ABBL-iTHEMSジョイントアストロセミナー
56 イベント
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セミナー
Dynamics of the very early universe: towards decoding its signature through primordial black hole abundance, dark matter, and gravitational waves.
2024年7月5日(金) 14:00 - 15:15
Riajul Haque (Postdoctoral Researcher, Department of Physics, Indian Institute of Technology, India)
I will start my talk with a brief overview of the standard reheating scenario. Then, I will discuss reheating through the evaporation of primordial black holes (PBHs) if one assumes PBHs are formed during the phase of reheating. Depending on their initial mass, abundance, and inflaton coupling with the radiation, I discuss two physically distinct possibilities of reheating the universe. In one possibility, the thermal bath is solely obtained from the decay of PBHs, while inflaton plays the role of the dominant energy component in the entire process. In the other possibility, PBHs dominate the total energy budget of the universe during evolution, and then their subsequent evaporation leads to a radiation-dominated universe. Furthermore, I will discuss the impact of both monochromatic and extended PBH mass functions and estimate the detailed parameter ranges for which those distinct reheating histories are realized. The evaporation of PBHs is also responsible for the production of DM. I will show its parameters in the background of reheating obtained from two chief systems in the early universe: the inflaton and the primordial black holes (PBHs). Then, I will move my discussion towards stable PBHs and discuss the effects of the parameters describing the epoch of reheating on the abundance of PBHs and the fraction of cold dark matter that can be composed of PBHs. If PBHs are produced due to the enhancement of the primordial scalar power spectrum on small scales, such primordial spectra also inevitably lead to strong amplification of the scalar-induced secondary gravitational waves (GWs) at higher frequencies. I will show how the recent detection of the stochastic gravitational wave background (SGWB) by the pulsar timing arrays (PTAs) has opened up the possibility of directly probing the very early universe through the scalar-induced secondary gravitational waves. Finally, I will conclude my talk by elaborating on the effect of quantum correction on the Hawking radiation for ultra-light PBHs and its observational signature through dark matter and gravitational waves.
会場: via Zoom
イベント公式言語: 英語
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Surrogate Modeling for Supernova Feedback toward Star-by-Star Simulations of Milky-Way-sized Galaxies
2024年5月10日(金) 14:00 - 15:15
Keiya Hirashima (東京大学 大学院理学系研究科 天文学専攻 博士課程)
Galaxy simulations have found the interdependence of multiscale gas physics, such as star formation, stellar feedback, inflow/outflow, and so on, by improving the physical models and resolution. The mass resolution remains capped at around 1,000 solar masses (e.g., Applebaum et al. 2021). To overcome the limitations, we are developing a new N-body/SPH code, ASURA-FDPS, to leverage exascale computing (e.g., Fugaku), handle approximately one billion particles, and simulate individual stars and stellar feedback within the galaxy. However, the emergence of communication costs hinders scalability beyond one thousand CPU cores. One of the causes is short timescale events localized in tiny regions, such as supernova explosions. In response, we have developed a surrogate model using machine learning to duplicate supernova feedback quickly (Hirashima et al., 2023a,b). In the presentation, I report the fidelity and progress of the simulations with our new machine-learning technique.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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How Stars End Their Lives
2024年4月26日(金) 14:00 - 15:15
Philipp Podsiadlowski (Professor, University of Oxford, UK)
While the basic evolution of stars has been understood for many decades, there are still major uncertainties in our overall understanding of how stars end their lives, both in the context of low- and intermediate-mass stars (including the Sun) and massive stars. I will first review some of key principles that govern the structure and evolution of stars and then present recent progress that has been made for both groups of stars. I will argue and present numerical simulations that show that all stars become dynamically unstable when they become large giant stars, which leads to sporadic, dynamical mass ejections. Low- and intermediate-mass stars may lose all of their envelopes as a consequence, leaving white-dwarf remnants. More massive stars experience core collapse, leaving a neutron-star or black-hole remnant, possibly associated with a supernova explosion. I will show how the dramatic recent progress on understanding the core-collapse process, for the first time, allows us to connect the late evolution of massive stars with the resulting supernova explosions and the final remnants and discuss how observations with current gravitational-wave detectors (such as LIGO) will allow us to test this theoretical connection.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Do plants have bones? Silica phytoliths and their role and fate in the development of terrestrial plants and human civilizations
2024年3月1日(金) 14:00 - 15:15
Mikhail Blinnikov (Professor, St. Cloud State University, USA)
Silicon is the second most common element in the Earth’s crust. Some families of higher plants evolved mechanisms for soluble silica to be carried by xylem from groundwater and deposited as plant opal in or around plant cells as phytoliths thought to play a role in the structural support and defense against herbivores. While known since the early 19th century, phytoliths remain an intriguing class of microfossils whose formation and role in plants and their preservation in soils and sediments are a subject for a lot of active research. I outline some emerging themes in phytolith analysis including phytoliths’ role in global biogeochemical cycles, plant-herbivore interactions, and their tracing of evolution of cultural plants, especially cereals such as rice (Oryza), wild rice (Zizania), maize (Zea), wheat (Triticum) and millet (Panicum), all relevant to global archaeology. Some emerging research on phytoliths connects their changes in shapes to plant taxonomy of some families such as grasses and opens up avenues for further investigation of their active construction in the cells of some taxa by yet undiscovered genetically mediated mechanisms. New image analysis techniques and some advanced microscopy methods will allow us to further the field of phytolith study using deep machine learning algorithms and true 3D analysis of their shapes, something where contribution from other branches of science are most welcome.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Relativistic Jet Simulations and Modeling on Horizon Scale
2024年2月8日(木) 13:00 - 14:30
水野 陽介 (T.D. Lee Fellow / Associate Professor, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)
Relativistic jets are launched in the vicinity of the central black holes and emit powerful radiation across the electromagnetic spectrum. According to our current understanding, relativistic jets are launched by directly tapping the rotational energy of spinning black holes via the so-called Blandford-Znajek process. In addition to the spin of the black hole, numerical simulations showed the amount of accreted magnetized flux has a major impact on the formation of relativistic jets. We have investigated the radiative signatures of self-consistently launched relativistic jets using 3D general relativistic magneto-hydrodynamical simulations and general relativistic radiative transfer calculations in horizon scale to the connection with large-scale structure. We discuss our findings and comparison with observations.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Probing structure of neutron stars through X-ray bursters
2024年1月12日(金) 14:00 - 15:15
土肥 明 (理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 基礎科学特別研究員)
Type-I X-ray bursts are rapidly brightening phenomena triggered by the nuclear burning of light elements near the surface of accreting neutron stars. Most of the X-ray bursters show irregular behavior of light curves. However, some X-ray bursters are somehow quite regular, i.e., constant recurrence time and constant shaper of light curves, and are often called Clocked bursters, which are powerful sites to probe uncertainties of many model parameters such as accretion rate, the composition of accreted matter, reaction rates, neutron star structure, and temperature. In this study, we focus on the uncertainties of the equation of states, which determines the latter two properties. Based on our numerical models covering whole areas of neutron stars, we will present their impact on X-ray burst light curves. Furthermore, we will discuss the possibility of constraining the equation of states from Clocked bursters such as GS 1826-24 and 1RXS J180408.9-342058.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Exploring material strengths of dust aggregates in planet formation by numerical simulations
2023年12月8日(金) 14:00 - 15:15
辰馬 未沙子 (数理創造プログラム 研究員)
The planet formation process is the growth from sub-micrometer-sized cosmic dust grains to thousand-kilometer-sized planets. This growth process has broadly two phases: the growth from dust grains to kilometer-sized planetesimals, mainly driven by intermolecular forces like van der Waals forces and hydrogen bonds, and the subsequent growth from planetesimals to planets, governed by gravitational forces. However, the planetesimal formation process encounters various challenges, including fragmentation and bouncing resulting from collisions among dust aggregates. To gain insights into the planetesimal formation process and how to avoid these obstacles, I have been focused on measuring and formulating the material strengths of dust aggregates using grain simulations. In this talk, I will introduce my works on the material strengths of dust aggregates and their applications to kilometer-sized bodies in the solar system, such as comets and asteroids.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Early Formation of Dark Matter Halos
2023年11月24日(金) 14:00 - 15:15
デリック・ビーティ―・インマン (数理創造プログラム 研究員)
Cosmological observations have led to an extremely precise understanding of the large-scale structure of the Universe. A common assumption is to extrapolate large-scale properties to smaller scales; however, whether this is correct or not is unknown and many well-motivated early Universe scenarios predict substantially different structure formation histories. In this seminar I will discuss two scenarios where nonlinear structures form much earlier than is typically assumed. In the first case, the initial fluctuations are enhanced on small scales leading to either primordial black holes clusters or WIMP minihalos right after matter-radiation equality. In the second, I will show that an additional attractive dark force leads to structure formation even in the radiation dominated Universe. I will furthermore discuss possible observations of such early structure formation including changes to the cosmic microwave background, dark matter annihilation, and when the first galaxies form.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Neutrino production in AGN cores: Constraints from Kinetic Plasma Simulations
2023年10月6日(金) 14:00 - 15:15
Amir Levinson (Professor, Tel Aviv University, Israel)
Accreting black holes power a variety of high-energy astrophysical systems. The activation and mode of operation of these engines has been subject of intensive research. In recent years the structure of the multi-flow emanating from the putative, giant black hole in the M87 galaxy was probed down to near horizon scales in unprecedented detail, shedding new light on the physics of accretion and the processes responsible for the formation and dissipation of relativistic jets by the black hole. I shall review recent progress in observational and theoretical studies of accreting black hole engines.
会場: セミナー室 (359号室) (メイン会場) / via Zoom
イベント公式言語: 英語
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Progenitors and Explosion Properties of Supernova Remnants Hosting Central Compact Objects
2023年10月6日(金) 10:00 - 11:30
チェルシー・ブラウン (Ph.D. Student, Department of Physics and Astronomy, University of Manitoba, Canada)
Presented is a systematic, global study of Galactic supernova remnants (SNRs) hosting Central Compact Objects (CCOs) aimed at addressing their explosion properties and supernova progenitors. With the Chandra and XMM-Newton telescopes, a spatially resolved X-ray spectroscopy study is performed on seven SNRs that show evidence of shock-heated ejecta. Using an algorithm, we segmented each SNR in the sample into regions of similar surface brightness. These regions were fit with one- or two-component plasma shock model(s) in order to separate the forward-shocked interstellar medium from the reverse shock-heated ejecta which peak in the X-ray bands for elements including O, Ne, Mg, Si, S, Ar, Ca, and Fe. We subsequently derived the explosion properties for each SNR in the sample and found overall low explosion energies (<10^51 erg). To address their progenitor mass, we compare the measured abundances from our spectroscopic modelling to five of the most widely used explosion models and a relatively new electron-capture supernova model. Additionally, we explore degeneracy in the explosion energy and its effects on the progenitor mass estimates. However, no explosion models match all of the measured ejecta abundances for any of the SNRs in our sample. Therefore, we present our best progenitor mass estimates and find overall low progenitor masses (<=25 solar masses) and we highlight the discrepancies between the observed data and the theoretical explosion models.
会場: via Zoom
イベント公式言語: 英語
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Collective Plasma Effects in Relativistic Radiation-Mediated Blast Waves
2023年9月8日(金) 14:00 - 15:15
Arno Vanthieghem (Princeton-NINS Postdoctoral Research Fellow, Department of Astrophysical Sciences, Princeton University, USA)
Relativistic radiation-mediated shocks (RRMS) dictate the early emission in numerous transient sources such as supernovae, low luminosity gamma-ray bursts, binary neutron star mergers, and tidal disruption events. These shock waves are mediated by Compton scattering and copious electron-positron pair creation. It has been pointed out that a high pair multiplicity inside the shock transition leads to a lepton-baryon velocity separation, prone to plasma instabilities. The interaction of the different species with this radiation-mediated microturbulence can lead to coupling and heating that is unaccounted for by current single-fluid models. Here, we present a theoretical analysis of the hierarchy of plasma microinstabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Our results are validated by particle-in-cell simulations that probe the nonlinear regime of the instabilities and the lepton-baryon coupling in the microturbulent electromagnetic field. Based on this analysis, we derive a reduced transport equation for the particles that demonstrates anomalous coupling of the species and heating in a Joule-like process by the joined contributions of the decelerating turbulence, radiation force, and electrostatic field. We will then discuss the effect of finite magnetization on the general dynamics and recent efforts toward a more self-consistent description of the coupling. In general, our results suggest that radiation-mediated microturbulence could have important consequences for the radiative signatures of RRMS.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Evidence against a strong first-order phase transition in neutron star cores: impact of new data
2023年8月1日(火) 13:30 - 15:00
Len Brandes (Ph.D. Student, Technical University of Munich, Germany)
Information on the phase structure of strongly interacting matter at high baryon densities can be gained from observations of neutron stars and their detailed analysis. Bayesian inference methods are used to set constraints on the speed of sound in the interior of neutron stars, based on recent multimessenger data in combination with low-density constraints based on chiral effective field theory and perturbative QCD constraints at asymptotically high densities. A detailed re-analysis is performed in order to clarify the influence of the latter constraints on the inference procedure. The impact of the recent new heavy (2.35 M_sol) black widow pulsar PSR J0952-0607 and of the unusually light supernova remnant HESS J1731-347 is inspected. One of the consequences of including PSR J0952-0607 in the database is a further stiffening of the equation-of-state, resulting in a 2.1 solar-mass neutron star in a reduced central density of less than five times the equilibrium density of normal nuclear matter. A systematic Bayes factor assessment quantifies the evidence (or non-evidence) for small sound speeds, necessary for a strong first-order phase transition, within the range of densities realized in the core of neutron stars. Given the presently existing database, it can be concluded that the occurrence of a strong first-order phase transition in the core of even a 2.1 solar-mass neutron star is unlikely, while a continuous crossover cannot be ruled out.
会場: via Zoom (メイン会場) / セミナー室 (132号室)
イベント公式言語: 英語
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A dynamical model for IRAS 00500+6713: the remant of a type Iax supernova SN1181 hosting a double degenerate merger product WD J005311
2023年7月7日(金) 14:00 - 15:15
黄 天鋭 (東京大学 ビッグバン宇宙国際センター (RESCEU) 博士課程)
Iras 00500+6713 is a bright nebula in the infrared, and X-ray observations show it consists of diffuse region and strong illuminated central region. In addition, optical spectral observations have recently revealed that fast wind with about 15,000 km/s is blowing from the massive white dwarf at the center. The properties of this nebula and white dwarf are very similar to those theoretically predicted by the binary white dwarf merger. In addition, its position on the celestial sphere and the extent make it a prime candidate for the remnant of SN 1181, a historical supernova. In this study, we propose that such a multilayered structure is formed by the collision between the remnant of SN 1181 and the stellar wind blowing from the central white dwarf, and succeeded in constructing a model that is consistent with the multi-wavelength observations. The results show that the progenitor of SN 1181 is a binary white dwarf with 1.3-1.9 solar mass and that their merger triggered an explosion that ejected mass with 0.2-0.6 solar mass to form the present object. The extent of the X-ray source concentrated in the center reveals that these winds began blowing within the last 30 years, and we will discuss this property as well.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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セミナー
Quantum uncertainty of fields and its effect on entanglement generation in quantum particles
2023年5月31日(水) 14:00 - 15:15
杉山 祐紀 (九州大学 大学院理学府 物理学専攻 博士課程)
The unification of gravity and quantum mechanics is one of the important problems. To elucidate the theory of quantum gravity, it is becoming more and more important to get any hint of the quantum nature of gravity. In particular, the quantum-gravity-induced-entanglement of masses (QGEM) scenario, which is expected to observe the quantum nature of non-relativistic gravity, has recently attracted great attention. In this talk, we show the effect of relativistic fields on entanglement generation based on quantum field theory. We also discuss the relationship between the entanglement generation and quantum uncertainty of the fields.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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セミナー
X-ray study on the synchrotron emission in Kepler's SNR
2023年5月19日(金) 14:00 - 15:15
Vincenzo Sapienza (東京大学 大学院理学系研究科 物理学専攻 博士課程)
Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high energy electrons accelerated at the shock front. We performed a spatially resolved spectral analysis of the young Kepler's SNR, where we identify two different regimes of particle acceleration. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with dense CSM. We also studied the temporal evolution of the synchrotron flux, from 2006 to 2014. A number of regions show a steady synchrotron flux and equal cooling and acceleration times. However, we found some regions where we measured a significant decrease in flux from 2006 to 2014. Our results display a coherent picture of the different regimes of electron acceleration observed in Kepler's SNR. Also If I will have time during the seminar it will be nice to present also some preliminary results I will have in the SN 1987A project.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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Towards EeV Neutrino Astronomy with GRAND
2023年4月18日(火) 14:00 - 15:15
クミコ・コテラ (Director of Research, Institute of Astrophysics, France)
We are living exciting times: we are now able to probe the most violent events of the Universe with diverse messengers (cosmic rays, neutrinos, photons and gravitational waves). One challenge to complete the multi-messenger picture resides in the highest energies, as no ultra-high energy neutrinos have been observed yet. This challenge could be undertaken by the GRAND (Giant Radio Array for Neutrino Detection) project, which aims at detecting ultra-high energy particles, with a colossal array of 200'000 antennas over 200'000 km2, split into ~20 sub-arrays of ~10'000 km2 deployed worldwide. In this talk, we will present preliminary designs and simulation results, plans for the ongoing, staged approach to construction, and the rich research program made possible by the proposed sensitivity and angular resolution.
会場: コモンルーム (246-248号室) / via Zoom
イベント公式言語: 英語
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Cosmic magnetism and its effects on the observed properties of ultra high-energy cosmic rays
2023年3月10日(金) 14:00 - 15:00
Ellis Owen (大阪大学 大学院理学研究科 宇宙地球科学専攻 宇宙進化グループ JSPS特別研究員)
Ultra high-energy (UHE) cosmic rays (CRs) from distant sources interact with intergalactic radiation fields, leading to their spallation and attenuation through photo-hadronic processes. Their deflection and diffusion in large scale intergalactic magnetic fields (IGMFs), in particular those associated with Mpc-scale structures, alter the cumulative cooling and interactions of a CR ensemble to modify their spectral shape and composition observed on Earth. In this talk, I will demonstrate the extent to which IGMFs can affect observed UHE CRs, and show that source population models are degenerate with IGMF properties. Interpretation of observations, including the endorsement or rejection of any particular UHE CR source classes, needs careful consideration of the structural properties and evolution of IGMFs. Future observations providing tighter constraints on IGMF properties will significantly improve confidence in assessing UHE CR sources and their intrinsic CR production properties.
会場: via Zoom / コモンルーム (246-248号室)
イベント公式言語: 英語
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Understanding kilonova spectra and identification of r-process elements
2023年1月20日(金) 14:00 - 15:00
土本 菜々恵 (東北大学 大学院理学研究科 天文学専攻 博士課程（学術振興会特別研究員）)
Binary neutron star (NS) merger is a promising site for the rapid neutron capture nucleosynthesis (r-process). The radioactive decay of newly synthesized elements powers electromagnetic radiation, as called kilonova. The detection of gravitational wave from a NS merger GW170817 and the observation of the associated kilonova AT2017gfo have provided with us the evidence that r-process happens in the NS merger. However, the abundance pattern synthesized in this event, which is important to understand the origin of the r-process elements, is not yet clear. In this talk, I will first introduce an overview and current understanding of kilonova. Then, I will discuss our recent findings of elemental features in photospheric spectra of kilonova toward identification of elements.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Mergers of neutron star-neutron star (or black hole) binaries as r-process sites
2023年1月13日(金) 14:00 - 15:00
和南城 伸也 (Senior Scientist, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Germany)
The discovery of an electromagnetic counterpart (kilonova) associated with GW170817 confirms that binary neutron star (NS) mergers are at least one of sites of r-process nucleosynthesis. However, there is no observational evidence that black hole (BH)-NS mergers are r-process sites. In this talk, we overview the latest work of nucleosynthesis based on long-term hydrodynamics simulations of NS-NS and BH-NS mergers covering early dynamical and late post-merger mass ejections. We also briefly discuss a possible constraint on nuclear equations of state.
会場: via Zoom / コモンルーム (246-248号室)
イベント公式言語: 英語
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セミナー
Modelling Optical Signals from Magnetar-Driven Supernovae
2022年12月20日(火) 14:00 - 15:00
Conor Omand (Postdoctoral Researcher, Department of Astronomy, Stockholm University, Sweden)
Many energetic supernovae are thought to be powered by the rotational energy of a highly-magnetized, rapidly-rotating neutron star. The emission from the associated luminous pulsar wind nebula (PWN) can affect the system in different ways, including accelerating the ejecta, ionizing the ejecta, and breaking the spherical symmetry through hydrodynamic instabilities or large scale asymmetries. Modeling the observables from these processes; the light curves, spectrum, and polarization; is essential from understanding the nature of the central engine. I will present the results of a radiative transfer study looking at the effects of a PWN on the supernova nebular spectrum, and the preliminary results from a more physically motivated light curve model for parameter inference, and a study examining the polarization that arises due to hydrodynamic instabilities in the ejecta of engine-driven supernovae.
会場: コモンルーム (246-248号室) / via Zoom
イベント公式言語: 英語
56 イベント