談話会 | 2024年度

第648回
Speaker: Ernesto Palomba (Director of Research, Institute of Astrophysics and Planetary Science-INAF, Rome, Italy)
Date: 3/31 Monday
Time: 14:00 – 15:00
Place: Room 504 seminar room, building 4
Title: Space Exploration of Asteroids: Unveiling the Solar System’s Building Blocks
Abstract: This seminar, tailored for astrophysics professors and students, will delve into the fascinating realm of asteroid exploration and the crucial role these celestial bodies play in understanding the formation and evolution of our Solar System. We will explore the scientific significance of asteroids as primordial remnants, preserving valuable clues about the early conditions and materials that shaped our planetary system. The session will highlight the groundbreaking achievements and latest findings from recent missions, including Japan’s Hayabusa 2, which successfully returned samples from the C-type asteroid Ryugu, and NASA’s OSIRIS-REx mission, which collected material from the near-Earth asteroid Bennu. We will discuss the implications of these sample return missions for our understanding of asteroid composition, the origin of volatiles, and the potential delivery of prebiotic molecules to early Earth. Furthermore, the seminar will look towards the future of asteroid exploration with a focus on ESA’s upcoming HERA mission, which will study the binary asteroid system Didymos and its moon Dimorphos in conjunction with NASA’s DART mission, providing crucial data for planetary defense strategies and furthering our knowledge of asteroid dynamics. This seminar aims to provide a comprehensive overview of the current state of asteroid exploration and its pivotal contributions to planetary science.

第647回
Speaker: Eva Laplace (University of Leuven）
Date: Monday 3/31
Time: 10:30 – 11:45am
Place: Room 504 seminar room, building 4
Title: Pulsating Red Supergiants: A New Perspective on Type II Supernova Light Curve Diversity
Abstract: The origin of the different sub-types of hydrogen-rich (Type II) supernovae and their connection to stellar and binary physics remains uncertain. Type II-L supernovae are often believed to originate from stars that have lost parts of their hydrogen-rich envelopes. However, such models struggle to reproduce all features of the observed light curves. We propose an alternative scenario by considering the naturally induced pulsations of red supergiants (RSGs) towards the end of their evolution. To investigate this, we self-consistently model these pulsations of RSGs in 1D. The pulsations are naturally induced by a κγ-mechanism in the envelope. The models are then exploded at different phases during the pulsation cycle to calculate the light curve of the resulting supernova. We find that for the same progenitor star, the pulsations significantly alter the envelope structure and the resulting supernova light curve, as well as the location of the progenitor in the Hertzsprung-Russell diagram, depending on the pulsation phase. The strength of these effects increases with increasing RSG mass. We show that some observed diversity in the light curves of Type II supernovae may be explained by considering the pulsations of RSGs. We discuss our model in the context of two recent supernovae, SN 2023ixf and SN 2024ggi, which show signs of a pulsating progenitor star. Considering the explosion of pulsating RSGs offers an explanation for their light curves.

第646回
Speaker: Tom Van Doorsselaere（KU Leuven）
Date: 3/28 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Global solar coronal models driven with Alfven and kink waves
Abstract: The solar corona has a mysterious heating source, which may be magnetohydrodynamic (MHD) waves. In recent years, the so-called AWSOM models are a new generation of solar atmospheric models, which incorporate the heating and forces of Alfven waves on top of more classical effects. They are outperforming older models capturing most structures of the solar corona. However, they are still lacking in the polar open field regions, where
artificial heating is still necessary to match the observations. In this talk, I will highlight our description of wave heating by kink waves. We develop a new formalism that allows to describe the kink wave in a similar way as Alfven waves. In this new development, we generalise the Elsasser variables to Q-variables in order to follow waves that are not Alfven waves. In the talk, I will explain the governing equations, and
highlight early outcomes of the proof-of-concept in 1D, 2D and 3D configurations. I will show that kink waves may play the role of heating the polar open field regions, replacing the artificial heating terms required in the earlier models.

第645回
Speaker: Colin Goldblatt (University of Victoria)
Date: 2/17 Monday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: The Habitable Zone & the runaway greenhouse
Abstract: The Runaway Greenhouse describes a maximum limit on the amount of thermal radiation an Earth-like planet can emit to space. This limit arises as evaporation of water on a warming planet causes the greenhouse effect to become stronger. A runaway greenhouse would make Earth turn into Venus. It describes the “inner edge” of the circumstellar “habitable zone”, where planets with liquid water that can support Earth-like life can exist. I will review some of the atmospheric physics that causes the runaway greenhouse, show some high spectral resolution calculations of this, and perhaps also talk about runaway greenhouses with other gases or on different planets.

第644回
Speaker: Ken Chen (ASIAA)
Date: 2/14 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Feeding the Beast: How Supermassive Black Holes Grow at Galactic Centers
Abstract: Understanding the formation of the supermassive black holes (SMBHs) present in the centers of galaxies is a crucial topic in modern astrophysics. Observations have detected SMBHs with masses of 10^9 solar masses in the high redshift galaxies with z ~ 7. However, how SMBHs grew to such huge masses within the first billion years after the Big Bang remains elusive. One possible explanation is that SMBHs grow quickly through the frequent mergers of galaxies, which provides sustainable gas to maintain rapid growth. In this talk, I will present new hydrodynamics simulations of the SMBHs’ growth with their host galaxies using the GIZMO code. In contrast to previous simulations, we have developed a giant molecular cloud (GMC) model by separating molecular gas particles from atomic gas particles and then evolving them independently. During major mergers, we show that the more massive molecular gas particles’ cloud bears stronger dynamical friction. Consequently, GMCs are substantially accreted onto the galactic centers that grow SMBHs from ~10^7 to ~10^9 solar masses within 300 Myr, which explains the rapid growth of SMBHs, and this accretion also triggers a violent starburst at the galactic center.

第643回
Speaker: Yuta Notsu (Laboratory for Atmospheric and Space Physics, University of Colorado Boulder)
Date: 12/27 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4
Title: Recent developments of stellar flare studies though multi-wavelength observations
Abstract:
Flares are frequent energetic explosions in the stellar atmosphere, and are thought to occur by impulsive releases of magnetic energy stored around starspots. Large flares (so called “superflares”) generate strong high energy X+UV emissions and coronal mass ejections (CMEs), which can greatly affect the planetary environment and habitability.
Recent Kepler/TESS photometric data have revealed the statistical properties of superflares on G, K, M-type stars. Young rapidly-rotating stars (e.g., “Young Suns”) and cooler stars (“M dwarfs”) tend to have frequent flares, which can be more “hazardous” for the habitable planets. However, we still do not know the emission mechanisms of superflares, and how large CMEs are associated with superflares on these active stars. Then recently, these active flare stars have been investigated in more detail through multi-wavelength campaign observations. In this presentation, I first introduce a quick brief history of superflare studies started from the Kyoto group, and then overview our various recent observation results of flares (especially including X-ray, UV, and radio data), and discuss future prospects on further multi-wavelength observations, and the importance of more collaborations with solar-based observation/modeling studies, and (exo)planetary science.

第642回
Speaker: Mauricio Elias Chavez (Instituto de Astronomi Ensenada, UNAM)
Date: 11/8 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: X-ray analysis of the AGN populationin the Deepest XMM‐Newton Field
Abstract:
In this project, we present results from a systematic X-ray analysis combined with multiwavelength photometric and spectroscopic data to identifyand characterize the complete AGN population detected in the XMM-Newton Ultra Narrow Deep Field (XMM-UNDF) survey, one of the deepest XMM-Newton surveys to date.Thanks to optical follow-up observations with the 10.4m Gran TelescopioCanarias, combined with WISE/2MASS IR data, we identified 225 AGNs based on a set of multiwavelength criteria. We estimated their photometric and spectroscopic redshift distribution (up to z~2.5), X-ray luminosities, X-ray/optical-IR flux ratios, and absolute magnitudes.
For the 23 brightest AGNs (> 500 counts), we calculated a mean black hole mass of log (MBH) = 7.26±0.68 Msun, estimated using the X-ray scaling method. We investigated the relationships among MBH, Lx, and X-ray variability, quantified by the Normalized Excess Variance (σ_rms^2). Our analysis revealed an anti-correlation in both MBH -σ_rms^2 and Lx-σ_rms^2 relations. The first is described as σ_rms^2 ∝ MBH^(-0.26±0.05), while the latter follows the same behavior with σ_rms^2 ∝ Lx^(-0.31±0.04). These results support the idea that the luminosity-variability anti-correlation is a byproduct of an intrinsic relationship between the BH mass and the X-ray variability. We also found a strong correlation among the Eddington ratio (λEdd), the hard X-ray photon index, and the parameter
log (A) related to the fraction of scattered seed photons f, where f = A/(1+A).
For a sample of 115 AGNs with X-ray, Optical and IR emission, we searched for characteristic multi‐wavelength signatures of obscured and highly obscured (Compton‐Thick or CT) AGNs, based on their spectral energy distribution (SED), and X‐ray spectroscopic features such as Fe Kα emission line intensity and reflected component. Our analysis confirms the “Iwasawa-Taniguchi effect”, an anti‐correlation between the luminosity and the FeKα line equivalent width, possibly associated with the decreasing of the torus covering factor as the AGN luminosity increases, suggesting that this relation prevails in the most obscured AGNs. Finally, we did not observe an increase in the intrinsic column density at higher redshifts and a low contribution of a cosmic filament at z~0.4.

第641回
Speaker: 大栗真宗 (千葉大学)
Date: 10/30 Wednesday
Time: 15:30 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: 銀河団時間変動重力レンズの発見と応用
Abstract:
ハッブル宇宙望遠鏡を用いた銀河団中心部の深観測により、重力レンズ超新星や焦線通過といった新しい重力レンズ現象が発見された。ジェームズウェッブ宇宙望遠鏡の観測によってその観測数はさらに増えつつある。これらの現象の理論的解釈や天文学、宇宙論への様々な応用の研究の現状を紹介する。

第640回
Speaker: 竹内 駿 (富士通オーストラリア/マッコーリー大学)
Date: 10/17 Thursday
Time: 10:00 – 11:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: 機械学習の高度化と天文学との関連
Abstract:
ディープラーニングの登場を機に、AIの進化が目覚ましい。天文分野でも、天体分類やサロゲートモデルなどAIは積極的に活用されており、実験・理論・数値シミュレーションに続く「第4の科学」としてその地位を広げている。技術の面では、画像認識や自然言語処理に関するAIは特に社会との親和性が高く、ChatGPTを始めとする技術革新とその実用化が急速に進んでいる。本講演では、天文学とも関連が深い画像認識に焦点を置き、State-of-the-Artモデルの基礎技術から、近年のAI国際会議で採択された我々の研究成果を紹介する。それらは人の行動を対象とした画像認識の学習手法に関する論文であるが、技術課題や提案手法のポイントは対象分野に依存しておらず、天文学コミュニティにも有益だろう。
また、天文学出身で現在AI分野に身を置く立場から、機械学習と天文学との関連にも触れる。機械学習のモデリングや評価方法の考え方はもちろんだが、理学と工学を始めとした学問的背景の違いなど、天文学研究とAI研究には異なる点も多い。AIから発展する天文学研究、天文学から発展するAI研究のそれぞれのトピックを例示し、先端学習技術の天文学応用への新たな視座を提供したい。

第639回
Speaker: A.S. Brun (CEA and Observatory of Paris-Saclay, France)
Date: 10/11 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: Magnetochronology of solar type star dynamos
Abstract:
We present an extensive study on the dynamo origin of solar-type star’s magnetism, based on a series of 3D MHD global numerical simulations of rotating magnetized convection.  We quantify how thecombination of rotation and convection via the Rossby number influences the type of magnetism established (short or long cycles, statistically steady activity) and their expected differential rotation (solar-like, anti-solar, cylindrical or almost solid).  We also compute the magnetic energy spectra and show how the dipole and quadruple magnetic fields evolve as the Sun ages over secular time. When compared to the observational trends we found a good overall agreement. This gives rise to a correspondence between magnetic activity and age, i.e. the so-called magnetochronology. We further assess the amount of energy needed to maintain such angular velocity profiles and magnetic activity.  We find that between 0.1 and 3% of the stellar luminosity can be converted into magnetic energy, giving plenty of energy for surface eruptive events to occur.  Interesting regimes at low and high Rossby number are also identified that need to be further studied.  This large survey allows us to propose a possible solution to why the Sun possesses a long decadal magnetic cycle period and a conical differential rotation.  Such key properties are recovered in a specific range of Rossby numbers, which opens up the possibility to use this dimensional parameter to define a theoretical path in parameter space towards more and more turbulent models while retaining key forces  balances thought to operate in the Sun. We will present such new theoretical path and discuss how well it compare to recent observational constraints.

第638回
Speaker: Jonay I. Gonzalez Hernandez (Instituto de Astrofisica de Canarias, Spain)
Date: 8/30 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: The origin and evolution of black hole binaries and supernovae
Abstract:
Black hole X-ray binary (BHXB) systems offer a unique opportunity to test angular momentum losses, in particular, those driven by magnetic braking, gravitational radiation and mass loss. During more than 10 years we have been getting spectroscopic observations of BHXBs with large optical telescopes such as Keck, VLT and GTC with medium- and high-resolution spectrographs, and we have been able to empirically track the orbital evolution of secondary stars in
these X-ray binary systems. These systems also allow us to investigate the link between the explosive end of massive stars and the formation of black holes and neutron stars. The chemical composition of secondary stars can allow us to study the nucleosynthesis in the possible supernova vent, and to extract information on supernova explosion properties and its progenitor.
We have also been studying Galactic type-Ia supernovae (SNe Ia), which are known to be the best cosmological distance indicators at high redshifts. Their use led to the discovery of the currently accelerating expansion of the universe. These SNe are known to occur when a white dwarf made of carbon and oxygen accretes sufficient mass to trigger a thermonuclear explosion. My group have been trying to search for the companion of the progenitor of historical Galactic SN Ia with the aim of clarifying the origin of these cosmological candles, using high-resolution spectra taken at Keck and VLT, as well as the HST and Gaia.
I will summarize the properties of these BHXB systems and these SNIa remnants and the work I have been doing on their origin and evolution.

第637回
Speaker: Yuhito Shibaike (NAOJ, Project Research Fellow)
Date: 7/19 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: Obtaining constraints on properties of forming planets from the dust continuum emission of their circumplanetary disks
Abstract:
Recently, direct imaging of forming planets has been achieved in multiple wavelengths. Dust millimeter continuum emission from a gas accreting planet in the wide gap of the pre-transitional disk of PDS 70 has been detected by ALMA, considered as the first evidence of a circumplanetary disk (CPD). We developed an evolution and emission model of dust in a CPD and find positive correlations between the flux density of the dust emission and the three planet properties, the planet mass, the gas accretion rate, and their product called MMdot. We then obtained a constraint on the MMdot of PDS 70 c as > 0.4 MJ^2/Myr, corresponding to the planet mass of > 5 MJ and the gas accretion rate of > 0.02 MJ/Myr, from the observed flux density. This is the first successful case of obtaining constraints on the planet properties from dust millimeter continuum observations of CPDs. In this talk, I will first present how we obtained the constraints on PDS 70 c and then show some predictions for other planet candidates. I will also summarize what we can understand from the comparisons of observations and theoretical predictions of CPDs.

第636回(2)
Speaker: Juan Echevarria (UNAM)
Date: 7/12 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: New Observations of DW Cnc: evolution of the period, spin and beat-period signals
Abstract:
New spectroscopic and photometric study of the 86 min orbital period intermediate polar DW Cancri are presented. We show that the lost spin signal has re-appeared in accordance with our predictions. However, as we will show, there is still no strong evidence of the beat-period signature.

第636回(1)
Speaker: Daichi Hiramatsu (Harvard U.)
Date: 7/12 Friday
Time: 15:00 – 16:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: Tracing the Final Moments of Massive Stars through Supernovae
Abstract:
Supernovae are the terminal explosions of massive stars with influences on every astrophysical scale. Advanced wide-field and high-cadence transient surveys routinely discover supernovae near the moment of explosion. Coupled with prompt and continuous follow-up facilities, these observations have revealed unprecedented features of dense circumstellar medium in various spatial scales as traced by the expanding supernova ejecta. Such circumstellar medium is thought to originate from mass-loss activities in the final years to decades of stellar evolution; however, their inferred densities exceed the expectations from standard theory by many orders of magnitude. In this talk, I will first introduce standard stellar evolution and supernova explosion mechanisms, and then describe novel observational probes and modeling techniques of supernovae interacting with circumstellar medium to reconstruct their explosion properties and progenitor mass-loss histories. Finally, I will discuss our on-going largest sample study of interacting supernovae and emerging pictures of dramatic dying breaths of massive stars.

第635回
Speaker: Ben Wilcock (Blue Skies Space LTD.)
Date: 7/9 Tuesday
Time: 16:30 – 17:30
Place: Room 504 seminar room, building 4 + ZOOM
Title: Mauve: a UV-Vis satellite dedicated to monitoring stellar activity and variability
Abstract:
Mauve is a satellite equipped with a 13-cm telescope and a UV-Visible spectrometer (with an operative wavelength range of 200-700 nm) conceived to measure the stellar magnetic activity and variability. The science program will be delivered via a multi-year collaborative survey program,
with thousands of hours each year available for long baseline observations of hundreds of stars, unlocking a significant time domain astronomy opportunity. Mauve’s mission lifetime is 3 years with the ambition of 5 years, and will cover a broad field of regard (~46.4 to 31.8 degrees
in ICRS) during this period. This facility was conceived to support pilot studies and new ideas in science and is fully dedicated to time-domain astronomy. The main surveys to be executed by Mauve are long baseline observations of flare stars, Herbig Ae/Be stars, exoplanet hosts, as well as contact binary variables (RS CVn variables, symbiotic stars,
Algol-type stars, etc.). Besides these major science themes,
the spectrometer’s data can be utilized to support and complement existing and upcoming facilities as a pathfinder, or conduct simultaneous/follow-up observations.

第634回
Speaker: 恒任 優(BHI/ハーバード大)
Date: 7/5 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: 偏光画像計算から探る、ブラックホール直近領域の円盤降着過程とジェット駆動機構
Abstract:
地球規模での超長基線干渉計(VLBI)観測の実現により、ブラックホールジェットの磁気駆動機構の研究は新地平に突入した。この好機にあたり必要なのは、観測画像と理論モデルを結びつける具体的指標である。これを踏まえ我々は、磁気流体モデルに基づいた一般相対論的輻射輸送計算を行い、ブラックホールの偏光画像を理論予測した。第一の結果として、直線偏光と円偏光成分が、ジェットに沿ってそれぞれ下流側と上流側に分布することがわかった。この直線偏光ー円偏光分離は降着円盤とジェットの電子温度分布やブラックホールへの質量降着率に依存するため、偏光観測からブラックホールー円盤ージェット構造を探査することが可能となる。第二に我々は、光学的に厚い非熱的プラズマに特有の直線偏光ベクトルの90度フリップが、ジェットの根元および光子リング上で検出されうることを示した。これらはブラックホール直近における電子のエネルギー分布の強い決定指標となる。最後に直近の結果として我々は、ブラックホールに突き刺さりジェットを駆動する磁気フラックスの変動が、数百シュバルツシルト半径まで広がって観測されるジェットの幅の変動に強く反映されることを示した。この結果は、ジェット観測から地平面スケールでの力学が推定可能であり、逆に地平面スケールでの変動からジェット形状の変化が予測できることを意味する。

第633回
Speaker: Kirpal Nandra (MPE)
Date: 6/26 Wednesday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: New Insights into the Hot Universe from the eROSITA All-sky Survey
Abstract:
The eROSITA telescope array was launched aboard the Spectrum-RG satellite on July 13th 2019 and has since performed an X-ray all sky survey of unprecedented depth. The resulting catalogues and data represent a rich scientific resource impacting broadly on many areas of astrophysics. The design-driving science behind eROSITA is the accumulation of a large sample of X-ray emitting clusters of galaxies which, via the evolution of their mass function, can be used to constrain the cosmological parameters. eROSITA’s cluster cosmology program has been a spectacular success, impacting significantly on the so-called “S8 tension” between CMB and weak lensing measurements. In addition to clusters, eROSITA traces the hot gas reservoirs in the Universe on smaller scales, in individual galaxies and galaxy groups. Here again, the data have provided new insights into the circumgalactic medium and the effects of feedback. The point source population revealed by eROSITA is no less rich, with almost a million accreting supermassive black holes being found across a wide luminosity and redshift range. Due to its particular scanning pattern, the survey has also revealed new phenomena in the time domain, including tidal disruption events and quasi-periodic eruptions. In this talk, selected scientific highlights derived from the all-sky survey will be presented and the content of the first data release will be described.

第632回
Speaker: Dimitris Stamatellos (University of Central Lancashire)
Date: 5/24 Friday
Time: 16:00 – 17:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: The formation of giant planets by gravitational instabilities in protostellar discs
Abstract:
The question about the origin of planets and life has fascinated humanity since ancient times. In the last three decades thousands of exoplanets, i.e. planets around stars other than our Sun, have been discovered, challenging our understanding of how planets form.  I will present radiative hydrodynamic computational models of the process of planet formation in young protostellar discs, and I will discuss whether gravitational instability may be responsible for the fast formation of giant planets, particularly focusing on planets around red dwarfs, which are targeted by planet characterization missions. I will also discuss the properties of disc-instability planets and how young planets evolve after they form. I will argue that their fates are quite diverse and depend on the detailed physics of their interaction with their parent discs.

第631回
Speaker: Marcin Sawicki (Saint Mary’s University)
Date: 5/20 Monday
Time: 13:30 – 15:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: CANUCS: 200 hours of JWST spatially-resolved spectroscopy and
imaging behind five gravitationally-lensing clusters
Abstract:
The CANUCS program, a 200-hour JWST Guaranteed Time Observations project, leverages the remarkable spatial resolution of JWST, boosted by gravitational lensing, to probe distant galaxies in unparalleled spatial detail. As the most comprehensive JWST program targeting gravitationally-lensing clusters, CANUCS uses NIRCam imaging and NIRISS and NIRSpec spectroscopy to study the formation and growth of galaxies at Cosmic Dawn and Cosmic Noon.
This talk will give an overview of the CANUCS project with a particular focus on what we are learning from our spatially-resolved data on distant galaxies from z~1 to z~8.

第630回
Speaker:  Teo Muñoz-Darias (IAC, Tenerife, Spain)
Date: 5/15 Wednesday
Time: 13:30 – 14:15
Place: Room 504 seminar room, building 4 + ZOOM
Title: Accretion and outflows in stellar-mass black holes
Abstract:

Accreting stellar-mass black holes in X-ray binaries represent a unique window to study black hole accretion in relatively cleaner environments and shorter time-scales than those typical of AGN and quasars. In the last decades, observations of the X-ray and radio continuum have been used to investigate in great detail the coupling between accretion and ejection processes in these objects. However, we have only recently started to systematically study the changes in their X-ray, optical and infrared spectral lines.  I will briefly review the state-of-the-art of the field, to then focus on the dramatic changes that we are observing in the optical and infrared spectra of X-ray binaries by using dedicated programs in some of the largest telescopes in the world (e.g. GTC-10.4m and VLT-8.2m). I will show how this variability in the spectral lines can be associated in most cases with the presence of massive outflows, as it can be inferred from the detection of wind-related observational signatures, such as P-Cyg line profiles.  I will discuss the relation between the different kinds of X-ray binary outflows (radio-jets, X-ray and optical-infrared winds), their impact on black hole accretion (including some extreme cases), and how their main observational properties seem to be universally correlated with those of the accretion flow.

Speaker:  Montserrat Armas Padilla (IAC, Tenerife, Spain)
Date: 5/15 Wednesday
Time: 14:15 – 15:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: UltracompCAT and the hunt for new ultra-compact X-ray binaries
Abstract:
The ultra-compact family of low-mass X-ray binaries (LMXBs) consists of those systems with orbital periods shorter than 80 minutes. Such short periods imply small Roche lobes, into which only degenerate (hydrogen- poor) donor stars can fit. They are unique laboratories for studying accretion processes in hydrogen deficient environments, in addition to some fundamental stages of binary evolution. They will also be primary sources for low-frequency gravitational wave studies with the upcoming LISA mission.  I will present UltraCompCAT, a new catalogue of ultra- compact and short orbital period X-ray binaries, in which we have compiled all the available multi- wavelength
information for the current population of these objects. This allows us to study, for the first time, this tantalising family of X-ray binaries as a whole. I will review the state-of-the-art in the field, and then discuss some of the global and most fundamental properties of this family.

第629回
Speaker: Philipp Podsiadlowski (University of Oxford)
Date: 5/7 Tuesday
Time: 13:30 – 15:00
Place: Room 504 seminar room, building 4 + ZOOM
Title: How Massive Stars End Their Lives
Abstract:
I will provide first a brief summary of the key principles that govern the structure and evolution of stars. While stars less than 8 times the mass of the Sun ultimately eject their envelopes and become white dwarfs, more massive stars form iron cores and ultimately collapse to either form a neutron star or a black hole. This may or may not be connected with a supernova explosion. Observations of supernovae and their remnants provide important observational clues on how massive stars end their lives. There has also been dramatic progress from hydrodynamical simulations on the core-collapse process. I will show that, for the first time, we are now able to combine detailed stellar structure calculations with a simple model for the supernova process to predict the final fate of massive stars. This makes predictions about the mass distribution of black holes that can be tested with gravitational-wave detections of merging black holes, as are being found in large numbers with the LIGO gravitational-wave detector. We will therefor be able to test both the theory of massive stars and the complex physics of supernova explosions.

第628回
Speaker: Antonio Rodriguez (Caltech)
Date: 4/15 Monday
Time: 15:00 – 16:30
Place: Room 504 seminar room, building 4 + ZOOM
Title: The nearest black holes are quiet (but white dwarfs are not)
Abstract:
While the Milky Way is predicted to harbor around 100 million stellar-mass black holes (BHs), only about two dozen have been dynamically confirmed. The vast majority of such systems were identified through their powerful X-ray outbursts, which approach Eddington luminosities, but the newest two systems were identified through their gravitational influence on their companion stars using precise astrometry from Gaia. In this talk, I will show why we expected to see X-rays from at least one of these two newest BHs, and what the implications are of not having seen any X-rays from either system. In the second half of the talk, I will show successful X-ray detections ― recent discoveries of accreting magnetic white dwarfs (WDs) from the combination of X-ray and optical data.
Magnetic WDs are interesting since they make up over a third of accreting systems in the form of cataclysmic variables, yet the origin of their magnetism is uncertain. I will
conclude by showing that the near X-ray future is bright, discussing both individual systems and large samples from a combination of the deepest all-sky X-ray surve (SRG/eROSITA) with the deepest all-sky optical time-domain survey (the Zwicky Transient Facility).

第627回
Speaker: Jason Xavier Prochaska　(UC Santa Cruz)
Date: 4/4 Thursday
Time: 15:00 – 16:30
Place: Room 504 seminar room, building 4 + ZOOM
Title: Probing the Universe with Fast Radio Bursts
Abstract: I will highlight recent results leveraging well-localized fast radio bursts (FRBs) to study cosmology and galaxy formation in our universe. The fundamental signals inherent in FRBs – dispersion measure (DM), rotation measure (RM), and fluence – offer unique constraints on properties of the matter along the sightline to Earth. In turn, we may map out the cosmic web, constrain the density of gas surrounding galaxies, and infer the magnetic fields of the interstellar medium from a diverse population of galaxies. I will describe the standard observational and analysis techniques and conclude by emphasizing areas poised for tremendous growth in the next few years due to the ongoing or upcoming commissioning of new facilities and systems (e.g. CRACO on ASKAP, DSA-110, and the CHIME Outriggers project).