投稿者: KatoRyo

Is Stephen Hawking’s evaporation of black holes wrong? – Black Hole as a new candidate for dark matter –

Speaker: Kazunori Kohri (DoS, NAOJ)

Abstract:

Last year, Prof. Gia Dvali, an expert in quantum gravity theory, criticized Stephen Hawking’s proposal for calculating the evaporation of black holes due to Hawking radiation, insisting that the initial conditions assumed were not appropriate, and if he added the back reaction (the Memory Burden Effect) from the entropy of Hawking radiation emitted by the black hole itself to the calculation, the lifetime of the black hole should be extended even further. Researchers around the world were surprised. My group (Thoss, Burkert and Kohri, MNRAS, 2024, arXiv:2402.17823) was quick to incorporate this correction and pointed out that primordial black holes that have existed since the early Universe may have remained and become dark matter without evaporating. I will explain this topic, which is still the subject of heated debate, in an easy-to-understand way for non-experts in this seminar. https://arxiv.org/abs/2402.17823

Diversity in planetary atmospheres shaped during the magma ocean stage

Speaker: Keiko Hamano (DoS, NAOJ)

Abstract:

Earth, Venus, and Mars are classified as the same planet category, “terrestrial planets”, in the solar system, but their atmospheres and surface environments are currently very different. Geochemical evidence and theoretical studies suggest that Earth, and likely the other terrestrial planets, began in a molten state—a global “magma ocean”—due to the energy released during planetary accretion. As these molten planets formed and solidified, fundamental planetary structures—such as the core, mantle, and atmosphere—were established, providing the initial conditions for subsequent planetary evolution.
In this presentation, the focus will be on the atmosphere that forms above a magma ocean, which is one of the key factors in understanding the diversity of planetary atmospheres. The research explores how basic planetary parameters, such as distance from the Sun (star) and planetary mass, can influence the early atmosphere throughout the magma ocean stage. Additionally, I will discuss the future directions of my research at NAOJ and welcome feedback and comments from the audience to help advance the research efforts.

Magnetic fields in star formation

Speaker: Junhao Liu (DoS, NAOJ)

Abstract:

Star formation is a key research field in modern astrophysics. Magnetic fields are crucial in influencing the key physical processes of star formation in molecular clouds. Here I review research progresses on the important questions regarding the role of magnetic fields in star formation, such as their impact on the star formation rate, cloud formation, cloud collapse, gas fragmentation, angular momentum transfer… Additionally, I will present our ongoing study on the formation mechanism of ~1000 au condensations in clustered massive star formation regions, leveraging a synergy between ALMA dust polarization survey and numerical simulations.

General relativistic radiation MHD simulations of super-Eddington accretion flows around a magnetized neutron star

Speaker: Akihiro Inoue (Osaka University)

Abstract:

Since neutron stars have strong magnetic and gravitational fields, they serve as natural laboratories for extreme plasma physics. When the neutron star is in a binary system, periodic X-ray pulsations are produced by the plasma accretion onto the neutron star. Such objects are called X-ray pulsars and thought to be ideal objects for investigating the extreme plasma physics. However, the magnetic field strength of the neutron star and the accretion rate remain uncertain since they cannot be observed directly.
We perform two-dimensional general relativistic radiation MHD simulations of super-Eddington accretion flows onto a magnetized neutron star, as modeling the X-ray pulsars at the super-Eddington rate. In our simulations, accretion disk, accretion flows along the neutron star’s magnetic field lines, and optically thick outflows driven by the strong radiation force appear. Such outflows can explain the thermal emission with a temperature of 1E+7 K and a blackbody radius of ~100 km detected in the super-Eddington X-ray pulsar, Swift J0243.6+6124. By comparing our models with observations, the magnetic field strength at the neutron star surface in Swift J0243.6+6124 is restricted to (0.3-4)E+12G. In this talk, we also present recent three-dimensional simulations, in which a non-axisymmetric accretion flows form.

Star Formation in Sub-Solar Metallicity Environments: Insights from the outer Galaxy and the Magellanic Clouds

Speaker: Yu Cheng (NAOJ)

Abstract:

The formation of stars in low-metallicity environments is a central problem in galaxy formation and evolution, particularly for understanding the high-redshift universe. However, studies of detailed star formation processes in such environments have been limited due to large distances and the challenge of characterizing low-mass sources. Here, I review recent progress and present observational efforts targeting the outer Galaxy and Magellanic Clouds—the nearest regions with metallicities as low as 0.2–0.5 Z☉. We conducted a multi-wavelength campaign targeting the outer Galaxy source Sh2-284, one of the lowest metallicity star-forming regions in the Milky Way. The synergy of multi-wavelength facilities, including ALMA and JWST, enables simultaneous characterization of dense gas and newly formed stellar populations. I will also present a sample study of massive young stellar objects (YSOs) in the Magellanic Clouds, followed by ALMA long-baseline observations that achieve unprecedented resolution at millimeter wavelengths (~1500 au) to probe fragmentation and kinematics. These results advance our understanding of star formation physics in conditions analogous to the early universe.

UV luminosity function of high-z galaxies using zZ-dependent IMF

Speaker: Keita Fukushima (Seoul National University)

Abstract:

To understand the overall picture of galaxy formation and evolution, it is essential to study the statistical properties of galaxies. The UV luminosity function of galaxies at redshift z>10, observed by the James Webb Space Telescope (JWST), shows that the bright end is more than an order of magnitude higher in number density than expected from the Schechter function. One theoretical model that explains this uses a top-heavy initial mass function (IMF) with an overabundance of massive stars.

In this study, we model an IMF dependent on redshift z and metallicity Z (Chon et al. 2022) and use the spectral synthesis code FSPS (Conroy, Gunn, & White 2009) to create spectral templates for star clusters. The zZ dependent IMF becomes top-heavy compared to the Kroupa IMF at low metallicities and high redshifts. In such cases, the UV luminosity is higher for young star clusters than with the Kroupa IMF but becomes lower at 10 Gyr due to a lower fraction of low-mass stars.

A galaxy formation model was developed by applying a semi-analytic model with metal enrichment to the merger trees obtained from the Shin Uchuu Simulation (Ishiyama et al. 2021). We then applied the spectral templates to this model to derive the UV luminosity function and compared it with observations, discussing the impact of the top-heavy IMF.

Wave dark matter

Speaker: Elisa Ferreira (Kavli IPMU)

Abstract:

In this colloquium, I will consider the prospect that dark matter is composed of ultra-light particles, exhibiting wave-like characteristics in galactic environments. We will delve into the underlying particle physics motivations and the intriguing wave phenomena associated with these candidates. Additionally, we will discuss the potential implications for astronomical observations and the strategies for experimental detection.

Recent JWST insights into Little Red Dots and unresolved mysteries

Speaker: Kohei Ichikawa (Waseda University)

Abstract:

The James Webb Space Telescope (JWST) has unveiled numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGN), even though their survey areas are only ~10^2 arcmin^2, which were previously considered too small for significant number of AGN discoveries before the JWST launch. This discovery highlights the presence of a dust-reddened AGN population, referred to as little red dots (LRDs), which are more abundant than X-ray selected AGNs that are less influenced by obscuration. In this talk, we summarize the recent observational properties of LRDs and discuss what we can learn from them about the growth of supermassive black holes (SMBHs) in the early Universe at z>=5. The large number density of LRDs indicates that the cosmic growth rate of BHs within this population does not decrease but rather increases at higher redshifts beyond z~6. The BH accretion rate density deduced from their luminosity function is significantly higher than that from other AGN surveys in X-ray and infrared bands. To align the cumulative mass density accreted by BHs with the observed BH mass density at z~4-5, as derived from the integration of the BH mass function, the radiative efficiency must be doubled from the canonical 10% value, achieving significance beyond the >2sigma confidence level.
This suggests the presence of rapid spins, with 96% of the maximum limit among these BHs, maintained by prolonged mass accretion instead of chaotic accretion with randomly oriented inflows. Accordingly, we propose a hypothesis that the dense, dust-rch environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts a potential association between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe. If time allows, we will explore the future prospects of wide-area surveys such as UNIONS and Euclid as well as VLA 3 GHz radio surveys, which could further uncover high-luminosity and/or low-z analogs of LRDs.

How should we explore the origin of astrophysical high-energy neutrinos using multi-messenger data?

Speaker: Wataru Iwakiri (ICEHAP, Chiba University)

Abstract:

Astrophysical high-energy neutrinos (HENs) are valuable tools for tracing the origins of high-energy cosmic rays. In the 2010s, the IceCube Neutrino Observatory, which has 5160 PMT detectors buried in the 1450-2450 m depth of the South Pole, enabled us to detect HENs above 100 TeV, allowing us to observe this new messenger. So far, the IceCube team has identified three objects as HEN sources: the TeV blazar TXS 0506+056, the Seyfert II galaxy NGC 1068, and a galactic plane. However, the primary source of the diffuse flux of astrophysical HENs observed by IceCube remains unclear. Correlation analyses of several years of accumulated data indicate that observed gamma-ray blazars contribute to less than 20% of the diffuse flux at most. Furthermore, observed HENs show no significant correlation with gamma-ray bursts and account for less than 0.4% of the diffuse flux. In this talk, I will discuss potential candidates for astrophysical HEN sources and how to reveal them using multi-messenger approaches.

Toward Understanding the Evolution of Exoplanetary Systems: Occurrence Rate and Stellar Age

Speaker: Kento Masuda (Osaka University)

Abstract:

The orbits and physical properties of exoplanets may continue to evolve long after their formation. For instance, it has long been discussed that the shortest-period planets may have orbits unstable to tidal dissipation, ultimately causing them to spiral into their host stars. Additionally, recent transit observations of low-mass planets have uncovered evidence of atmospheric escape, which may shed light on their internal composition. Understanding these evolutionary processes is vital for interpreting the diversity of exoplanetary systems and situating our solar system within a broader context. In this talk, I will present our ongoing efforts to statistically probe these changes by analyzing the dependence of exoplanet occurrence rates on stellar age. Specifically, I will discuss our recent work on deriving the age-dependent occurrence rates of giant planets orbiting Sun-like stars using isochronal ages, and will also mention insights into the rotation-based age estimation method gained from high-resolution spectroscopy of Sun-like stars in twin binaries.