第1708回 2020年7月14日(火) 16:15-17:15

Title: Development of Monte Carlo Based X-Ray Clumpy Torus Model and Its Applications to Nearby Obscured Active Galactic Nuclei

Speaker: Atsushi Tanimoto 谷本 敦 (Department of Physics, The University of Tokyo)

Language: English

The unification model of an Active Galactic Nucleus (AGN) indicates the ubiquitous presence of dusty clumps (clumpy torus) around the accreting Supermassive Black Hole (SMBH). This torus is a key structure to understand the mechanisms of the coevolution between the SMBH and the host galaxy. This is because the torus is considered as a mass reservoir that feeds mass onto the SMBH from the host galaxy. However, the basic properties of AGN tori (e.g., the spatial distribution of matter and the gas-to-dust ratio) are still unclear. In this talk, we present (1) constructing an X-ray spectral model from a realistic clumpy torus (XClumpy: Tanimoto et al. 2019) and (2) its applications to nearby obscured AGNs (Tanimoto et al. 2020). (1) We constructed the XClumpy model utilizing the Monte Carlo simulation for Astrophysics and Cosmology (MONACO) for the first time. We investigated the dependence of the X-ray continuum and the Fe Ka fluorescence line profile on the torus parameters. (2) We applied the XClumpy model to the broadband X-ray spectra of 10 obscured AGNs. The infrared spectra of these AGNs were analyzed by Ichikawa et al. with the CLUMPY code. Because the XClumpy model adopts the same clump distribution as that in CLUMPY, we can directly compare the torus parameters obtained from the X-ray spectra and those from the infrared spectra. The torus angular widths determined from the infrared spectra are systematically larger than those from the X-ray spectra. The difference correlates with the inclination angle determined from the X-ray spectrum. These results imply that the dusty polar outflows contribute to the observed infrared flux.

(This talk will be given online. Details will be announced via e-mail.)


第1709回 2020年10月13日(火) 16:15-17:15

Title: Enrichment of Heavy Elements in Dwarf Galaxies and the Milky Way

Speaker: Yutaka Hirai 平居 悠 (RIKEN 理化学研究所)

Language: English

Abundances of heavy elements in metal-poor stars help us understand their astrophysical sites and evolutionary histories of galaxies. High-dispersion spectroscopic observations have identified that abundances of neutron-capture elements such as Sr, Ba, and Eu show star-to-star scatters in extremely metal-poor stars in the Milky Way while there is an increasing trend toward lower metallicity in the abundance of Zn. However, their astrophysical sites and the enrichment in galaxies are not well understood. Here I will show the enrichment of heavy elements of dwarf galaxies and the Milky Way using N-body/smoothed particle hydrodynamics simulations. The results suggest that binary neutron star mergers can contribute to the enrichment of Sr, Ba, and Eu for [Fe/H] < −2. Likewise, this study finds that ejecta from supernovae by low-mass progenitors can form stars with high [Zn/Fe] and [Sr/Ba] ratios. In this talk, I will also discuss relationships among abundances of elements, kinematics of stars, and the Milky Way formation.

(This talk will be given online. Details will be announced via e-mail.)


第1710回 2020年10月27日(火) 16:15-17:15

Title: Observing the interplanetary dust grains as meteors

Speaker: Ryo Ohsawa 大澤 亮 (IoA 天文センター)

Language: Japanese (日本語)

Our solar system is filled with plenty of small dust grains, which are generated by small solar system bodies such as comets and asteroids. They are commonly referred to as interplanetary dust. The Earth is orbiting through the interplanetary dust. A number of dust grains are expected to continuously collide with the Earth’s atmosphere. Part of the grain’s kinetic energy is emitted in a form of light or used to ionize the ambient atmosphere, which is recognized as a meteor phenomenon. Measuring the total energy of a meteor brings an estimate of the mass of a single interplanetary dust grain corresponding to the meteor. Meteor observation is a powerful tool to investigate the mass frequency distribution of the interplanetary dust. Faint meteors have been widely observed by radar. A meteor head echo observation is very useful to detect faint meteors caused by small grains, as faint as 10 mag in the visible band. How faint meteors are detected by radar had, however, not been firmly confirmed in any simultaneous observations. This is mainly due to insufficient sensitivity of optical observations. Kiso Observatory, the University of Tokyo, has developed a wide-field mosaic CMOS camera, Tomo-e Gozen, which is able to monitor a wide area of the sky at up to 2 fps. The performance of Tomo-e Gozen enables us to measure the brightness of the same meteors detected by radar. In the presentation, we present the result of simultaneous observations with Tomo-e Gozen and the MU radar carried out in April, 2018. We concluded that 228 meteors were simultaneously detected by radar and in optical. By combining the previous observations, we derive the relationship to convert the radar cross sections into the optical brightness. This illustrates that the meteors detected by the MU radar are as faint as about 9.5 mag in the visible band, corresponding to 10 micrograms in mass. The total mass falling onto the Earth as meteors is estimated to be about 1000 kg/day.

(This talk will be given online. Details will be announced via e-mail.)


第1711回 2020年11月10日(火) 16:15-17:15

Title: Evolution of Low- and Intermediate-mass Stars with Neutrino Magnetic Moment

Speaker: Kanji Mori 森 寛治 (東大天文 D3/国立天文台 NAOJ)

Language: Japanese (日本語)

Observations of the neutrino oscillation have revealed that neutrinos have masses. Particle theories predict that massive neutrinos have magnetic moment, although its value is experimentally unknown. Since the Standard Model of particle physics assumes massless neutrinos, detection of neutrino magnetic moment (NMM) is a key to physics beyond the Standard Model. If NMM is finite, an additional energy loss channel becomes open in stellar plasma and thus stellar evolution is affected. In the first part of this talk, I adopt the effect of NMM on 7—10 M stars, which form a loop during central He burning. It is found that the blue loop is eliminated when NMM is adopted. Since a part of stars in the blue loop can be observed as Cepheid variables, the elimination of the blue loop results in the lack of observed Cepheids. In order to avoid the elimination of the blue loop, NMM should be in the range of 2×10-10μB to 4×10-11μB, where μB is the Bohr magneton. In the second part, I adopt the effect of NMM on 1 M stars. Recently, it is pointed out that observed Li abundances in low-mass red clump stars are significantly larger than model prediction. I found that the additional energy loss induced by NMM helps internal mixing in stars near the tip of the red giant branch and enhance the Li abundance. It is concluded that NMM of (2—5)×10-12μB mitigates the Li problem in low-mass stars.

(This talk will be given online. Details will be announced via e-mail.)


第1712回 2020年12月15日(火) 16:15-17:15

Title: New Theoretical Insights into the Nonthermal Sky Based on Observations of Very-High-Energy Particles

Speaker: Takahiro Sudoh 須藤 貴弘 (天文学教室 D3)

Language: English (英語)

The Milky Way has natural particle accelerators capable of producing TeV-PeV particles. Observations of gamma rays are key to understanding the nature of them. In this talk, I will discuss theoretical implications from recent observations of very-high-energy gamma rays. In particular, we discuss implications from the detection of “TeV halos”, a source class recently identified around nearby pulsars. We show that pulsar emission can dominate the TeV gamma-ray sky, explaining the majority of source counts and diffuse emission from the Milky Way. We also show that recycled/millisecond pulsars can dominate the radio emission in quiescent galaxies, producing a mass-dependence in radio-SFR correlation, which is reported by recent radio observations. We discuss the implications of our findings for our understanding of cosmic rays, gamma rays, and multiwavelength sky.

(This talk will be given online. Details will be announced via e-mail.)


第1713回 2020年12月22日(火) 16:15-17:15

Title: New Gravitational Wave Astronomy and Detection

Speaker: Takayuki Tomaru 都丸 隆行 (国立天文台 NAOJ/東大天文)

Language: Japanese (日本語)

Gravitational wave (GW) astronomy was opened by the first direct detection of GW by advanced LIGO in 2015. Until now, 50 GW events has been reported. This is almost 1 event detection per week, and most of the events are binary black-hole merger. Since GW observation is a really new field, the sciences are spread over physics and astronomy. In the view point of multi-messenger astronomy, present GW observation is insufficient because its localization is too poor. For example, only one follow-up observation by electromagnetic wave telescopes was successfully achieved in GW170817, and no successful follow-up observation was achieved in Observation-3a (O3a). This comes from poor angular resolution of GW telescopes, and the global observation network by GW telescopes, two aLIGO detectors, aVirgo, KAGRA and future LIGO-India, is critical in future observation. Large-scale cryogenic gravitational wave telescope (KAGRA) project in Japan started from 2010, when is about 15 years behind from LIGO and VIRGO construction. KAGRA construction was completed in 2019, however, KAGRA could not join in O3. We will join it in O4. We still continues our effort to increase the KAGRA sensitivity, 25Mpc binary neutron star range in O4, and intend to improve event localization. And an advantage of KAGRA is that it equips some advanced technologies and features. Two major features in KAGRA are underground site to reduce seismic noise and cryogenic sapphire mirror to reduce thermal noise. And in this year, we also successfully developed a new technology to reduce photon noise (quantum noise) by using TAMA300 in NAOJ. These technologies can give large impact to future GW detectors in the world. In this seminar, I’d like to explain new GW astronomy and detector technologies in KAGRA.

(This talk will be given online. Details will be announced via e-mail.)


第1714回 2021年1月12日(火) 16:15-17:15

Title: Collective neutrino oscillation with symmetry breaking in supernova neutrinos

Speaker: Masamichi Zaizen 財前 真理 (天文学教室 D2)

Language: Japanese (日本語)

High neutrino fluxes are emitted from core-collapse supernovae after core bounce, including various information inside the environments. However, the neutrino self-interaction becomes potentially dominant near the core and collective flavor conversion is induced. The neutrino oscillation prevents us from understanding original flavor states in the neutrino observation. The neutrino-neutrino interaction is a 7-D non-linear problem and it is hard to simulate it correctly. It has been numerically calculated using bulb model, which requires some symmetries and assumptions in position and momentum space. The fact that high dense background matter can suppress the collective behaviors under the bulb model has been reported by many previous works. On the other hand, linear stability analyses have recently revealed that spontaneous symmetry breaking produces more unstable modes and can overcome the matter-induced suppression. Then we extended the axial-symmetric bulb model into three-dimensional momentum-space and investigated the impact on collective flavor conversions numerically. We found non-axisymmetric modes evolve and lead to flavor conversions at epochs when it is completely suppressed under the axial-symmetric case. However, we also found the non-axisymmetric flavor instability cannot grow sufficiently due to the dense background matter at certain time snapshots. These behaviors alter the suppression description and provide additional time evolution in the signal prediction at current and future neutrino detectors.

(This talk will be given online. Details will be announced via e-mail.)


第1715回 2021年1月26日(火) 16:15-17:15

Title: クェーサーの輝線で探る宇宙の化学進化

Speaker: Hiroaki Sameshima 鮫島寛明 (東大・天文センター)

Language: Japanese (日本語)

初期宇宙における星形成の様子を明らかにすることは、現代天文学が抱える大きな課題の一つである。星は質量等によって異なる種類の超新星爆発を起こし、様々な重元素を周囲に供給する。特に放出されるα元素と鉄の組成比[α/Fe]は超新星爆発の種類によって大きく異なるため、その値が宇宙論的タイムスケールでどの様に進化したかを調査することで、星形成の歴史についても情報が得られると期待される。そこで我々は、遠方にあっても明るく、複数の強い金属輝線を持つクェーサーを使って、過去の宇宙における[α/Fe]の進化の様子を調査している。従来この分野ではFeII/MgII輝線強度比が直接組成比を反映しているという仮定のもとで観測が進められてきたが、我々は光電離シミュレーションを介して輝線強度から組成比に変換する方法を考案し、化学進化モデルとの直接比較を初めて可能にした。本講演ではスローンデジタルスカイサーベイのアーカイブデータを用いた 0.7 < z < 1.6 での結果に加えて、チリ共和国の新技術望遠鏡を使って近赤外線分光観測した z ~ 2.6 のクェーサー6天体の結果を紹介する。また、天文学教育研究センターが現在取り組んでいる、チリ共和国北部 の標高5640 mに口径6.5 mの望遠鏡を設置するTAOプロジェクトと、それに伴う本研究の将来計画についても紹介する。

(This talk will be given online. Details will be announced via e-mail.)