Workshop of Theoretical Astrophysics on December 2020

Detailed Program

You can see the titles and abstracts below:

Speaker: Shin Mineshige
- Time: 14:05-14:25, December 3rd (Thu)
- Title: Super-Eddington Accretion Flow: What is next?
- Abstract: I will outline the remaining issues to be explored in the field of super-Eddington accretion flow.

Speaker: Takaaki Kitaki (Zoom)
- Time: 14:25-14:45, December 3rd (Thu)
- Title: Outflow from super-Eddington flow: where it originates from and how much impact it gives?
- Abstract: It is widely believed that super-Eddington accretion flow can produce powerful outflow, but where it originates from and how much mass and energy are carried away to which directions? To answer to these questions, we newly perform a large-box, two-dimensional radiation hydrodynamic simulation, paying special attention lest the results should depend on adopted initial and boundary conditions. We could achieve a quasi-steady state in an unprecedentedly large range, r = 2 rs - 600 rs from the black hole. The accretion rate onto the central black hole is M._BH ~ 180 L_Edd/c², whereas the mass outflow rate is ~ 24 L_Edd/c². The ratio is much less than those reported previously. By careful inspection we find that most of outflowing gas which reach the outer boundary originates from the region at R < 140 rs, while gas at 140 rs - 230 rs forms failed outflow. Therefore, significant outflow occurs inside the trapping radius ∼ 450 rs. The power ratio between the total mechanical luminosity and the isotropic X-ray luminosity is L_mec/L_X^ISO ~ 0.05 - 0.08, in good agreement with the observations of Ultra-Luminous X-ray sources surrounded by optical nebulae.

Speaker: Nacho Botella
- Time: 14:45-15:05, December 3rd (Thu)
- Title: A comprehensive study of gas evoultion around an AGN.
- Abstract: There have been observational measures of supermassive black holes (SMBH) in the early universe that defy our understanding of accretion physics. Our objective of this project is to tackle the question of the formation of such gigantic object so early in time. For that I will show how we use radio-hydrodynamical (RHD) simulation tools to see, with high definition, what happens in the gas surrounding these objects. Because the study of these objects cannot be done solely from an astrophysics standpoint, we need the cosmological data of the universe at the time of birth of the SMBH.

Speaker: Norita Kawanaka (Zoom)
- Time: 15:20-15:40, December 3rd (Thu)
- Title: What Determines Unique Spectra of Super-Eddington Accretors?: Origin of Optically Thick and Low Temperature Coronae in Super-Eddington Accretion Flows
- Abstract: While the typical optical depth and temperature of the coronae seen in black holes with sub-Eddington accretion rates are $\tau \lesssim 1$ and $k_{\rm B} T \sim 100~{\rm keV}$, respectively, it is inferred from the spectral fitting that the coronae in black holes with super-Eddington accretion rates such as ultraluminous X-ray sources (ULXs), GRS 1915+105, and narrow-line Seyfert 1 galaxies (NLS1) are relatively cool ($k_{\rm B} T \lesssim 10{\rm keV}$) and optically thick ($\tau \gtrsim 3$). To understand their physical origin, we investigate the emission properties of the corona which is formed by the gas blown off the super-Eddington inner disk by radiation pressure. We assume that the corona is heated by the reconnection of magnetic loops emerged from the underlying disk. We show that this radiation pressure driven wind can act as an optically thick corona which upscatters thermal soft photons from the underlying disk, and that with a reasonable parameter set we can theoretically reproduce the coronal optical depth and temperature which are inferred by spectral fittings of observational data. By contrast, the coronal optical depth cannot be so high in sub-Eddington cases, since the coronal material is supplied from the disk via evaporation and there is a maximum limit on the evaporation rate. We support that the low temperature, optically thick Comptonization should be a key signature of super-Eddington accretion flow.

Speaker: Yuh Tsunetoe
- Time: 15:40-16:00, December 3rd (Thu)
- Title: Polarization Images around the SuperMassive Black Holes in M87* and Sgr A*
- Abstract: With unprecedented angular resolution, the Event Horizon Telescope (EHT) has opened up a new era of black hole studies. With the EHT polarimetry in mind, We have previously calculated the expected polarization images of M87*. There, we demonstrated that circular polarization (CP) images, as well as the linear polarization (LP) maps, can convey quite useful information to us, such as the flow structure and magnetic field configuration around the black hole. Recently, we made new predictions for the cases in which disk emission dominates over jet emission, bearing Sgr A* in mind. As a result, we obtain ring-like images and triple-forked images around the black hole for face-on and edge-on cases, respectively. We also find significant CP components on the images both with positive and negative signs, amplified through the Faraday conversion. These results indicate that future full polarization EHT images of M87* and Sgr A* are a quite useful tracer of the magnetic field structure.

Speaker: Masayuki Furuno (Zoom)
- Time: 16:00-16:20, December 3rd (Thu)
- Title: The spectrum calculation of low luminosity accretion flow for detecting isolated Black Holes
- Abstract: It is expected that there will be many black holes in the galaxy. To know how many black holes exist in galaxies and how they are distributed is important in discussing the theory of black hole formation and stellar evolution. However, only X-ray binaries are currently observed, and the number is at most dozens. On the other hand, theoretical predictions indicate that there are as many as 10^8 (e.g., Caputo+2017), and most of the undiscovered ones are isolated black holes (Fender+2013). In this seminar, I will explain the outline of IBH researches and the calculation result of the spectrum when IBH rushes into the gas cloud existing in the galaxy, causes gas accretion.

Speaker: Keiichi Maeda
- Time: 9:40-10:00, December 4th (Fri)
- Title: ALMA View on Activity of a Massive Star in the Final One Year
- Abstract: We are organizing the effort for quick ALMA follow-up observations of nearby supernovae (SNe) to extract key information of evolution of massive stars in the final decade, or even in the final year, before the SN. In this talk I will show an example of one nearby SN. Our analyses of the data from the ALMA telescope at 100 GHz, as combined with the lower frequency data obtained through different telescopes (e.g., VLA), clearly show non-smoothed distribution in the very vicinity of the SN, corresponding to the fluctuation of the mass-loss properties in the final sub-year time scale. The time scale thus we have derived indicates that the unknown final activity could be driven by the change of the nuclear fuel in the last stage.

Speaker: Ryoma Ouchi
- Time: 10:00-10:20, December 4th (Fri)
- Title: Are 56Ni-poor Stripped envelope supernovae really deficient?
- Abstract: The nuclear decay of 56Ni is one of the most important power sources of supernovae (SNe). Recent works have been indicating that the 56Ni masses estimated for Stripped Envelope SNe (SESNe) are systematically higher than those estimated for SNe II . Although this may indicate the distinct progenitor structure or explosion mechanism between these types of SNe, the possibility remains that this may be caused by observational bias. One important possible bias is that SESNe with the low 56Ni mass are dim, so that they can be escaping from the detection. By investigating the distributions of the 56Ni mass and distance for the samples collected from the literature, we have found that the SESN sample suffers from a significant observational bias, i.e; the objects with low 56Ni masses are missed, especially at distant locations. This indicates that the different 56Ni masses between SNe II and SESNe may be, at least partly, explained by the observational bias. We also conducted mock observations assuming that the Ni mass distribution for SESNe is intrinsically the same with that for SNe II. From this analysis, we have found out that the 56Ni distribution of the (detected) SESNe samples becomes more massive than the assumed intrinsic distribution, which explains the general trend of the SESNe sample collected from the published literature.

Speaker: Kenta Taguchi
- Time: 10:20-10:40, December 4th (Fri)
- Title: Towards Quick Follow-up Observation for Classical Novae Using Seimei Telescope
- Abstract: A classical nova is a transient which brighten ~10 mag in $\lesssim 1$ day. Many lines having P Cygni profile is found in a nova spectrum around the optical peak ($\gtrsim$ 1 days after beginning of the nova outburst), which suggests that there should be an optically thick wind blowing outwards. However, Arai et al (2015) reported a spectrum taken during the brightening stage. Their spectrum has not lines of P Cygni profile but emission lines. It means that there is an unknown stage in early nova, and therefore it is said that our understanding of nova is not enough. More and more nova spectra taken in super-early stage and their physical interpretation are needed! We are tackling both of them, but in this talk I deal with former. That mean, I will talk about our program of ToO (Time of Opportunity) observation using Seimei telescope. We were not able to quickly observe enough targets in the first year of Seimei, but recently we are better observers. Until now, we managed to take some dwarf novae, a nova in M31, and some transients having not been analyzed the data very quickly.

Speaker: Roberto Iaconi
- Time: 10:55-11:15, December 4th (Fri)
- Title: First steps in simulating common envelope involving massive stars
- Abstract: I will discuss some of the numerical difficulties in reproducing the common envelope interaction between massive stars.

Speaker: Tomoki Matsuoka (Zoom)
- Time: 11:15-11:35, December 4th (Fri)
- Title: Radio Emission from Ultra-stripped Supernovae as Diagnostics for Properties of the Remnant Double Neutron Star Binaries
- Abstract: An ultra-stripped supernova (SN) is an explosion of a helium or C+O star whose outer envelope has been stripped away by a companion neutron star. A double neutron star (DNS) binary is believed to be left after the explosion, which will emit the gravitational wave later at the coalescence. Recent detections of a few candidates for the ultra-stripped SN have constrained the properties of the explosion and the progenitor, but little information is given as to whether the remnant DNS binary will merge within the cosmic age. A large fraction of the material stripped away from the helium star through the binary interaction is expected to escape from the system and form circumstellar material (CSM). The CSM should be traced by radio emission induced by the collision with the SN ejecta. Based on the stellar evolution models previously developed, we calculate the expected radio luminosities from ultra-stripped SNe. We find that high radio luminosity at its maximum can be an indicator of small separation of a DNS binary leading to its merger within the cosmic age. Our results can be used to optimize the strategy for the radio follow-up observations such as observational epochs and frequencies.

Speaker: Kohki Uno
- Time: 11:35-11:55, December 4th (Fri)
- Title: My Future Research Plan
- Abstract: Having presented my recent research at the last Tuesday seminar, at the workshop, I will talk about my plans for future work. Now, I develop one-dimensional radiation hydrodynamics simulation code. In this talk, I will present about its progress and what I want to study using the code.

Speaker: Kazuya Iwata
- Time: 11:55-12:15, December 4th (Fri)
- Title: Fundamental researches on chemical / thermonuclear detonation wave
- Abstract: In the present talk, I'm going to about my research activities so far on chemical detonation wave related to aerospace propulsion device, and my work-in progress research ignition process of thermonuclear detonation of surface He layer in the scenario of double-detonation considered to be one of the potential progenitor in Ia type supernovae.

Speaker: Takanori Sasaki
- Time: 13:40-14:00, December 4th (Fri)
- Title: A Comprehensive Model of Satellite Formation of Rocky, Gas, and Ice Planets
- Abstract: The solar system contains different types of planets: small rocky planets, gas giants, and ice giants, six of which have satellite systems of varying orbital configurations, masses, and compositions. Some satellites have internal oceans and atmospheres. Plans to explore the satellites of Jupiter and Mars have been discussed. While many exoplanets have been discovered, their satellites are becoming detectable. Data on satellites inside and outside the solar system are expected to be expanded in the future. The formation of a satellite system is essentially a thermal and dynamical evolution of a mixed solid/liquid/gas phase circumplanetary disk, and the diversity of the composition of each phase gives rise to the diversity of the satellite system. In this talk, I will introduce a theoretical viewpoint to unify the conventional theories to construct a comprehensive model of satellite formation for all types of planets.

Speaker: Akihiro Yamanaka
- Time: 14:00-14:20, December 4th (Fri)
- Title: The possibility of giant satellite formation around super Jupiter-mass planet
- Abstract: Among 5747 extrasolar planets, the number of known habitable planets is only 46 (ExoKyoto, Kopparapu+ 2013). Being a habitable world requires the body to be in the habitable zone, and be a terrestrial body. If an extrasolar satellites (exomoons), orbiting a gas giant at stellar habitable zone is large enough to have a substantial atmosphere, it could be habitable. Since satellite is heated by illumination and tide from the host planet in addition to stellar illumination, it is possible for an exomoon to be habitable even beyond stellar habitable zone. Using a common mass scaling law of gas giant planets (Canup & Ward 2006), it has been suggested that super Jovian planets can have such massive satellites. We investigate the possibility of large satellite forming around super Jupiter-mass planet via core accretion scheme by semi-analytical simulations.

Speaker: Yohdai Kihara (Zoom)
- Time: 14:20-14:40, December 4th (Fri)
- Title: 円盤の熱進化を考慮した天王星周りの衛星形成
- Abstract: 天王星周りには27個の衛星が存在する。そのうち5つの主要な衛星が衛星系全体の質量の99%を担っているそして天王星に限らず、巨大惑星まわりの衛星達がどのようなメカニズムで形成されたのかについて様々なモデルが存在する（Canup & Ward. 2006;Sasaki et al. 2010 ）。しかし他の太陽系内惑星とは異なり天王星の赤道傾斜角は98 度傾いており、その自転軸の周りを衛星が回転している。そのため上記の論文のモデルをそのまま適用しても、軸を傾けるためのメカニズムを別に考えなければならない。その系を説明する方法として、二つの原始惑星が衝突し、その衝突によって自転軸が曲げられかつ破片が自転軸周りに散らばりデブリ円盤が形成されるというジャイアントインパクト説が提唱された。実際にこの仮説の元SPH計算が行われた結果、赤道傾斜角の傾きと衛星の元となるようなデブリ円盤が惑星周りにできることが説明された（Slattery et al.1992 ）。そのデブリ円盤をもとにN体計算で今の衛星系を形成できるかが検証されたが、初期円盤の面密度は中心惑星より外側のほうが小さくなるので、外側の衛星の質量が実際の観測結果より小さくなることが分かった（Ishizawa et al. 2019）。上記の外側の衛星の質量が観測より小さくなってしまうという点を解消するために、ジャイアントインパクトによってできたガス円盤の熱進化を考慮した天王星周りの初期円盤モデルが提唱された（Ida etal. 2020 ）。本発表ではこの円盤モデルの物理過程について、および円盤の熱進化を考慮することによってなぜ天王星周りの衛星形成の説明にとって都合がいいのかを紹介する。また我々が行っている実際に観測から分かっている衛星系の再現を目的とした、円盤の熱進化を考慮したN体計算についても紹介する。

Speaker: Naho Fujita
- Time: 14:55-15:15, December 4th (Fri)
- Title: Orbital evolution of close-in super-Earths via atmospheric escape
- Abstract: Recently, a lot of super-Earths close to their host stars have been discovered especially around M dwarfs. Orbital distributions of planets in multiple close-in super-Earth systems are different each other. The mass-radius relationship of the super-Earths indicates that some of them have a substantial atmosphere, while others have almost no atmosphere. The origin of these differences remains unclear. Close-in super-Earths would undergo atmospheric escape via the energy-limited hydrodynamic escape driven by stellar X-ray and UV irradiations. Considering the angular momentum of a system, the super-Earths those experience atmospheric escape and lose their mass must move outward. However, little attention has been given to the orbital evolution of the planets due to the mass loss by hydrodynamic escape of their atmosphere. In this study, we aim to gain a better understanding of the origin of multiple close-in super-Earth systems by calculating the atmospheric mass loss and the orbital evolution of the planets. In this talk, I will also introduce the relationship with the future observations of super-Earths around M dwarfs.

Speaker: Keiya Murashima
- Time: 15:15-15:35, December 4th (Fri)
- Title: Toward a global SPH simulation of an icy moon with internal ocean
- Abstract: There are some traces of existence of internal ocean in some icy moons, such as a plumes of vapor of Europa and Enceladus. Since liquid water would be essential for the origin of life, it is important to understand the development of inner sea, especially the temperature distribution/evolution inside the icy moons. It is considered that the balance between the tidal heating caused by tidal acceleration and radiative cooling sustains liquid water beneath the surface of an icy moon. Thus, we aim to simulate tidal heating an internal ocean of an icy moon by numerical fluid calculations using Smoothed Particle Hydrodynamics (SPH) method , which is a particle-based method for simulations of fluid dynamics. We added viscosity terms into the governing equations of SPH. However, we found two problems in calculating the rigid body rotation by using SPH including viscous term. In order to settle problems, we modified the formulation of viscosity and introduce Density Independent SPH (DISPH).

Speaker: Herman Lee
- Time: 15:35-15:55, December 4th (Fri)
- Title: On the origin of SNR 3C397 - a comparison with SN-SNR hydrodynamical evolution models
- Abstract: 3C397 is a Galactic SNR with a highly elongated, box-like morphology in X-ray which seems to be caused by a strong interaction with a dense medium in the surrounding environment, suggesting a core-collapse (CC) origin. However, recent X-ray spectroscopic studies have revealed a super-solar chemical abundance from the remnant with significant enhancements seen in isotopes like 55Mn and 58Ni, which instead favors a near-Chandrasekhar mass Ia progenitor. In this study, we compare a broadband X-ray spectrum from an analysis of archival Suzaku XIS data with a diverse grid of Ia and CC SN-SNR evolution models based on available SN explosive nucleosynthesis simulations. We contrasted bulk properties of the modeled X-ray spectra like line centroids and flux ratios against the observation results, and found that 3C397 is most consistent with an evolved near-Mch Ia SNR interacting with a dense medium (2 - 5 particles/cc), which is atypical among known Ia objects.

Speaker: Ryosuke Kobashi (Zoom)
- Time: 15:55-16:15, December 4th (Fri)
- Title: Long-term evolution of non-thermal emission from SNRs in various environments
- Abstract: Supernova remnants (SNRs) are thought to be one of the origins of high-energy accelerated particles, so-called cosmic rays (CRs), in our galaxy. Non-thermal emission from SNRs, especially gamma-ray emission and radio emission, is thought to be originating from CRs locally accelerated at the SNR shocks. To understand how much CRs a SNR can produce at every age and throughout its lifetime, self-consistent simulations of the time-evolution of non-thermal emission is useful. Information from SNR non-thermal emission can also be useful for understanding the circumstellar environment of SN progenitors as well as their pre-SN mass loss histories. Yasuda & Lee (2019) simulated the time-evolution of non-thermal emission from SNRs until 5,000 yr, without including the effect of radiative cooling and associated physics which occur after the shock has slowed down, so discussion on the production of CR from SNRs all the way through the radiative phase (>~10,000 yr) was impossible. In this study, as a natural extension of the Yasuda & Lee study, we implemented these effects as well as additional important components such as re-acceleration of CRs for the study of both core-collapse and type Ia SNRs. Within our parameter space, our results showed that most of non-thermal spectra are mainly dictated by the condition of surrounding of SNRs, and the effect of radiative cooling on the non-thermal emission is evident in some specific cases.