for the central compact object.

  第1687回 2019年6月4日(火) 16:00-17:00 (language: English)

Type Ia supernova progenitors
山口 弘悦 (宇宙科学研究所) / Hiroya Yamaguchi (ISAS/JAXA)
Type Ia supernovae, widely believed to result from thermonuclear explosions of white dwarfs, are extremely important phenomena in the Universe, owing to their roles as distance indicators in cosmology and major sources of iron. However, many of their fundamental aspects, e.g., how their progenitors evolve and explode, remain elusive. I will review recent observational and theoretical work addressing this issue, and discuss future prospects for both electromagnetic and gravitational wave observations.

  第1688回 2019年6月11日(火) 16:00-17:00 (language: English)

How AGN radiative feedback may shape black hole-galaxy co-evolution
石橋 和貴子 / Wakiko Ishibashi (University of Zurich)
Active Galactic Nucleus (AGN) feedback is widely invoked in galaxy evolutionary models, while evidence of such AGN feedback in action is now observed in the form of galactic outflows. However, the physical mechanism driving AGN feedback remains ill-understood, and whether galactic outflows are powered by jets, winds, or radiation, is still a source of much debate. We consider AGN feedback driven by radiation pressure on dust. We show that such radiative feedback is capable of accounting for the observed dynamics and energetics of galactic outflows, provided that radiation trapping is properly taken into account.
Feedback from the central black hole is usually invoked to quench star formation in galaxies (the standard negative feedback paradigm). We consider the alternative possibility of triggering star formation in the host galaxy, within the feedback-driven outflows (a form of positive feedback). Such “AGN feedback-driven star formation” may contribute to the size and morphological evolution of galaxies over cosmic time. Recently, there has been growing observational evidence for such star formation occurring inside galactic outflows. I will discuss the multiple roles of AGN feedback in galaxy evolution, and how radiative feedback may ultimately shape the co-evolutionary path.

  第1689回 2019年6月18日(火) 16:00-17:00 (language: English)

How to Create Flare-productive Sunspots?
鳥海 森 (宇宙科学研究所) / Shin Toriumi (ISAS/JAXA)
Solar flares, especially the strong ones, emanate from active regions including sunspots, where high magnetic non-potentiality resides in a wide variety of forms. In this talk, I will introduce some of our recent works on the flare-productive sunspots: (1) the statistical survey on flare-productive spots [Toriumi et al. 2017]; (2) the 3D MHD simulations of such flaring spots [Toriumi & Takasao 2017]; and (3) the flare loop modeling and reproduction of SXR light curves [Reep & Toriumi 2017]. Based on these studies, we show that the twist and interaction of subsurface magnetic fields give rise to the flare-producing sunpots. In this talk, I will also discuss possibilities to apply our solar knowledge to the investigation of stellar activities.

  第1690回 2019年6月25日(火) 16:00-17:00 (language: English)

GALACTIC WINDS: Physics, Phenomenology, and Implications
Timothy Heckman (Johns Hopkins University)
I will review galactic winds driven by populations of massive stars. I will describe the basic physics that drives winds and dictates their impact on the interstellar and circum-galactic medium. I will describe observations of the multi-phase outflow in the proto-typical starburst galaxy M 82. I will then discuss the systematic dependence of wind properties on such key parameters as the star-formation-rate, galaxy size, and mass (circular velocity), and show that there explain the strong rise in the incidence rate of winds with redshift. I will also compare these empirical scaling relations with sub-grid recipes commonly adopted in models and simulations. Finally, I will describe observations that demonstrate the impact of winds on the chemical evolution of galaxies and on the properties of the circum-galactic medium.

  第1691回 2019年7月2日(火) 16:00-17:00 (language: English)

Machine Learning for Classification of Astronomical Data
Tilman Hartwig (物理学専攻 / Department of Physics)
I will give an overview of machine learning techniques and present decision trees as very efficient machine learning tool to classify astronomical data. A labelled training sample is split according to available features by requiring that each split minimises the information entropy of the assigned classes. This elegant mathematical formulation allows us to construct decision trees with supervised learning, which can then be applied to classify new observations.
Eventually, I will present recent results of my own research: by classifying the chemical abundance patterns of metal-poor stars in the Milky Way, we can derive the multiplicity of the first generation of stars in the Universe. Furthermore, this approach provides the feature importance to identify crucial chemical elements to classify metal-poor stars, which can be used to optimise future spectroscopic surveys of Milky Way stars.

  第1692回 2019年7月3日(水) 15:00-16:00 (language: English)

Direct Detection of Nano-Herz Gravitational Waves with Pulsar Timing Array
高橋 慶太郎 (熊本大学) / Keitaro Takahashi (Kumamoto University)
Pulsar timing array is a unique way to detect gravitational waves of nano-Hz frequencies and will greatly contribute to the multi-wavelength gravitational-wave astronomy. The major sources of nano-Hz are super-massive black hole binaries and cosmic strings, and we can probe structure formation of the universe in a different way from the other methods. In this talk, I will review the basic idea of pulsar timing array and the current status, and then give prospects for a future project, SKA (Square Kilometre Array).

  第1693回 2019年7月16日(火) 16:00-17:00 (language: English)

The shadow of M87’s super-massive black hole revealed by EHT
本間 希樹 (国立天文台) / Mareki Honma (NAOJ)
On 10 April 2019, the first-ever image of a black hole shadow has been released. The image, taken by the Event Horizon Telescope toward M87’s core, shows a ring-like structure corresponding the photon-sphere of the black hole. The image also shows a clear depression of its intensity at the ring center, the shadow of a black hole, visually demonstrating for the first time that even light cannot escape from a black hole. In this talk, I would like to summarize the EHT results on M87, by describing the properties of images as well as details of observations and data analysis.

  第1694回 2019年7月23日(火) 16:00-17:00 (language: English)

Young star clusters: a nursery of merging binary black holes
Ugo Niccolò Di Carlo (University of Insubria, Italy)
Searching for distinctive signatures, which characterize different formation channels and environments of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitational wave detections. In this talk, I will discuss the formation of merging BBHs in young star clusters (SCs), which are the nursery of massive stars. In particular, I will show the results of a large set of N-body simulations of SCs with a large primordial binary fraction and novel population-synthesis prescriptions. The simulated SCs have fractal initial conditions, to mimic the clumpiness of star forming regions. I will discuss the impact of star cluster dynamics on the properties (total masses, mass ratios and delay times) of merging BBHs. More than 50% of merging BBHs formed by dynamical exchange in our simulations. BBHs formed via exchanges are significantly more massive than BBHs formed from primordial binaries, reaching chirp masses larger than 30 Msun. In our simulations, dynamical exchanges are the only channel able to form merging BBHs similar to GW170729, the most massive gravitational wave event observed by LIGO-Virgo. Finally, stellar collisions lead to the formation of intermediate mass black holes (IMBHs) with mass up to 440 Msun. IMBHs represent <~ 0.1 % of all black holes in our simulations.