Title: The Effect of Misalignment between the Rotation Axis and Magnetic Field on the Circumstellar Disk
Speaker: 平野 信吾（天文学教室）／Shingo Hirano (Department of Astronomy)
The formation of circumstellar disks is investigated using three-dimensional resistive magnetohydrodynamic simulations in which the initial prestellar cloud has a misaligned rotation axis with respect to the magnetic field. We examine the effects of (i) the initial angle difference between the global magnetic field and the cloud rotation axis and (ii) the ratio of the thermal to gravitational energy. We study 16 models in total and calculate the cloud evolution until ∼5000 yr after protostar formation. Our simulation results indicate that an initial nonzero promotes disk formation but tends to suppress outflow driving for models that are moderately gravitationally unstable. In these models, a large-sized rotationally supported disk forms and a weak outflow appears, in contrast to a smaller disk and strong outflow in the aligned case.
Title: Observational study on high-mass star formation through development of own instruments
Speaker: 西村 淳（天文学教育研究センター）／Atsushi Nishimura (Institute of Astronomy)
Finding new aspects of the universe through observations using own developed instruments is the most exciting experience for observational astronomers. Although the telescopes of big projects like ALMA provide opportunities to tackle excellent sciences, there is some room to flourish as well for smaller projects such as single dish telescopes in radio astronomy because they have advantages especially on their wider field of view. In this seminar, I introduce two projects in which I have been involved. One is the 1.85-m mm-submm telescope developed by Osaka Prefecture University aiming to observe CO(2-1) at 230 GHz (1.3 mm). Thus far, more than 1800 deg2 are mapped including the Galactic plane and nearby molecular clouds, and the project provides a good reference of the ratio between CO(1-0) and CO(2-1) toward the giant molecular clouds. The other is the FUGIN project which is a CO(1-0) survey toward the Galactic plane using the FOREST receiver (4-beam) installed on Nobeyama 45-m telescope. By grace of its high dynamic range (wide area with high resolution) data, we find that most of the giant molecular clouds associated with the high mass star formation region experienced cloud-cloud collision. Finally, I briefly introduce the latest activities on developments of future multi-beam receivers.
Title: Observational studies of neutral carbon in our Galaxy
Speaker: 山岸 光義（天文学教育研究センター）／Mitsuyoshi Yamagishi (Institute of Astronomy)
Language: Japanese (with English slides)
Carbon is one of the major atoms in the Universe. It is valuable to examine carbon atoms and carbon-bearing molecules in the various interstellar environment to gain a deep understanding of interstellar chemistry. Emission lines from a neutral carbon atom ([CI]) are observed in the high-frequency ranges (492, 809 GHz). Since excellent atmospheric conditions are required in high-frequency observations, intensive studies of [CI] have been difficult before the ALMA era. In this talk, I will talk about two recent studies of [CI] with ALMA and ASTE in the different interstellar environments in our Galaxy (rho Oph A PDR and gamma-ray SNR W51C). Based on the results, I will discuss the origins of the [CI] emission in a star-forming region and the effects of cosmic-ray on interstellar chemistry.
Title: Challenges and opportunities in young planet discoveries from Kepler to TESS
Speaker: Jerome de Leon（天文学教室/Department of Astronomy）
Studying young exoplanets is important because they inhabit a very important part of the exoplanet evolutionary timescale where several yet unknown processes are expected to shape their final outcome. However, finding and characterizing transiting exoplanets orbiting young stars is notoriously difficult. This is due to intrinsic high stellar activity in young stars that induces photometric variations and radial velocity jitter often a few orders of magnitude
larger than the planet signal. This problem is compounded by the fact that most of the young stars usually reside in star clusters or moving groups where the false positive rate is high due to the crowded field. Despite the small sample of young planets discovered from the K2 mission however, there is tentative evidence that the planetʼs size depends with the host starʼs age.
To confidently corroborate or refute such trends and to have a complete understanding on how exoplanets form and evolve from birth to maturity, the community needs a statistically
significant sample of transiting planets orbiting bright young stars. As the TESS mission continues to survey the sky until 2022, new transiting planets around young stars will be discovered.
The brightness of the host star would facilitate precise measurements of the fundamental parameters of the host stars and planets which in turn lead to better models to constrain their bulk density and atmospheric composition.
Title: Probing supernova interiors with neutrinos
Speaker: 諏訪 雄大（総合文化研究科）／Yudai Suwa (Graduate School of Arts and Sciences)
Neutrinos are guaranteed observable from the next Galactic supernova (SN). Optical lights and gravitational waves are also observable but can be difficult to observe if SN location in the galaxy and the explosion details are not suitable. The key to the next SN observation will be understanding various physical quantities using neutrinos first and then connecting them to other signals. In this seminar, I will introduce our current research on neutrino observation, particularly on long-time evolution. I will discuss how we can use neutrinos to determine the mass and radius of neutron stars that form in supernova explosions, which would be useful in probing other physics such as the explosion mechanism with other messengers.
Title: Mechanisms of Star Formation Suppression in the Strongly Barred Galaxy NGC1300
Speaker: 前田 郁弥（天文学教育研究センター）／Fumiya Maeda (Institute of Astronomy)
Language: Japanese (with English slides)
Star formation activity in many barred galaxies is significantly different between in arms and in bars. In the arms, HII regions are associated with dust lanes, and giant molecular clouds (GMCs) coexist with the dust lanes. However, massive star formation is notably absent in the bars: Prominent HII regions are often not seen, while there are remarkable dust lanes along the bar which implies the existence of GMCs. Physical mechanisms which could suppress star formation in bar regions have been long debated and fall into the following three scenarios recently: (1) Gravitationally unbound molecular clouds in bar regions. (2) A large amount of diffuse molecular gases in bar regions. (3) Fast cloud-cloud collisions in bar regions. To unveil which of the scenarios is the most dominant cause for the suppression, we focus on the nearest strongly barred galaxy NGC 1300 where the massive star formation in the bar regions is remarkably suppressed compared to the arm regions. In this seminar, I will talk about the examination results for each scenario by CO observations using ALMA and Nobeyama 45-m telescope and discuss the cause of the star formation suppression.
Title: Morphological dependences of star formation activity in local galaxies
Speaker: 小山 舜平（天文学教育研究センター）／Shuhei Koyama (Institute of Astronomy)
Language: Japanese (with English slides)
It is well known that there is a strong correlation between star formation activity and morphology in nearby galaxies. One possible explanation for this correlation is the influence of galaxy morphology on star formation activity. For example, Martig et al. (2009) has proposed “morphological quenching”, in which star formation efficiency is reduced in early-type galaxies.
However, its influence on galaxy evolution has not been clarified observationally. In this talk, I will present the results of a study of the difference in star formation activity due to the difference in morphology (disk- or bulge-dominated) using observations with Nobeyama45m and CO(1-0) data from xCOLDGASS. I will also introduce my recent work on the relationship between morphology and star formation using MaNGA, an IFU survey, data.
Title: The Diversity of IGM-galaxy connection among galaxies at redshift z = 2-3
Speaker: 百瀬 莉恵子（天文学教室）／Rieko Momose (Department of Astronomy)
The link between the intergalactic medium (IGM) and galaxies is key to understand the evolution of baryonic matter and galaxies. In fact, several observational studies have confirmed the connection between the IGM HI and galaxies up to several tens comoving Mpc. However, understanding the variations on the IGM-galaxy connection over galactic properties, such as mass, SFR, and galaxy population, is still limited. Motivated by this, we have studied the IGM-galaxy connection and its dependence on those galactic parameters to shed more light on the link using both simulations and observations. In this talk, I will present the observational results obtained from Lyα forest tomography named the CLAMATO (Lee et al. 2016, 2018). I will also discuss the change of the IGM-galaxy connection in terms of galaxy evolution indicated from our results.
Title: The high-performance N-body code PeTar for studying the dynamical evolution of massive star clusters and gravitational waves
Speaker: Long Wang (天文学教室/Department of Astronomy)
Binary black hole (BBH) mergers in dense stellar systems like globular clusters (GCs) are important sources for the gravitational waves detected by Advanced LIGO. Star-by-star N-body simulation is the most reliable method to understand the formation rate, mass ratio and orbital parameters of these BBHs. But it is very time-consuming to produce such a model because of the expensive computing. To overcome this bottleneck, I have developed a high-performance N-body code, PeTar. It is the fastest code for modelling massive star clusters with a large fraction of binaries. Using PeTar, we carried out a series of N-body models for massive star clusters, and found that the initial mass function significantly affects the long-term evolution of GCs and properties of BBH mergers, while primordial binaries have a weak influence. For low-mass open clusters with tidal streams, the stochastic sampling of IMF makes it impossible to predict their long-term evolution. In addition, we implement PeTar in a hydrodynamic code, Asura-bridge, which allows us to study the star cluster formation with the complexity from binary dynamics, gas and feedback of massive stars. With the benefit of highly accurate binary dynamics from PeTar, we found when massive stars form, the violent close interaction can easily eject them from the birth place and significantly affect the surrounding star formation. This may explain the multiple populations discovered in young star clusters like Orion nebular cluster. In summary, the PeTar code opens a new window to solve a variety of challenging N-body problems.
Title: Star formation in The Galactic Disk: A Bubble-Filament Paradigm
Speaker: 犬塚 修一郎（名古屋大学）／Shu-ichiro Inutsuka (Nagoya University)
Recent observations have emphasized the importance of the formation and evolution of magnetized filamentary molecular clouds in the process of star formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds. Thus, the mass function and rotations of molecular cloud cores should be directly related to the properties of the filamentary molecular cloud, which determines the initial size and mass distribution of a protoplanetary disk around a protostar created in a core. In this talk I explain our current understanding of the star formation processes in the Galactic disk, and summarize various processes that are required in understanding the star formation rate/efficiency, and the stellar initial mass function. I will also address how massive stars form in dense molecular clouds identified as “hub-filament systems”, which can be understood in an extension of the filament paradigm.
Title: Monte Carlo radiative transfer code MONACO for X-ray astronomy—the framework design and its applications to molecular clouds, black holes, and neutron stars
Speaker: 小高 裕和（物理学教室）／Hirokazu Odaka (Department of Physics)
MONACO is a general-purpose X-ray radiative transfer code for linking between theoretical models and high-quality observational data. Precise treatment of radiative transfer is particularly important for accreting systems where optical depths are close to or larger than unity, and scattering and photon reprocessing become significant. In this talk, I review the design of the MONACO framework as well as photon interactions implemented in the simulation code, which include photoelectric effects, various types of scattering, and absorption and reemitting processes within photoionized plasma driven by intense X-rays from accreting objects such as black holes and neutron stars. We applied this simulation framework to X-ray reflecting molecular clouds in the Galactic Center region, accretion flow onto magnetized neutron stars, and outflows from accreting black holes for interpreting high-resolution X-ray spectra. Finally, I would like to discuss future prospects for XRISM, the next-generation X-ray observatory scheduled for launch in 2023.
Title: Basic modern signal processing techniques and astronomy
Speaker: 池田 思朗（統計数理研究所）／Shiro Ikeda (The Institute of Statistical Mathematics)
In the last 30 years, signal processing technologies have been updated and many interesting new methods have been proposed. Starting from a classical PCA (Principal Component Analysis) and FA (Factor Analysis), I would like to introduce new methods, such as ICA (Independent Component Analysis), NMF (Nonnegative Matrix Factorization), RPCA (Robust PCA) and sparse modelling. There are interesting applications of them in astronomy and I would like to introduce some of them, especially those I am involved in.
Title: The absolute magnitude and color of red clump stars
Speaker: 小野里 宏樹（国立天文台）／Hiroki Onozato (National Astronomical Observatory of Japan)
Language: Japanese with English slides
Red clump (RC) stars are low mass stars in the core-helium burning phase with relatively high metallicity. They are widely used as a standard candle because the dispersion of luminosity and color of RC stars is small, and the number of RC stars is numerous. To make RC stars as an even better standard candle, it is important to know the accurate absolute magnitude and color of them. We are trying to obtain the accurate absolute magnitude and color of RC stars by two approaches. One is to determine the absolute magnitude and color of RC stars in the solar neighborhood, where the effect of interstellar extinction is small. We used the thirty millimeter telescope at Subaru Telescope Okayama Branch Office to observe bright RC stars. The other is an investigation of age and metallicity dependence of absolute magnitude and color using star clusters in the Magellanic Clouds. Observations were conducted with IRSF/SIRIUS, and archive data from VMC survey were used. In this talk, I will present the result of these observations and comparison with theoretical models.
Title: Radiation hydrodynamics simulations of star cluster formation
Speaker: 福島 肇（筑波大学）／Hajime Fukushima (University of Tsukuba)
Star formation in giant molecular clouds (GMCs) proceeds under UV feedback from massive stars. Recent observations of nearby galaxies showed that the duration time of the star formation after massive star formation is shorter than their lifetime (< 3-5Myr, e.g., Kruijssen et al. 2019). They indicated that pre-supernova feedback (photoionization or stellar wind) was responsible for inducing the disruption of clouds. However, the dependency of photoionization feedback on physical properties, such as the compactness of clouds, is still unclear. In this talk, we show the impact of photoionization feedback on star cluster formation. We use the hydrodynamics code with the adaptive mesh refinement code, SFUMATO (Matsumoto 2007), with the solver for radiation transfer (Fukushima & Yajima 2021). The star formation efficiencies increase with the surface densities of clouds. In simulations, the bound fraction of stars rapidly increases when the total stellar mass becomes higher than 10-20% of clouds. We show that this condition is related to massive star cluster formation conditions. In addition, we consider the effects of FUV radiation on star cluster formation. FUV feedback is especially effective when the surface densities of clouds are less than 50 Msun/pc^2.