專題演講 2022

週四中午定期舉辦專題演講

地點:教學研究大樓 S101講堂

時間:12:20-13:20

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(物理系與地球科學系專題演講時間地點請見系網公告)

With an increasing number of gravitational wave events, understanding
features of gravitational waves becomes crucial for uncovering the physics in
extreme spacetime and testing general relativity. Gravitational waves emitted by
a binary black hole system consist of inspiral, merger and ringdown. The backwards
one-body (BOB) model is a time-domain waveform model for late-merger and
ringdown. In this talk, I give an introduction to this model, focusing on how BOB was
built and what we can learn from this model

Zoom meeting information
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Meeting ID:821 222 3533
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M-type subdwarfs are metal-poor Very Low Mass stars with low luminosity (L<0.05L), and they were named “subdwarfs” because they are located below the dwarfs of the main sequence on the H-R diagram. They are Galactic fossils with lifetimes much longer than the Hubble time and crucial touchstones of the star formation and metal enrichment histories of the Milky Way. These faint low-mass stars were originally discovered through their large proper motion and low luminosity, and were subsequently found to share similar kinematics as the inner halo and thick disk stellar populations. However, it was difficult to obtain their spectra for a long time because of their local scarcity and intrinsic faintness until spectroscopic surveys came such SDSS and LAMOST. In this talk, I would review the spectroscopically identification of M-subdwarfs, and present our work of M-subdwarfs with the LAMOST survey including the spectral analysis, the stellar parameter estimation, multiplicity study and kinematics. With the continuous observation of LAMOST, combined with Gaia DR3, the spectral sample of subdwarfs will be enlarged which would play a very important role in the further understanding of their physical properties and also constraining the stellar atmosphere model.

線上演講連結:https://zoom.us/j/97478209618 

In astronomy, the wide-field sky surveys referred to repeated imaging of the sky using telescopes that have a large field-of-view. In this talk, I will introduce several such sky surveys that our institute at National Central University (NCU) has been involved in, or going to participate soon. These wide-field sky surveys include the Pan-STARSS1 (PS1) project and the Palomar Transient Factory (PTF, and its successor, intermediate-PTF or iPTF) in the past 10 years; the on-going Zwicky Transient Facility (ZTF) project; and the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) in the coming decade. Several selected scientific highlights or discoveries from these surveys (excluding LSST) will also be presented.

Understanding how galaxies form and evolve has been one of the most important topics in astrophysics. In the past two decades, astrophysicists have realized that the so-called feedback mechanisms which regulate mass and energy into and out of galaxies play a fundamental role in driving galaxy evolution. In this talk, I will first introduce the current challenges of understanding the physics of feedback and demonstrate that the properties of gas around galaxies, the circumgalactic medium (CGM), are essential to overcome these challenges. I will present my research that probes the CGM by utilizing the power of statistical analysis applied to big datasets of large sky surveys. The results have motivated not only new theoretical investigations of galaxy formation astrophysics but also the development of new sky surveys. Finally, I will describe one of such surveys, the Dark Energy Spectroscopic Instrument (DESI) survey, in which I participate. I will summarize the main scientific goal of the DESI survey, i.e. to unveil the evolution of dark energy.

https://www2.phy.ntnu.edu.tw/index.php/2022/02/09/110-2-seminar-1110504/

Online seminar link 2:20 pm:
https://us02web.zoom.us/j/85099069221?pwd=WC9tbHo4bnhCa05yZzRtYmh2RnFJdz09

在太陽系起源的研究中,和其它很多科學議題一樣,可以有兩個方法。一個方法是先從簡單的模型假設出發,再逐步加入各種參數發展越來越複雜的理論用來解釋觀察現象。另一個方法是從觀察現象開始,再逐步反推。如果中間出現瓶頸,便可能是因為欠缺了些物理原則元素或者觀察和實驗資料而未有突破。在科學史上,第二個方法,雖然是進展比較延慢和曲折,反而往往是勝出者。我們用一些自己知道的研究工作作為例子說明。

Several tight scaling relations were recently revealed in the dark matter problem, from galaxies to galaxy clusters. In spiral galaxies, a tight correlation was found between dynamical and baryonic acceleration with a characteristic acceleration scale g† =1.2×10-10 ms-2, called the radial acceleration relation (RAR). Besides, the low acceleration limit of the RAR implied the baryonic Tully-Fisher relation (BTFR), which has been confirmed with the same acceleration scale g†. To explore these correlations on larger gravitationally bound systems, we investigate dynamical and kinematical scaling relations in three different samples, including 20 CLASH clusters, 29 HIFLUGCS clusters, and 54 MaNGA brightest cluster galaxies (BCGs). For the first time, we discovery the existence of a parallel RAR on BCG-cluster scale, albeit with a larger acceleration scale g‡. Additionally, we also confirm the kinematic implication with the corresponding scale g‡, i.e., mass–velocity dispersion relation (MVDR). Consequently, the baryonic mass is proportional to the flat velocity dispersion with a slope of four. Notably, the MVDR on BCG-cluster scales provides a strict test, which disfavors the general prediction of the slope of three in the dark matter model.

Neutron stars are one of the most extreme objects in the Universe. They are engines that power many short, sporadic, and energetic events in all electromagnetic wavebands. The Crab pulsar occasionally emits giant radio pulses (GRPs) that are sudden radio bursts that are several orders of magnitude brighter than regular pulses and with microsecond time scales. GRPs are one of the most promising candidates of mysterious fast radio bursts (FRBs). For a long while, GRPs have been observed only in the radio band, but an excess of visible light was found in 2003. We have conducted simultaneous observations of the Crab pulsar with a multi-wavelength campaign and found a ~4% X-ray enhancement coinciding with GRP occurrence. This indicates total energy is much higher than previously expected. This result, together with the recently discovered galactic FRB in a magnetar SGR 1935+2154, does not favor the GRP-FRB model. Our recent studies of bursts of a few magnetars suggest that X-ray short bursts may have different origins. Future observations and systematic studies of radio and X-ray bursts would help understand the activities of neutron stars.

https://www2.phy.ntnu.edu.tw/index.php/2022/02/09/110-2-seminar-1110406/

One of the fundamental constraints on studying galaxy evolution is that we are not able to monitor individual galaxies to track the evolution. The stars in galaxies provide the fossil records on the build-up processes of galaxies. I will demonstrate that with current observing facilities, we are able to track the mass assembly histories of individual galaxies in the distant Universe from their stellar populations. We also have gathered a statistical sample to understand early galaxy evolution as a population, as well as identify galaxies in the key phase of galaxy evolution for follow-up studies. The measurements achieve a high precision to test the implementation of state-of-the-art numerical simulations of galaxy evolution. With the coming survey projects and observing facilities, the archeological method will be able to track the evolution of galaxies in even earlier Universe, and with better statistics. 

宇宙中幾乎每個星系中央都有具有一個超大質量黑洞(supermassive black hole),它的質量非常巨大(從數百萬到幾數十億倍的太陽質量),而科學家也非常好奇這麼龐大的天體會不會對周圍環境有影響?會不會與早期星系的生成演化有關聯?在這個演講中,我會先介紹超大質量黑洞有哪些物理特徵,再來會介紹近年來研究超大質量黑洞研究上的重大事件:包括人類以事件視界望遠鏡 (event horizon telescope)拍到的第一張黑洞影像,還有銀河系中央超大質量黑洞的研究(2020年諾貝爾物理獎)。最後我會討論超大質量黑洞可能生成的原因?它們與宇宙中的星系演化有沒有關聯?還有我們可以使用什麼工具和儀器來作這樣的研究?

Water is a peculiar liquid with many abnormal properties, maximum density at 4 oC is a famous example. A 40-year-old puzzle is about supercooled water. In 1976 C.A. Angell, then at Purdue University, experimented to see how far they could supercool water, and how the liquid would behave at extremely low temperatures. What they saw surprised everybody: As water dipped below −20 °C, its isothermal compressibility began to soar, a sign that its density was fluctuating wildly at the molecular scale. The liquid seemed on the verge of some dramatic transformation. But whatever that transformation was, Angell couldn’t actually see it; it occurred at temperatures below the homogeneous nucleation temperature, where the liquid state was too short-lived for the researchers to measure. In the early 1990s, Gene Stanley came up with a compelling explanation. Stanley’s theory hinged on the concept of critical points, special points in a phase diagram where two thermodynamic phases of matter—say, liquid and gas—meld into one. Water has a well-known critical point at about 374 °C and 218 atm, above which liquid water and water vapor become indistinguishable. Stanley proposed that water has a second critical point, hidden deep in the supercooled
regime. At temperatures below that point, there exist two distinct liquid phases of different densities; above that point, the liquid phases merge. In Stanley’s interpretation, the density fluctuations in Angell’s experiment represented a kind of fluctuation between the two
phases of water. However, this created a big controversy among theoreticians, two schools fighting each other, David Chandler(Berkeley) was much against the 2nd critical point concept. Then in 2003, Sow-hsin Chen(MIT) and I started a decade-long experimental program(mainly by neutron scattering) to study the supercooled water under nanoconfinement. We can supercool nano-confined water down to 180 K, still maintaining the liquid state. This is because in nanoscale, water cannot freeze. In this talk, I will tell this story of resolving the water controversy, mainly from our own data.
Also, an important question of water is to understanding solubility of a hydrophobic molecule under nanoconfinement which impact on several related problems, (a) solubility of methane in water within nanopores of rock under fracking condition, (b) understanding how hydrophobic effect would be changed in confined water, (c) catalysis of gaseous molecule under confinement. Finally, I will speculate on some implications of confined water in several fields: (a) Its role in origin of life, (b) Geological Shale Gas by Fracking, (c) Pulling water out of thin air in desert. (d) Gas hydrate as energy source. 

https://web.ntnu.edu.tw/~chenlw/ntnuescolloq.html

In the unified theory, accretion disc is believed to be the central engine of the active galactic nuclei (AGN), but we have limited knowledge of how it works. The historic standard thin disc model can explain the spectral energy distribution of quasar continuum spectrum. The magneto rotational instability further provides a promising mechanism to drive the turbulence so that the accretion disc can sustain long enough and effectively accrete. However, physically interpreting the observed stochastic variability remains challenging. We aim to study the dependence of the variability of QSOs on luminosity, wavelength and thermal time scale in their accretion disks. We use over 6,000 of the most luminous known QSOs with light curves of almost nightly cadence spanning > 5 years of observations from the NASA/ATLAS project, a data set, which provides 20 billion magnitude pairs for a bootstrap analysis. We find that the results depend on which time scales are included in the analysis, and once we only consider time scales > 6 months, we find a robust result. This result is extremely consistent with the predictions for thermal time scales from our calculations of thin accretion disk models, which predict log t_thermal ∝ 0.6 × log L + 2.25 × log λrest.

I will give an outsider’s view of the path from supersymmetric sigma model to the topological quantum field theory, and how that leads to Gromov-Witten theory. If time allows, some selected topics in Gromov-Witten theory will be discussed.