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
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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.
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.
Online seminar link 2:20 pm:
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.
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.
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.
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.