The CAG regular seminar is held on Thursday noon
Venue: Education and Research Building S101
Please fill the online registration form to order your free lunchbox. Due to the pandemics, the lunchbox will be distributed after the talk.
(See announcements on departmental websites for colloquia of Department of Physics and Department of Earth Sciences)
Solar magnetic fields are the main driver of many observed activities on the Sun and in the interplanetary space. How the magnetic fields are generated and why they change polarity every 11 years have been one of the most important subjects in the solar physics.
However, the signal of the magnetic field below the surface is weak and difficult to detect because the gas pressure is much larger than the magnetic pressure inside the Sun. In this study, the aim is to probe the variation of the magnetic fields below the surface using solar meridional flows. Solar meridional flows are axisymmetric flows on the meridional planes, and exist in the entire convective zone. We use SOHO/MDI helioseismic data from 1996 to 2010, which includes two solar minima and one maximum. The time-distance method is first applied to the data to determine the travel-time difference between northward and southward propagating waves. The travel-time difference is then related to the meridional flow based on the ray path theory. Finally, an inversion procedure is applied to the travel-time difference to obtain the meridional flow speed at the solar minimum and maximum.
The results show that the flow pattern in the entire convective zone changes significantly from solar minimum to maximum and that the flow change is related to the active latitudes, which are the centroid locations of the surface magnetic fields. This suggests that the change of the meridional flow pattern is closely related to the change of the magnetic fields. Therefore, the solar cycle variation of the meridional flow is a promising tool to probe the solar cycle variation of the solar magnetic fields in the convective zone.
online participation https://meet.google.com/xtz-dsam-mrk
Particles coagulation and fragmentation are ubiquitous (raindrop formation, air pollution, combustion, polymerisation, astrophysics) and mathematically described by the Smoluchowski coagulation and the fragmentation equations. Solving these equation accurately while preserving tractable computational costs is a tremendous numerical challenge, yet critical in astrophysics for understanding the formation of the planets. In particular, low-order numerical schemes do strongly overestimate the formation of large particles. We present a novel high-order discontinuous Galerkin algorithm (Lombart & Laibe, 2021) that addresses all these issues. This new algorithm paves the way to perform the first 3D simulations of dusty protoplanetary discs that include realistic coagulation/fragmentation.
We present the first cosmological constraints using the cluster abundance of a sample of eROSITA clusters, which were identified in the eROSITA Final Equatorial Depth Survey (eFEDS). In a joint selection on X-ray and optical observables, the sample contains 455 clusters within a redshift range of 0.1 < z < 1.2, of which 177 systems are covered by the public data from the Hyper Suprime-Cam (HSC) survey that enables a uniform weak-lensing mass calibration. In a framework of empirical modelling and blind analysis, we simultaneously model the cosmology, the X-ray selection, and the observable-to-mass-and-
The mystery that whether extra-solar planets (exoplanets) exist was resolved around 1990s. A tremendous effort through both ground-based and space telescopes has led to more than 5000 confirmed exoplanets. The continuous flow of new exoplanet detections, the diversity of exoplanets, the observations of exoplanet atmospheres, and the configurations of multi-planet systems have triggered even more international projects on both detecting and characterizing exoplanets. I will give a brief review on this subject and also introduce the on-going effort and results from my group.
In 2019, the first event horizon scale images of black hole is announced by the event horizon telescope, a global radio telescope network. In this talk, selected mathematical concepts behind the black hole images will be shared. From the mathematics behind radio telescope observation, to the mathematics for the spacetime idea and general relativity, and finally the mathematics for the comparison between observation and theoretical models.
An obscuring torus or disk made of dust and molecular gas is the key component in the unified model for active galactic nuclei (AGN). The dusty molecular gas flows from the galactic scale of ~100 parsec to the subparsec environment through a disk with moderate scale height. Such a cold disk may therefore be considered the AGN-feeding region. When the energy delivered by the AGN is sufficiently large to unbind the cold gas from black hole’s gravitational potential, providing the conditions to launch the winds and outflows. In this talk, I will present the recent studies in the nuclear regions in the nearby Seyfert galaxies. The results allow us to discuss the nuclear star formation and the outflows within a physical scale of 10-200 pc around a supermassive black hole. In addition, I will introduce an efficient astronomical visualization tool specially for analyzing the radio data, named CARTA (Cube Analysis and Rendering Tool for Astronomy), and share the experience working as a software engineer.
Galaxies are at the front seats where astrophysics meets cosmology. We can use galaxies as test particles to constrain our cosmological models. However, galaxies are also subject to complex astrophysics associated with baryons. So detailed understanding of galaxies will impact our cosmological studies. Here I will introduce our works on a special class of galaxies, who are rich in dust and traditionally invisible from optical surveys made in the optical wavelengths. I will use examples to demonstrate that they are important building blocks of the universe, including analyses on their contributions to the cosmic infrared background radiation, and their dark matter masses.
On April 13th, 2029, a Potential Hazardous Asteroid (PHA) 99942 Apophis will visit Earth at a distance about 36700 ± 9000 km, which is almost the altitude of geosynchronous satellites. Such a close approach of an asteroid will provide an extraordinary opportunity to study a PHA in great detail and to raise public awareness of planetary defense. Here we propose a mission concept “ACE (Apophis Close Encounter)” which aims to rendezvous with Apophis prior to its 2029 flyby and to shepherd the asteroid for several months to investigate its possible evolution induced by the tidal force of Earth. The mission is scheduled to launch in August 2028 and expected to end in late August or early September of 2029 with a lifetime of around 280 days.
Online seminar: https://meet.google.com/qwo-hywy-aib
Abstract: 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
Join Zoom meeting：
Meeting ID：821 222 3533
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.
Online seminar: https://zoom.us/j/
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.