專題演講 2021

2021 年秋季隔週四中午定期舉辦專題演講




The missing energy-density problem will define much the basic research agenda towards understanding of the Universe. We will review the compelling evidence that the Problem does exist, and survey the diverse approaches scientists are pursuing to try to make advances. We will also recall comparable episodes in history which may help to illuminate the landscape among darkness.

Gamma Ray Bursts (GRBs) were discovered more than half a century ago. Their nature remained highly mysterious since then until the detection of their multi-wavelength afterglows and host galaxies in late 90’s, which eventually allowed to measure the redshift of GRBs and to establish their cosmological distance scale. These advances were based on extensive efforts of GRB monitoring and follow-up observations. The Gamma-ray Transients Monitor (GTM), whose main goal is to monitor GBRs in the energy band from 50 keV to 2 MeV, is a secondary payload on board Formosat-8B (FS-8B), a Taiwanese remote-sensing satellite scheduled to launch in 2024. GTM consists of two identical modules located on two opposite sides of FS-8B. Each module has four sensor units facing different directions to cover half of the sky. The two modules will then cover the whole sky, including the direction occulted by the Earth. Each sensor unit is composed of a GAGG scintillator array (50 mm × 50 mm × 8 mm) to be readout by SiPM with 16 pixel-channels. GTM is expected to detect about 40 GRBs per year. 

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Tidal effects have important imprints on gravitational waves emitted during the final stage of the coalescence of binaries that involve neutron stars. Dynamical tides can be significant when neutron star oscillations become resonant with orbital motion. Understanding this process is important for accurately modeling gravitational wave
emission from these binaries, and for extracting neutron star information from gravitational wave data. In this talk, I give a brief introduction to the concept of dynamical tides and outline recentprogress. In particular, I describe the impact of dynamical tides on gravitational-wave parameter estimation.

Numerous Active Galactic Nucleus (AGNs) have been found from our local universe to distant, early universe. It is considered to be the active phase of a galaxy, particularly the supermassive black hole (SMBH) at the center. When the SMBH is actively accreting materials, the AGN emits light which can be brighter than the entire light from the host galaxy adding together. For many years, astronomers have been speculating that there is an accretion disc surrounding the SMBH serving as the main source of AGN light. One of the most common and simple model for accretion disc is the thin disc model, which assumes fluids rotate on a circular shape with certain viscosity. The light emitted from the disc follows the black body emission with a temperature profile that approximately decreases with increasing radius. However, this model can not be proven by any direct image so far due to the tiny scale and distance of AGNs. In other words, no one has ever seen what an accretion disc really looks like. The best way to study the geometry of an accretion disc is through the light variabilities of AGNs. By gathering several years of light curves from thousands of quasars (the most powerful type of AGNs) using the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, we have established a relation between quasar variability, luminosity, wavelength, and time differences. In this talk, I will present what we can learn from this relation and its possible implication. Given the abundant data we are using, this result will be the latest understanding for AGN accretion disc before the data from the Rubin Observatory Legacy Survey of Space and Time (LSST) becomes available.

Understanding the origin of stellar initial mass function (IMF) is a central issue in the study of star formation. The past studies of dense cores reported that the slope of the stellar IMF and core mass function (CMF) are consistent, suggesting that the fundamental mass distribution of stars is determined during the early stage of the core formation. However, dense cores have been observed and studied only in the inner part of the Galactic plane (including the solar neighborhood), which has similar metallicity to that of the solar neighborhood. Thus, an important question to address is whether the same relation between the CMF and stellar IMF holds true even in low-metallicity environments. To solve this question, we performed CO and dust continuum ALMA high-resolution (~0.1 pc scale) mapping observation toward a massive star-forming molecular cloud in the outer Galaxy, which has much lower gas density and lower metallicity (~20 % of the solar neighborhood value) than those in the solar neighborhood . As the results, we successfully detected ~0.1 pc-wide filament structures and ~0.1 pc-scale dense core structures in the outer Galaxy for the first time. We also found that the slope of the CMF in the outer Galaxy is similar to that of the universal IMF. These results suggest that the star-formation processes in the low-metallicity environment follow a universal law.

In the theory of structure formation in the Universe, galaxy clusters are thought to grow by accreting surrounding material, resulting in strong, so-called virial, shocks. Such a shock is expected to accelerate relativistic electrons, thus generating a spectrally-flat leptonic virial ring. A significant (5.8σ) virial shock signal was identified near the expected shock radius, $\sim 2.5R_{500}$, by stacking gamma-ray data from Fermi LAT around 112 nearby clusters. We provide new virial shock signals over a wide range of wavelengths, better localizing the phenomenon and measuring the shock properties. Understanding these shocks has implications for astrophysics, cosmology, and plasma physics; in particular, we estimate the energies that the shock deposits in relativistic electrons and magnetic fields.

The Earth is unique amongst the telluric planets as it has abundant surface water, thriving ecosystems, an oxygen-rich atmosphere, and is geologically dynamic due to plate tectonics. In contrast, Venus does not have surface water, no ecosystems, a CO2-rich atmosphere, and does not have Earth-like plate tectonics. In spite of these differences, Earth and Venus have many geological similarities such as, density, composition, and size. The major geological difference is the presence of evolved silicic igneous rocks (e.g., granite, rhyolite, anorthosite) that comprise the continental crust of Earth. A large portion (70-80%) of the surface of Venus is featureless lava plains which lie within ± 1 km of the mean planetary radius. The remainder of the surface comprises mesolands and highlands. The mesolands have a median elevation (1-2 km) between the highlands and lowlands and preserve tectonomagmatic features such as coronae and chasmata (troughs). The highland regions (> 3 km) represent 8% to 10% of the surface and consist of crustal plateaux, tesserae terrane, volcanic edifices, and large-scale compression-related mountains. The formation of the highlands is debated but, it is possible that they could be representative of proto-continental crust. The surface composition of Venus was measured at seven different locations across the volcanic plain and highland regions. The compositions are similar to terrestrial basalt but, the rock at the Venera 8 landing site has anomalously high Th (6.5 ± 2.2 ppm) and U (2.2 ± 0.7 ppm) concentrations that are similar to granite or diorite suggesting the crust of Venus is differentiated. Petrological modeling of parental magma compositions similar to basalt identified on Venus can yield silicic compositions similar to granite/rhyolite and the rock at the Venera 8 landing site. Moreover, the initial plagioclase that crystallize in the models have anorthite (An% = mol. Ca/Ca+Na+K) fractions typical of non-primordial anorthosites of Earth. The modeling results indicate that silicic rocks are likely present and that Venus has a significant proportion of either proto-continental or sialic crust.

The arrival of ISOs — and specifically the appearance of 1I/’Oumuamua — points to a significant number density of free-floating bodies in the Solar neighborhood. We study the details of ‘Oumuamua’s pre-encounter galactic orbit and find that it encountered our Sun at very nearly its maximum vertical and radial excursion relative to the galactic plane. ‘Oumuamua’s kinematics, moreover, strongly resemble those of nearby Young Associations. To obtain an order-of-magnitude a-priori age estimate, we compared ‘Oumuamua’s orbit to the orbits of 13,066 F-type stars drawn from the Gaia DR2, and with a simple diffusion model, we find tau_`O ~35 Myr. We then compare ‘Oumuamua’s orbit with the trajectories of nearby moving groups, confirming that its motion is consistent with its membership in the Carina and Columba associations, whose ages are 45 Myr. We conducted Monte Carlo simulations that trace the orbits of test particles ejected from the stars in the Carina and Columba associations. We find that to achieve the inferred Pan-STARRS number density of 0.2 ISOs per au^3, the required ejection mass is ~ 100 M_{Jup} per known star within the associations. The Pan-STARRS observation is thus in significant tension with scenarios that posit ‘Oumuamua’s formation and ejection from a protostellar disk.

How old are Saturn’s rings? Could life exist elsewhere in the solar system? Thanks to the 13-year-exploration of the Cassini-Huygens mission at Saturn, we are a step closer to answer these fundamental questions. In this talk, I will review the current understanding of Saturn’s ring system and the plausible connection to the origin and evolution of Saturn’s icy satellites. Among them, I will focus on Enceladus, a 500-km-sized icy moon constantly emitting 200 kg of water vapor and ice grains from its south-polar fissures each second, and why it is considered one of the potentially habitable Ocean Worlds in the solar system. The presentation will be summarized with highlights about the interactions between the ring-moon system and its host planet, connections to various astrophysical phenomena, and future solar system explorations.

In this talk I first review the current known approaches of generating theoretical waveform templates, and their limitations on the efficiency and extrapolating capability. Motivated by these limitations, we construct a deep learning generative model of gravitational waveforms based on the scheme of variational auto-encoder. I will present our preliminary results to demonstrate its potential to bypass the above limitations of the conventional approaches.