Speaker: Shin Mineshige
Title: Warm corona model for AGN soft excess
Abstract: We discuss the origin of the soft excess observed in most of AGN spectra below ~2 keV on the basis of the so-called warm corona model. The issues to be answered are (1) what determines the warm corona temperature, (2) what are the conditions to form warm coronae, and (3) why no clear line features are observed.
Speaker: Yuh Tsunetoe
Title: Polarimetric Features of the Black Hole-Disk-Jet Images: Towards Unified View on AGN Jets
Abstract: Based on theoretical polarimetric images based on radiative transfer calculation, we propose that both of the linear and circular polarization components contain the useful information on the magnetic field configuration and other plasma properties. In the polarimetric images around the supermassive black hole, polarization components produce a characteristic distribution along the jet, linear polarization vectors in the downstream and circular polarization components in the upstream, due to combination of two Faraday effects, rotation and conversion. Further, we also suggest that the polarimetric images present more diverse features depending on the observer's viewing angle. These results imply that a unified analysis of linear and circular polarization will lead to a better understanding of the AGN jet mechanisms from the viewpoint of unified interpretation of AGNs.
Speaker: Masayuki Furuno
Title: Stability of Disk Corona System
Abstract: Observations of active galactic nuclei (AGN) have shown that, apart from the component of thermal emission from the accretion disk, there is an excess in the soft X-ray region and a continuous component in the hard X-ray region. A model of a layered corona coexisting on the accretion disk is one of the explanations for this observation. In a previous study, magnetic reconnection was assumed as the heating mechanism of the corona and presented spectral energy distributions for a disk corona model (Liu et al. 2003). In our study, using the fraction f of energy dissipated in the corona calculated from the same code as in this previous study, the thermal equilibrium curve of the disk corona system from the Shakura-Sunyaev model is shown to be more stable than that of the disk in the absence of a corona.
Speaker: Shogo Yoshioka
Title: Large-scale outflow structure of super-Eddington accretion
Abstract: The super-Eddington accretion flow has two key signatures: it can shine in excess of the Eddington luminosity, and it has powerful outflows due to the increase in radiation force. In order to precisely evaluate the impacts by super-Eddington accretors to their environments, it is essential to assure a large enough simulation box and long computational time to avoid any artifacts from numerical settings as much as possible. We carry out axisymmetric two-dimensional radiation hydrodynamic simulations and study the large-scale outflow structure and radiation properties of super-Eddington accretion flow for a variety of accretion rates. We find that the mechanical luminosity grows more rapidly than the radiation luminosity with an increase of accretion rate. The reason is that the higher accretion rate is, the more vertically inflated becomes the disk surface, which makes radiation fields more confined in the region around the rotation axis, thereby strongly accelerating outflowing gas. We analyze physical quantities along each outflow trajectory, finding that the Bernoulli parameter (Be) is not a good indicator to discriminate pure and failed outflows, since it is never constant because of continuous acceleration by radiation-pressure force. Pure outflow can arise, even if Be < 0 at the launching point. As a further topics, we also discuss the progress of our ongoing spectral calculations and RMHD simulations.
Speaker: Keiichi Maeda
Title: Type "Iax" Supernovae as a failed white-dwarf explosion?
Abstract: Type Iax supernovae (SNe Iax) represent an enigmatic variant of type Ia SNe. While SNe Iax are believed to share its origin with canonical SNe Ia, as being related to a thermonuclear white-dwarf (WD) explosion, the difference in their origins has not been fully understood. One of the most striking observational features whose origins are yet to be explained is persistent photospheric spectra (i.e., allowed transitions) over several 100 days, which is totally different from canonical SNe showing transition to nebular spectra (i.e., forbidden transitions). In this talk, we show the results of in-depth analysis of late-time spectra of SN Iax 2019muj, and provide a very-likely answer to this problem - existence of very dense and iron-rich core. With constrains on masses and kinematics of major elements seen in the core, we develop a simple toy model to explain the origin of the inner region with such unprecedented properties; the initial explosion is partly failed and leaves a remnant WD polluted by newly-synthesized 56Ni, which then energizes the remnant WD through radioactive decay and later launches the second, dense and inner ejecta component.
Speaker: Kenta Taguchi
Title: Initial Spectra of the Galactic Nova V1405 Cas: Possible Sub-solar-mass ONeMg White Dwarf
Abstract: A nova is a transient event occurring in a close binary of a white dwarf (WD) and a late companion star, being caused by the thermonuclear runaway of the H-rich accreted material from the companion star onto the WD. Here we report our initial spectra of the very slow Galactic nova V1405 Cas (= Nova Cassiopeiae 2021) taken 9.88, 23.77, 33.94, 71.79, and 81.90 hours after the discovery. The first spectrum shows He II and N III emission lines. It is the first detection of these highly-ionized ionized lines within 24 hours of the discovery of the very slow novae. In the later epochs, these emission lines have almost disappeared, and lower ionized lines, including Al II (6237 A), become prominent. As long as we know, this aluminum line had never been found as an emission line in past nova spectra. It indicates that the ratio Al/H is much excessed (more than ten times larger) than the solar system and that an ONeMg WD is preferred as the host to a CO WD. However, the light curve indicates the WD mass is significantly less than the solar mass, deviating from the standard single-star evolution.
Speaker: Qiliang Fang
Title: Aspherical distribution of the explosive burning ash of core collapse supernovae
Abstract: A core-collapse supernova (CCSN) is the explosion that announces the death of a massive star. Outstanding questions remain about the explosion mechanism, which is reflected in the geometry of the innermost ejecta and its relation with the progenitor mass. Here we present late phase spectra of a large sample of CCSNe from stripped envelope stars, and use the profiles of the calcium and oxygen lines, which are emitted from the explosive burning ash and the unburnt material respectively, to explore the geometry of these two regions and their mutual relations. About half of the objects show clear signatures of substantial deviation from a spherically-symmetric explosion. Furthermore, in these aspherical SNe, the calcium and oxygen profiles show a clear anti-correlation: either double-peaked calcium with single-peaked oxygen or single-peaked calcium with double-peaked oxygen. This anti-correlation can be naturally explained by varying the viewing angle, if the explosion creates a bi-modal distribution in the iron-rich burning ash. The combined analysis of the line profiles and the progenitor mass further suggests the degree of the asphericity grows with the
mass of the carbon-oxygen core, posing a strong constrain on the explosion mechanism of CCSNe.
Speaker: Kohki Uno
Title: Beyond David Branch: Machine-Learning Classification of Supernova Spectra
Abstract: While type Ia supernovae (SNe Ia) have been used as a powerful distance indicator to measure the expansion history of the Universe, their astrophysical nature has not been fully understood. One important issue is how their observational diversities are related to different populations, which might affect the precision of the cosmological distance measurement. In this work, we apply a cluster analysis in the framework of unsupervised machine learning (ML) to a sample of SN Ia spectra. It not only reproduces the known major populations, but also hints the existence of a previously-hidden population. Surprisingly, while it emerges from the normal-velocity SN class in the classical classification, its typical environment within host galaxies is similar to that found for the high-velocity SN class. Our findings provide brand-new information in our understanding the origin of SN Ia diversity, highlighting a power of the unsupervised ML for astrophysical data analysis.
Speaker: Satoko Inutsuka
Title: 1D Simulations of Type IIn Supernovae Interacting with Symmetric/asymmetric CSM
Abstract: ype IIn supernovae(SNe IIn) are subclass of core-collapse supernovae. They have narrow H emission lines in their spectrum. Generally, their luminosity are high, and also show long rise time. Those feature are caused by interaction with CSM(circumstellar matter). According to Soumagnac et al.(2019), there are two populations of SNe IIn, that are (i) luminous and have evolving photosphere and (ii) dimmer and steady (~10^15cm) photosphere. Our study aims to explain the two populations. We are doing 1D hydrodynamic+ radiative transfer simulation using SNEC code(Morozova et al. 2015) for different geometry of CSM, i.e. spherical and disk-like. In this seminar, I will talk the result of simulation with spherical CSM.
Speaker: Mao Ogawa
Title: Numerical Calculations of Euler Equation with HLLC methods
Abstract: Fluid dynamics simulations are important in understanding various physical phenomena in the universe. For example, the time evolution of the shock waves can be solved by Euler equation, which is the basic equation of fluid dynamics. However, it is hard to solving them because of numerical instability. In this study, we calculated the time evolution of the shock front using the HLLC method, a numerical scheme that can correctly solve the contact discontinuity. We make a numerical code by ourselves for shock tube, sedov solution, and time evolution of circumstellar material (CSM) around supernovae. In this talk, I will talk about result of them.
Speaker: Masaki Tsurumi
Title: Triple evolution: an important channel in the formation of type Ia supernovae
Abstract: Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear explosions in white dwarfs (WDs). Commonly considered formation pathways include two merging WDs, and a single WD accreting material from a H or He donor. Since the predicted SNe Ia rates from WD in binaries are thought to be insufficient to explain the observed SNe Ia rate, it is important to study similar interactions in higher-order multiple star systems such as triple systems. They use the evolutionary population synthesis code Multiple Stellar Evolution (MSE) to study stellar evolution, binary interactions and gravitational dynamics of the triple-star systems. In this talk, I will introduce their results and they imply that triples, when considering their entire parameter space, yield an important contribution to the overall SNe Ia rate.
Speaker: Haonan Ye
Title: The tidal deformation of the White Dwarf
Abstract: Tidal interactions affect not only the mass distribution of the planet, but also the motion of the planet along its orbit, and even the waveform of gravitational waves. In Newtonian mechanics, TLN is the proportional constant between the tidal field exerted on a planet and the multipolar moment of its mass distribution. In relativistic cases, the tidal field will result in a tidal moment, which causes the TLN not to be a constant. As the progenitor of the Type Ia SNe, White Dwarf are the most think of the great source of GW, by analyzing the tidal deformability of WD we may be able to obtain the evolution of white dwarfs and have a significant impact on other areas of cosmology.
Speaker: Herman Lee
Title: Three-dimensional hydrodynamic simulations of core-collapse and Type-Ia supernova remnants for the era of micro-calorimeter X-ray spectroscopy and imaging
Abstract: In preparation for upcoming super-high resolution X-ray spectroscopy of supernova remnants (SNRs) in the next decade, we have developed a novel method for high efficiency and low computational cost 3-D hydro simulations specializing on the calculation of broadband X-ray emission from SNRs or young SNe of various types of progenitors. Using a hybrid Eulerian-Lagrangian approach on either a fixed or non-inertial co-moving mesh (commonly used for cosmological simulations), our code can follow long-term evolution of SNRs and their plasma and ionization states over a large range of space and timescales. The outputs can be fed to a parallelized atomic physics code to compute the time-dependent spectra and so on. These can further be convolved by the responses of any observational instruments in question (such as XRISM, ATHENA) to provide predictions or direct comparison with data. Our team is particularly putting effort in advancing the approach to support science cases of future concept instruments such as the Line Emission Mapper (LEM) to propel X-ray astronomy of supernovae and SNRs.
Speaker: Ryosuke Kobashi
Title: Constraining the circumstellar environment of Tycho's SNR
Abstract: Among type Ia supernova remnants (SNRs), Tycho's SNR (Tycho) is considered to be a typical remnant from the viewpoints of almost spherical shape, unique density for ambient medium (Slane et al. 2014) and normal feature in terms of X-ray spectrum and light echo. However, recent results of Tycho’s radio and X-ray observations (Katsuda et al. 2010; Williams et al. 2016) show azimuthal variability of shock motion and CO observation (Zhou et al. 2016) suggest Tycho is surrounded by cloud swept by wind-like cavity. Moreover reanalysis of Tycho's Chandra data show that since around 2007 Tycho's shock is experiencing substantial deceleration that infer interaction with molecular cloud, and support this cavity-wall environment. Such non-uniform environment have implications on activities by a progenitor. From these information, knowing Tycho's age, we can strictly give a constraint on Tycho's environment and thus progenitor. In this study, from the angular size of forward shock we use the correspondent values for low-density in cavity and distance (to the earth) which is necessary to convert proper motions to physical values. Using the correspondent values, we calculate the evolution of Tycho's shock using hydro simulation with a range of parameters for environments, compare the results with observation data and search for best-fit parameters using chi squared for each azimuthal region, and obtain the picture of Tycho's CSM. In this presentation, I will present the preliminary results for what Tycho's CSM looks like.
Speaker: Takanori Sasaki
Title: Habitable Planet Formation around White Dwarf
Abstract: Several tens of white dwarfs are known to host circumstellar debris disks, thought to arise from the tidal disruption of rocky bodies originating in the star’s remnant planetary system. In the disk, outwards-spreading material flows beyond the Roche limit and coagulates into a new planet in a process analogous to the formation of the Moon. The recycling of a tidally disrupted super-Earth, for example, could yield an Earth-mass planet on a ∼10h orbit, located in the habitable zone for 2-to-10-Gyr-old white dwarfs. I will introduce some pioneering works of the planet formation around white dwarfs, and discuss the habitability of such a planet.
Speaker: Keiya Murashima
Title: Smoothed Particle Hydrodynamics Method
Abstract: Smoothed Particle Hydrodynamics (SPH) method was developed to simulate phenomena in astrophysics by Lucy (1977) and Gingold & Monaghan (1977). I will introduce theory and fundamentals of SPH. I will also show how to create initial conditions, which are often omitted in papers.
Speaker: Ryo Ikeda
Title: Estimation of the survival probability of (extra)terrestrial life
Abstract: Several exoplanets have been discovered to present, and it will be important to search for extraterrestrial life. In the paper that I will review , a new approach is introduced to estimate the probability that life on Earth has survived from birth to the present based on its terrestrial extinction history. Consequently, we found that the estimated probability is ~0.15.Also, this value affects a parameter in the Drake equation, which contributes to estimating the number of life-bearing exoplanets.
Speaker: Kaname Taniyasu
Title: Important results of JWST -detection biomarker in an exoplanet's atmosphere-
Abstract: To find a planet with extraterrestrial lives, we mainly discuss about whether liquid water exist on planet surface. However, Oxygen, Methane, Carbon dioxide, and Sulfur dioxide are also important as a basis for the existence of life. Such a detectable atmospheric species or set of species are called as “biomarker”.In this talk, I will introduce how biomarker is and NASA’s announcement that James Webb Space Telescope made the first identification of sulfur dioxide in an exoplanet’s atmosphere.
Speaker: Shumpei Nagoshi
Title: Probing the origin of broad line region by reverberation mapping of an extremely variable quasar
Abstract: While the unified model of quasar structure provides a concise description of diverse spectra, the physical origins of components (e.g., broad line region, dust torus, and narrow line region) still need to be solved. To explore the physical origin of the unified model, we focused on Changing-State Quasar (or Changing-Look Quasar) as a research target, as we expect that observing structural changes associated with state transitions can provide insight into the origins of each structure. We conducted reverberation mapping to the most variable CSQs with Seimei Telescope. I will talk about the result of it.
Speaker: Naho Fujita
Title: 会社紹介と近況報告
Abstract: 主にリクルーターとしての会社紹介を行います。その中で、自分が最近行っている活動などについても少し触れたいと思います。