投稿者: Yuko Matsushita

On the nature of extremely low-density exoplanets: Haze or Ring?

speaker: Kazumasa Ohno

Abstract:
Observational efforts in the last decade have discovered more than 5000 exoplanets. The most enigmatic exoplanet within the current sample may be extremely low-density (<0.1 g/cm^3) exoplanets called super-puffs, which are Jupiter-sized planets with masses lower than Neptune. Planetary interior structure model indicated that the extremely low bulk density could be explained by the presence of massive atmosphere, whereas it remains mystery why such low-mass planets could acquire massive atmospheres and why they could retain the atmospheres until today. Recent studies have suggested is that the observed radii of super-puffs are rather overestimated somehow, such as due to atmospheric dust or presence of circumplanetary ring. Here, we discuss the feasibility and observational implication for each scenario. We first use a microphysical model of grain growth in exoplanetary atmospheres to study how the atmospheric dust affects the transit observation of super-puffs. We found that the intense atmospheric escape suggested for super-puffs can bring the dust to upper atmosphere to enhance the observable radius if the dust is formed at relatively upper atmospheres, indicating that photochemically produced particles may be responsible for the anomalously large radius of some of super-puffs. Next, for the circumplanetary ring hypothesis, we have developed a method to include the ring in the calculations of atmospheric transmission spectra for arbitrary viewing orientation. We showed that the presence of ring tends to explain both anomalously large radius and flat atmospheric spectrum, in agreement with the current observation of super-puffs. I will discuss the condition with which each process can work to explain the anomalously large observed radius, and how we can distinguish the two hypothesizes from future observations by JWST.

Learning Representations of Galaxies from Observations and Simulations

speaker: Suchetha Cooray

Abstract:
In recent years, advances in machine learning have allowed astronomers to extract valuable insights from large datasets of observations and simulations. Traditionally, representing information of these datasets has been through summary statistics like the scaling relations, stellar mass functions, and correlation functions. However, we now know that there is plenty of information that may be missed out through the use of summary statistics. Therefore, there is a strong motivation to learn efficient representations made possible through unsupervised machine learning. In this talk, I will present two applications of representation learning on observations and simulations. The first application shows that the emissions of local galaxies can be represented by two parameters that correspond to the galaxy’s evolutionary stage and scale. The second application presents a data-driven model of star formation history (SFH) constructed by finding an efficient representation of simulated SFHs in cosmological simulations. This data-driven SFH model contains the physics included in the simulations and can be used to constrain observed SFHs. I will cover the machine learning techniques used to learn the efficient representations for anyone interested in such methods.

Core-collapse of white dwarfs with super Chandrasekhar mass

speaker: Masamitsu Mori

Abstract:
Recently, Fast Radio Bursts (FRBs) from globular clusters have been reported (Kirsten et al. 2022). A model has been proposed suggesting that young neutron stars (magnetars) may be involved in causing FRBs. However, these FRBs have been observed in globular clusters where star formation has ended. This implies the possibility that young magnetars were formed in the old stellar population.
As an explanation for this celestial phenomenon, it has been proposed that a white dwarf (WD) binary system merged and exceeded the Chandrasekhar mass limit, causing a collapse due to a decrease in degenerate pressure through electron capture reactions, leading to a gravitational collapse supernova explosion rather than a type Ia supernova explosion.
Therefore, we conducted a general relativistic neutrino radiation hydrodynamics calculation for a 1.6 solar mass WD and successfully achieved an explosion. In this simulation, we assumed hydrostatic equilibrium and used the Chandrasekhar mass before gravitational collapse as the initial conditions.
In this study, we used a one-dimensional simulation and performed calculations that implemented general relativistic gravity, a state equation based on nuclear physics, and a moment method for neutrino radiation transport. First, we confirmed stability under adiabatic conditions and then calculated the process leading to gravitational collapse through a decrease in the Chandrasekhar mass due to electron capture reactions by solving the neutrino radiation transport. Up to this point, we used Newtonian gravity to reduce numerical errors. Finally, we performed an explosion calculation with consideration of general relativistic gravity for the model that underwent gravitational collapse.
The explosion reproduced in this study was very small, with an explosion energy of $3 \times 10^{48}$ erg and an ejecta mass of $5 \times 10^{-4}~M_{\odot}$.

Rotation in star and planet forming regions

speaker: Nagayoshi Ohashi

Abstract:
One of the remarkable properties of stars is the ubiquity of spinning motion, which can be found in any astrophysical objects in our Galaxy. The rotation period of the Sun is about one month and those of other stars vary a lot. In case of neutron stars they are as small as milli-seconds. Rotation is also commonly observed in dense cores, and it plays essential roles in star and planet formation, i.e., rotation is crucial to form protoplanetary disks around young stellar objects (YSOs) and also to drive outflows/jets. Nevertheless, rotation in dense cores has been studied less systematically than infall and outflow motions to date
In my presentation, I will review previous key works studying rotation in dense cores and try to summarize the latest understanding of how angular momentum transfers from outer parts to inner parts of dense cores based on my personal view. As a related topic, I will also discuss the latest observational results of embedded disks with an emphasis on the eDisk ALMA large program.

Development of GPU-accelerated simulation code for planetesimal accretion

speaker: Yuki Kambara

Abstract:
During the planet formation process, interaction with the gas of protoplanetary disk is important. Gas drag changes the random velocity and semimajor axis of the planetesimals and also causes orbital migration. Recently, a model of gas evolution in protoplanetary disks has been proposed that takes into account the effects of MHD disk wind and photoevaporation. However, only very simplified gas disk models have been considered in simulations of planetesimal accretion. In this study, we investigate the effect of difference of gas disk evolution models on the accretion process of planetesimals. In order to perform N-body simulations considering various gas disk evolution models, I am developing a simulation code. Fourth-order Hermite method is implemented for time integration and GPU acceleration is also implemented. In this presentation, I will present the current situation of the development and the prospects for future simulations.

Dynamics of planetesimal rings

speaker: Yohsuke Mizutani

Abstract:
The existence of planetesimal rings in planet formation has attracted attention from both observational and theoretical perspectives. However, most of the studies discussing planet formation based on the assumption of planetesimal rings do not follow the growth of the planetesimal rings themselves. Moreover, although orbital evolution of individual planetesimals and planetesimal accumulation in planetesimal systems have been studied, the consideration these system as a ring and the study on the evolution of the ring itself have not yet been done. Purpose of this study is to understand the evolution of planetesimal rings. As a first step, we focused on the orbital evolution of the rings. While the motion of planetesimals is dominated by the gravity from the central star, the orbital evolution is affected by the viscous stirring and gas drag in the scope of this study. We investigated the evolution of planetesimal rings using N-body simulations. We focused on the orbital evolution of the rings, specifically the migration and diffusion of the rings, and the evolution of the orbital eccentricity and inclination of the rings’ planetesimals. We also investigated the dependence of orbital evolution on parameters by varying the initial parameters of the planetesimal rings. In this talk, I present evolution of these planetesimal rings shown for the first time by N-body simulations.

Unknown Ionization Sources in Local Dwarf Galaxies

speaker: Shun Hatano

Abstract:
We reconstruct ionizing spectra of dwarf galaxies in 13.6 — 100 eV range, exploiting Umeda+22 method. We compare this ionizing spectra with black hole (BH) disk models of Kawaguchi 03. We find that ionizing spectra of some dwarf galaxies can be explained by the BH disk models with BH masses of 1e3-1e5 Mo.

Due to the COVID-19 situation, the colloquium is running online for this semester.

Date	Speaker	Title	Time and direction
2022-9-28	Student talks Aditya, Giada	Predicting the redshift of gamma-ray loud AGNs and GRBs using Supervised machine learning Active galactic nuclei (Aditya) Abstract Quasars as high-redshift cosmological probes (Giada) Abstract	16:20- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Maria Dainotti)
2022-10-5	Zsolt Keszthelyi (NAOJ)	The effects of surface fossil magnetic fields on massive star evolution Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Kataoka)
2022-10-12	Satoshi KIKUTA (NAOJ)	UV & Lyα halos of Lyα emitters across environments at z = 2.84 Abstract	15:30- zoom DTA#1 (moderator: Mizutani)
2022-10-19	Mitsuru KOKUBO (NAOJ)	AGN Variability: reverberation phenomena and variability-based AGN selection Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Kataoka)
2022-10-26	Jeong-Gyu Kim (NAOJ)	Star formation efficiency and destruction of giant molecular clouds with stellar feedback Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Mizutani)
2022-11-2	Koh Takahashi (Tohoku University)	Novel modeling of magneto-rotational stellar evolution Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Nagakura)
2022-11-9	Alessio Traficante (INAF Rome)	From filaments to clumps: multi-scale dynamics in 70 micron quiet star-forming regions Abstract	15:30- zoom DTA#1 (moderator: Kobayashi)
2022-11-16	Student talks, Doi (D1), Sasaki (D1)	Constraints on the dust size distribution in the protoplanetary disk around HD 163296 by ALMA multi-band observations (Doi) Abstract Phenomenological turbulent effects in core collapse supernovae (Sasaki) Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Nagakura)
2022-11-23	Labr Thanks giving
2022-11-30	Seamus Clarke (Academia Sinica Institute of Astronomy and Astrophysics)	Filaments and their role in the star formation process Abstract	15:30- zoom DTA#1 (moderator: Kobayashi)
2022-12-7	Conflict with NAOJ Future Planning symposium
2022-12-14	Tadafumi Matsuno (University of Groningen)	Building blocks of the Milky Way revealed from the chemodynamics of halo stars Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Chiaki)
2022-12-16	Eiichi Egami (Arizona university)	The James Webb Space Telescope (JWST): Successful Commissioning and First Scientific Results Abstract	15:00- zoom DTA#2 /Instrument Development Bldg. 3 (hybrid /moderator: Fujii)
2022-12-21	Rironkon symposium
2022-12-28	New Year break
2023-1-4	New Year break
2023-1-11	Student talks, De Simone (D1)	A new perspective on cosmology through Gamma-ray Bursts and Supernovae Abstract	15:30- zoom DTA#1/Instrument Development Bldg. 3 (hybrid /moderator: Maria Dainotti)
2023-1-18	Student talks, Sato (D1)	Light curves of electron capture and low mass Fe core collapse supernovae (Sato) Abstract	15:30- zoom DTA#1 (moderator: Kobayashi)
2023-1-25	Tomomi Sunayama (Nagoya Univ.)	Towards an accurate cluster cosmology with photometric galaxy surveys Abstract	15:30- zoom DTA#1 (moderator: Fujii)
2023-2-1	Student talks, Yoshida (M2), Ishihara (D1)	Observational Constraints on the Dust Albedo Spectrum in a Protoplanetary Disk (Yoshida) Abstract Observational research of fragmentation process in nearby star-forming regions (Ishihara) Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid / moderator: Chiaki)
2023-2-8	Alberto J. Castro-Tirado (IAAS)	The Burst Observer and Optical Transient Exploring System in the multi-messenger astronomy era Abstract	15:30- zoom DTA#1 (moderator: Maria Dainotti)
2023-2-15	Student talks, Nishigaki (M2), Watanabe (M1)	Early Galaxy Formation Near and Far Probed with SDSS and JWST Data (Nishigaki) Abstract Elemental Abundance Ratios and Origin of Very High (Fe/O) in Extremely Metal-Poor Galaxies (Watanabe) Abstract	15:30- zoom DTA#1 (moderator: Fujii)
2023-2-22	Student talks, Okada (M2), Morii (D1)	Follow-up of metal-poor star candidates discovered by narrow-band survey (Okada) Abstract Dense Cores Embedded in 70µm-dark High-mass Clumps: CMF and Fragmentation (Morii) Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Ross)
2023-3-1	Student talks, Mizutani (M2), Hatano (M1)	Dynamics of planetesimal rings (Mizutani) Abstract Unknown Ionization Sources in Local Dwarf Galaxies (Hatano) Abstract	15:30- zoom DTA#1/Instrument Development Bldg. 3 (hybrid /moderator: Fujii)
2023-3-8	Student talks, Kambara (M1)	Development of GPU-accelerated simulation code for planetesimal accretion Abstract	15:30- zoom DTA#1 (moderator: )
2023-3-15	Annual meeting of Japan Society
2023-3-22	Ohashi Nagayoshi (ASIAA)	Rotation in star and planet forming regions Abstract	15:30- zoom DTA#1 /Instrument Development Bldg. 3 (hybrid /moderator: Kataoka)

Follow-up of metal-poor star candidates discovered by narrow-band survey

speaker: Hiroko Okada

Abstract：
Observational determination of the chemical abundance of metal-poor stars is important for understanding the chemical evolution of the early Universe. However, the metal-poor stars discovered by previous surveys are faint, and it is difficult to measure many elements with high precision. We performed photometric observations using the wide-field CMOS camera (Tomo-e Gozen) on the Kiso Schmidt telescope with narrow-band filters sensitive to stellar metallicity to search for bright metal-poor stars. Very metal-poor star candidates with [Fe/H]<-2 were selected for follow-up medium-resolution spectroscopy with the Nayuta telescope. We have established a method for analysing medium-dispersion spectra using 43 stars with metallicity measurements, and determined the metallicity and α-elemental abundance of ~300 metal-poor star candidates that we had followed up so far. As a result, nine new very metal-poor stars and two low-alpha stars were discovered. In this talk, I present the results of the follow-up and the metal-poor star candidate selection methods.