談話会 | 2025年度
定例の場合
・金曜日午後4時～5時（毎週行なうわけではありません）
・理学研究科4号館 5階 504号室（宇宙物理学教室講義室）
という時刻・場所で行なっています。 談話会でのトークを考えていらっしゃる方は、どの教室スタッフにでも良いので、コンタクトをお願いします。
世話人 Host: 前田 啓一 (email: keiichi.maeda_at_kusastro.kyoto-u.ac.jp)
第661回
Speaker: Tetsuya Hashimoto (National Chung Hsing University)
Date: 12/2 Tuesday
Time: 15:00 – 16:00
Place: Room 504 seminar room, building 4
Title: Observational cosmology using fast radio bursts
Abstract: Fast radio bursts (FRBs) are mysterious coherent radio pulses with millisecond timescales, most of which emerge from galaxies at cosmological distances. One of the unique observables of FRBs is the dispersion measure (DM), which is the free electron density integrated along the line of sight to an FRB. DM could offer a new independent method to constrain cosmology. This talk will include two topics about FRBs as cosmic probes:
(i) the missing baryon problem and (ii) the Hubble tension. The enigma of the missing baryons has been posing a prominent and unresolved problem in astronomy. FRBs’ DM revealed the missing baryons described in the Macquart (DM-z) relation. The scatter of this relation is anticipated to be caused by the variation of cosmic structure. However, this is not yet statistically confirmed. Here, we present the statistical evidence of the cosmological baryonic fluctuation by measuring the foreground galaxy number densities around localized FRBs with galaxy catalogues. We found a positive correlation between the excess of DM contributed by the medium outside galaxies and the foreground galaxy number density. Our findings indicate that baryonic matters outside galaxies exceed its cosmic average along the line of sight to high galaxy-density regions, whereas there is less amount of baryons along the line of sight to low-density regions, presenting the statistical evidence of the cosmological fluctuation of the missing baryons. Measuring the Hubble constant (H0) is one of the most important missions in astronomy. Nevertheless, recent studies exhibit the tension of H0 among different measurements.
DM measured by FRBs could open a new avenue to probe H0 because the cosmic average of the intergalactic component, DM_IGM, is proportional to H0. I will present a new methodology to constrain H0 by utilizing the temporal scattering of the FRB pulses. This method could reduce the systematic error of H0 measurements with FRBs by ~9%. This systematics is comparable to the Hubble tension, indicating the feasibility of our method to address the Hubble tension using future FRB samples. If time allows, I would like to discuss the new radio telescope in Taiwan, BURSTT, which is dedicated to FRB observations.
第660回
Speaker: Yuto Bekki (Max Planck Institute for Solar System Research)
Date: 11/28 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: The Dawn of Inertial-Mode Helioseismology
Abstract: The interior of the Sun can be studied with helioseismology, which analyzes oscillations observed on the solar surface. Conventionally, acoustic modes of oscillation have been used to probe the Sun’s internal structure and large-scale flows. Thanks to SDO/HMI data, low-frequency modes of inertial oscillations have been observed on the Sun. These modes can also be used as probes of the interior of the Sun. In this talk, we review recent advances in modelling the solar inertial modes, using both a linear eigenmode analysis of the differentially-rotating convection zone and using fully nonlinear numerical MHD simulations of the Sun. One remarkable result is that the high-latitude inertial modes interact dynamically with the Sun’s differential rotation and the magnetic field.
第659回
Speaker: Yusuke Tampo (South African Astronomical
Observatory)
Date: 10/17 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Cataclysmic variables in the era of the Rubin Observatory LSST
Abstract: The Rubin Observatory LSST will start its full operation in the end of 2025, and is expected to produce millions of alerts every night. We here have performed mock observation simulations to characterize the LSST discovery spaces of cataclysmic variables (CVs) with various amplitudes and timescales under a thin-disk approximation of CV distributoin in our Galaxy. With the planned cadence and depth, we found that only 20% of WZ Sge-type dwarf novae systems, representing the most energetic disk-driven outbursts in CVs, will be detected during outbursts by LSST. Polar subclass of magnetic CVs, characterised by 2–4 mag long-term modulations, can be unbiasedly recovered down to 23 mag with more than 100 detections over 10 years. We also find the detection rate of the fast (<1 d) micronova bursts found in some intermediate polars as 2.6%. LSST will also provide a good opportunity for systematically studying CVs beyond the Galactic disk populations, including globular clusters and Magellanic clouds. Overall, our results inform on how to organize follow-up observation strategies on CVs and other transients.
第658回
Speaker: Yuh Tsunetoe (Shanghai Astronomical Observatory)
Date: 10/6 Monday
Time: 10:30 – 11:30
Place: Room 504 seminar room, building 4
Title: Limb-brightened jets from anisotropic nonthermal electrons
Abstract: Very long baseline interferometry (VLBI) observations reveal that relativistic jets like the one in M87 have a limb-brightened, double-edged structure. Meanwhile, analytic and numerical models struggle to reproduce this limb-brightening. We propose a model in which we invoke anisotropy in the distribution function of nonthermal synchrotron-electrons such that electron velocities are preferentially directed parallel to magnetic field lines, as suggested by particle physics simulations. We implement our emission model in both general relativistic magnetohydrodynamic
(GRMHD) simulations and force-free electrodynamic (GRFFE) models and produce simulated jet images at multiple scales and wavelengths using general relativistic radiative transfer calculation. We find that the synchrotron emission is concentrated parallel to the local helical magnetic field and that this feature produces limb-brightened jet images on scales ranging from tens of microarcseconds to hundreds of milliarcseconds in M87. We also present theoretical predictions for event horizon-scale images that can be tested with next generation instruments, such as next-generation Event Horizon Telescope (ngEHT) and Black Hole Explorer (BHEX). Due to the scale-invariance of the GRMHD and GRFFE models, our emission prescription can be applied to other targets and serve as a foundation for a unified description of limb-brightened images of extragalactic jets.
第657回
Speaker: Yukio Katsukawa (NAOJ)
Date: 7/10 Thursday
Time: 15:30 – 17:00
Place: Room 504 seminar room, building 4
Title: ロケット・気球・衛星観測が切り拓く太陽プラズマ研究
Abstract: 本講演では、ロケット気球実験、人工衛星を用いた観測手法により、太陽プラズマ研究にどのような進展をもたらしているかを紹介する。特に、紫外線偏光分光観測を実現したCLASPロケット実験や、高解像度高精度偏光分光観測を実現したSUNRISE気球実験における搭載装置の開発と観測の成果を示し、太陽の磁場構造や大気ダイナミクスの理解がいかに深まったかを解説する。これらの実験から得られた知見は、恒星磁気活動の理解に寄与するとともに、紫外線分光望遠鏡を搭載するSOLAR-C衛星の開発にも活かされている。
第656回
Speaker: Chris Packham (University of Texas)
Date: 6/19 Thursday
Time: 10:30 – 11:30
Place: Room 504 seminar room, building 4
Title: Future Flagships to Revolutionize Astronomy: Habitable Worlds Observatory & TMT/GMT
Abstract: The coming decades will afford the chance to transform our knowledge of the universe and perhaps will reveal compelling evidence of life outside of the Earth. In this presentation I explore the facilities that will enable such advances, advancing the successes of the JWST and 8m ground based telescopes. Uniquely achieved through multi-national collaborations, leveraging the technical and scientific skills of those partners, as well as spreading the costs and risks, a revolutionary scientific future beckons.
第655回
Speaker: Shota Notsu (The University of Tokyo)
Date: 6/6 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: ALMA observations of complex organic molecules, water, and 13C17O in the disk around FU Ori type protostar V883 Ori
Abstract: Complex organic molecules (COMs) in protoplanetary disks are key to understand the origin of volatiles in primitive asteroids and comets. However, the chemistry of COMs in protoplanetary disks remains uncertain. We present the results of our Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations for the disk around the FU Ori type protostar V883 Ori, where the COMs sublimate from ices by accretion burst heating (Yamato, Notsu et al., 2024, AJ, 167, 66). We robustly identified ten oxygen-bearing COMs including 13C-isotopologues in the disk-integrated spectra. The radial distributions of the COM emission, revealed by the analyses of line profiles, show the inner emission cavity, similar to the previous observations in Bands 6 and 7. We found that the COMs abundance ratios with respect to CH3OH are significantly higher than those in the warm Class 0 protostellar envelopes of IRAS 16293-2422 and similar to the ratios in the comet 67P, suggesting the efficient (re-)formation of COMs in disks. We constrained the 12C/13C ratios of COMs in disks for the first time. The 12C/13C ratios of COMs are consistently lower (∼20–30) than the canonical ratio in the ISM (∼69), indicating the efficient 13C-fractionation of CO. The D/H ratios of CH3OCHO are slightly lower than the values in IRAS 16293-2422, possibly pointing to the destruction and reformation of COMs in disks. In addition, we also report our detection of the 13C17O (=the rarest and most optically thin stable CO isotopologue) and water emission lines by ALMA Band 7 in the V883 Ori disk. We discussed the gas column density distribution and position of water snowline from our observations. In our presentation, we investigate our studies for V883 Ori disk, and discuss the importance of ALMA observations for understanding water and complex organic chemistry in planet forming regions.
第654回
Speaker: Jim Fuller (Caltech)
Date: 6/2 Monday
Time: 15:30 – 16:30
Place: Science seminar house
Title: Drama preceding the deaths of massive stars
Abstract: A renaissance in the astrophysics of massive stars is currently underway, driven by asteroseismology, supernovae, and gravitational wave observations. I will discuss several processes that occur at the end of massive stars’ lives that influence their supernovae and compact remnants. I will present a new model for mass loss from red supergiants via dense chromospheric material supported by shock waves emanating from the stellar surface. This dense chromosphere may account for the puzzling circumstellar material inferred from early observations of many type II-P supernovae. Low-mass helium star supernova progenitors expand rapidly at the end of their lives, which can drive intense mass loss in the presence of a binary companion, and may be able to account for many type Ibn supernovae. Finally, new simulations of stable mass transfer measure the amount of angular momentum lost during this process, with key implications for orbital contraction and the formation of gravitational wave sources.
第653回
Speaker: Jong-Hak Woo (Seoul National University) and the SAMP Collaboration
Date: 5/29 Thursday
Time: 15:30 – 16:30
Place: Room 504 seminar room, building 4
Title: Are the black hole masses right? Unveiling the central engine of AGN by reverberation-mapping and implications to mass estimation
Abstract: Black hole (BH) mass is a key parameter for understanding BH growth and AGN physics. The method of determining BH mass has been rapidly evolved over the last 20 years. I will present the latest results of the reverberation mapping (RM) studies based on the 6 year SNU monitoring project, including velocity resolved-lag measurements and BLR kinematical structures. Combining with literature sample, the new calibration of the Hbeta BLR size – luminosity relation shows a slope of ~0.4, which is smaller than the popularly used ~0.5 slope, along with a systematic trend with Eddington ratios. These results indicate serious limitation and systematic effects on single-epoch BH mass estimation. I will present a new calibration and the updated mass, particularly for high-luminosity AGN. I will also discuss the relatively new disk-mapping method and the potential of the disk size-luminosity relation for mass estimation in the era of the large photometric surveys, i.e., LSST and SPHEREx.
第652回
Speaker: Kate Follette (Amherst College）
Date: Friday 5/23
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Direct Observational Constraints on Planet Formation and Accretion
Abstract: One of the most important and exciting recent advancements in exoplanet astronomy is the ability to directly constrain planet formation processes through direct imaging of still-forming planets. Breadcrumbs of formation physics are sprinkled amidst detection and characterization studies of protoplanets and young brown dwarfs, as well as their circumstellar/circumplanetary environments. In this talk, I will describe recent efforts to assemble a coherent picture of planet formation via study of accreting protoplanets and planetary mass companions, paying particular attention to the challenges and opportunities inherent in the process, as well as the bright future of this field.
第651回
Speaker: Matteo Guainazzi (European Space Agency)
Date: Friday 5/19
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: “What’s past is prologue”: from XRISM to NewAthena
Abstract: A number of key questions in modern astrophysics require accurate measurement of astrophysical plasma in the high-energy (X-ray) domain. A number of these quests are at the core of the science case of NewAthena, the large X-ray observatory under study by the European Space Agency (ESA), to be launched in the second half of the 2030s: how ordinary matter assembles into large-scale structures; how black holes grow and shape their environment, as well as the cosmological evolution of the galaxies hosting them; how do baryons evolve from the epoch of structure formation to the current Universe; what drives the most energetic and explosive phenomena in the Universe; and what is the nature of the electromagnetic counterparts of neutrino and gravitational wave sources. In this talk I will show how the initial results of the JAXA-led X-ray observatory XRISM, thanks to the revolutionary high-resolution spectroscopic capability of its instrument Resolve are already contributing to a better focusing of the NewAthena science case.
第650回
Speaker: Michal Michalowski (Adam Mickiewicz University)
Date: Friday 5/9
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Binary nature of supernovae type Ic revealed by molecular gas observations
Abstract: Supernova (SN) explosions are important for galaxy evolution because they enrich the interstellar medium with heavy elements and provide feedback which can halt further star formation. These effects depend on what stellar progenitors explode as a given SN type. Type Ic SNe (without hydrogen or helium lines in their spectra) can be explosions of either ~10 solar mass stars in binary systems or very massive (>30 Msun) stars. These models involve very different lifetimes of the SN progenitors, so predict very different states of molecular gas around the explosion. Exploiting this, I will show that millimetre carbon monoxide observations of galaxies hosting SNe provide evidence for the binary model of type Ic SNe. This is an important distinction from the point of view of stellar evolution as well as the galaxy’s future star formation. This finding can be implemented in sub-grid prescriptions in numerical cosmological simulations to improve the feedback and chemical mixing.
第649回
Speaker: Jian Jiang (University of Science and Technology of China)
Date: Wednesday 4/16
Time: 15:30 – 16:30
Place: Room 504 seminar room, building 4
Title: Fast Transient Studies in the First Year Operation of the Wide Field Survey Telescope
Abstract: Transients with fast brightness variance in UV/optical (“fast transients”), such as early-phase supernovae (SNe), fast blue ultraluminous transients (FBUTs or LFBOTs), kilonovae, are of great interest in time-domain astronomy. Given their rapid brightness variance and shallow depths of transient surveys with (sub-)meter class telescopes, fast transient observations are still limited. The Wide Field Survey Telescope (WFST or “Mozi”) is a 2.5 m optical telescope with a high throughput at blue wavelengths and equipped with a mosaic CCD camera covering a field of view of 6.5 square degree, making WFST one of the most powerful fast transient survey facilities in the world. The telescope carried out a pilot survey from Mar 6 to Jul 10, 2024 and has started the formal 6-year survey from Dec 14, 2024. In this talk, I’m going to introduce the current status of WFST and transient sciences through the WFST global time-domain network in the past year.