Seminars 2023

The CAG regular seminar is held on Thursday noon

Venue: Education and Research Building S101

Time: 12:20-13:20

  • 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
  • Internal seminars are given by CAG members and are held alternately with the regular seminar. Venue: Education and Research Building S801-2

The Outer Planet Atmospheres Legacy (OPAL) program with Hubble was started in 2014 with the goal of studying time-domain phenomena in Jupiter, Uranus, and Neptune, with Saturn added in 2018 once the Cassini spacecraft was de-orbited. Once a year, the OPAL program images each of our four outer planets, producing pairs of global maps in multiple filters, which are made available at the MAST archive. The OPAL team (Simon, Wong, and Orton) have used the data to discover new dark spots on Neptune, discover a UV-dark oval in Jupiter’s southern polar haze cap, measure changes in Jupiter’s Great Red Spot over time, detect fine-scale waves, chronicle shifts in haze and cloud layers on all four planets, measure jet streams, and study the structure and evolution of convective storms. The data have also provided a valuable resource enhancing the science return from the Juno and New Horizons spacecraft missions, also supplementing observations from a growing list of observatories including JWST, Kepler, Spitzer, VLA, Keck, ALMA, IRTF, VLT, and Gemini.

Wong et al. (2022) https://doi.org/10.1016/j.icarus.2022.115123
Simon et al. (2022) https://doi.org/10.3390/rs14061518
Wong et al. (2021) https://doi.org/10.1029/2021GL093982

The classical limit of scattering amplitudes offers a convenient strategy to calculate gravitational-wave observables for binary processes in the post-Minkowskian (PM) regime, in which the two objects are far apart and interact weakly. This link has received renewed attention after the recent calculation of the 2-to-3 amplitude for the scattering of two massive objects and the emission of a graviton at one loop, which gives access to the first subleading contribution to the gravitational waveform, the leading PM correction to the classic result obtained by Kovacs and Thorne in the 70s.
In this talk, I will revisit this connection and show how the two-massive-particle cut contributions to this waveform have the effect of implementing a simple change of frame, as follows naturally from the approach based on the eikonal exponentiation. I will then discuss the comparison between the waveform obtained from the amplitude approach and the predictions obtained from soft theorems, at low frequencies, and from the post-Newtonian (PN) formalism, at low velocities. This will highlight, in particular, how an appropriate choice of BMS supertranslation frame is crucial in order to obtain complete agreement up to and including 2.5PN order.

Zoom linkhttps://us02web.zoom.us/j/83086581981

我會先回顧2016年我在中央大學天文所主持的Python討論會,並說明為何會將它發展成為Astrohackers in Taiwan這個以「開放天文 拉近群眾與星空的距離」為宗旨的社群。接著,我會分享我連續七年在台灣Python年會上演講天文主題的經驗。最後,我將展示ChatGPT如何降低大眾接觸天文資料的門檻,並展望這種創新的天文教學方式。

Over the past decade, all-sky surveys have ushered in a new era in time-domain astronomy, unraveling the unexpected discovery of stellar transients and challenging our understanding of stellar explosions. At the forefront of this transformative journey is the Public ESO Spectroscopic Survey of Transient Objects (PESSTO), which has successfully operated for a decade. In this talk, I will explore our collaborative work within ePESSTO+, offering insights into various extreme stellar transients. From slow-rising supernovae to rapidly evolving events, electromagnetic counterparts to gravitational waves, and the characterization of superluminous supernovae with diverse features and their host environments. Additionally, I will share my experiences with students and junior researchers regarding research grant and fellowship applications.

The formation of planetesimals in protoplanetary discs (PPDs) via the core accretion model faces several obstacles, such as to so-called growth and drift barriers. In order to overcome these, mechanisms to effectively concentrate dust grains in the midplane of PPDs are required. The most popular of these mechanisms is the streaming instability (SI), which in its nonlinear saturation can produce dense clumps of dust particles, which can then collapse self-gravitationally to form planetesimals. However, for small dust grains the clumping by the SI is only strong enough if the metallicity Z (dust abundance) takes at least a few times the solar value (Z~0.01).

In the recent years, the possible occurrence of purely hydrodynamic instabilities in protoplanetary discs has received increased attention. The main reason is that the magneto-rotational instability (MRI), which had long been considered to be the primary source for mass accretion in PPDs, is likely weak or absent in large parts of PPDs, due to low ionization rates. While purely hydrodynamic instabilities are likely too weak to induce significant mass accretion, they can strongly affect the concentration of dust grains, and can thus be active elements in planetesimal formation.

In this talk I will focus on two such instabilities, namely the vertical shear instability (VSI) and the convective overstability (COS). I will present results of my recent work applying hydrodynamic simulations and linear analyses to understand the role of these instabilities in the process of planetesimal formation.

Astrometric observations of the moons of Jupiter and Saturn have revealed that their orbits expand over time faster than previously thought. This significant orbital “migration” implies that efficient mechanisms of tidal energy dissipation are at play in the interiors of gaseous planets. These findings profoundly affect our understanding of the formation and evolution of moons, but also of the spin-axis dynamics of their host planet. The coupling between the planet’s spin-axis dynamics (the “Cassini states”) and the moons’ orbital dynamics (the “Laplace plane”) can gradually tilt the planet and make the system converge to a highly unstable configuration. This mechanism has direct applications in the Solar system: Jupiter today is about to start the tilting phase, Saturn is probably halfway in, and Uranus may have completed the final unstable stage.

https://cantor.math.ntnu.edu.tw/index.php/2023/10/24/lecture20231206/

I’m a theoretical scientist of planet formation. Today I present the latest understanding of the formation of our solar system. I will also introduce JAXA’s planetary exploration missions, such as the Martian Moons eXploration (MMX) mission and JAXA’s future Saturn’s rings mission.MX) mission and future JAXA’s Saturn’s rings mission.

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

Medium-sized asteroids hit Earth frequently without major devastation, but there is more to an asteroid impact than meets the eye. 
Impacts can cause other hazards that can last for years, and a planetary defense preparedness strategy should address not just the initial impact but also these second-order cascading hazards. In this talk, I will introduce these potential “impactors” and talk about how to characterize them using the space- and ground-based facility. In the end, the successful planetary defense mission, DART, will be illustrated. 

Online seminar link
連線資訊(參與連結):

Webinar topic:
CAG-NSYSUPHY joint seminar: Yuxin Lin (MPE)
Hosted by Yueh-Ning Lee (NTNU)

Date and time:
2023年11月02日星期四 (時段) 18:00 | 1 hour | (UTC+08:00) Taipei

Join link:
https://ntnu.webex.com/ntnu-en/j.php?MTID=m700c460c4001d72848d6094d10a14b5b

Meeting number: 
2513 571 6057

Webinar password:
vXP6dFdpW32

Abstract:
Pre-stellar cores represent a crucial evolutionary phase of star and planet system formation, where the dense gas is on the verge of gravitational collapse. Characterising physical and chemical properties of pre-stellar cores is a prerequisite for establishing sophisticated models for a better understanding of star formation process. In this talk I will present our recent works : 1) we dissect the underlying density landscape of L1544, a prototypical pre-stellar core, with multi-transitions of molecules that show strong chemical segragation 2) we derive the deuteration fractionation of CH3OH with the first detection of CHD2OH towards pre-stellar cores and establish the chemical link to more evolved phase 3) using image combination techniques, we constrain the physical structures of three early-stage cores and quest after the depletion behavior of NH3 molecule

Closed kinematic chains of interconnected links, know also as linkages, serve to induce motion or transmit force in mechanical systems. We introduce a family of linkages consisting of n ≥ 7 identical links connected by revolute hinges. Each such linkage exhibits just one internal degree-of-freedom, manifested by an everting motion. From the standpoint of the Chebyshev–Gr¨ubler–Kutzbach mobility criterion, any linkage in this family with n ≥ 8 links is underconstrained and, thus, is deemed exceptional. In the limit as n → ∞, these linkages converge to a smooth, ruled M¨obius band with three half twists and three-fold rotational symmetry. The rulings of this surface are aligned with the unit binormal of its midline, which is a geodesic and has uniform torsion. We find that this M¨obius band can also be obtained by a stable isometric deformation of a helicoid with a certain number of turns. Also, helicoids with more turns can be isometrically deformed into stable M¨obius bands with more half twists. Among all stable M¨obius bands with k ≥ 3 half twists that can be so obtained, the one with k-fold rotational symmetry has the least bending energy. While knotted M¨obius bands can also be produced from helicoids, we find that they saddle points of the bending energy. Returning to the family of linkages mentioned above, we present various consequences of relaxing the requirement that the constituent links be identical, subject to a particular proportionality rule.

Venue: NTNU Gongguan campus Conference Center 2F Meeting Room (師大公館校區 綜合館二樓 交誼廳)

Venue: NTNU Gongguan campus S801-2
In the current culture of mathematics, a prominent approach of mathematicians to physics is to model the physical universe via partial differential equations and then to show these equations are well-posed in the sense of Hadamard (and possibly then to pursue further results concerning these equations).  In this talk I will introduce and illustrate this concept of well-posedness in the sense of Hadamard in the context of Newtonian gravity and the static Maxwell equations.  I will state a few theorems, though rest assured that there will be no proofs.

Gravitationally lensed quasars are valuable astronomical objects that offer unique opportunities for studying galaxy evolution and cosmology. Current and upcoming imaging surveys will contain thousands of new lensed quasars, augmenting the existing sample by an order of magnitude. We report the discovery of new lensed quasars (or candidates) in the HSC and CFIS surveys. Spectroscopic or high-resolution imaging follow up on these newly discovered lensed quasar candidates will further allow their natures to be confirmed.

Unusual time: 11-12 am

The DECam Ecliptic Exploration Project (DEEP) is an extensive survey of the trans-Neptunian solar system conducted using the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, utilizing the Dark Energy Camera (DECam). Through the implementation of a shift-and-stack technique, DEEP achieves an impressive mean limiting magnitude of r ∼ 26.2, resulting in an unparalleled combination of survey area and depth. This advancement significantly enhances our understanding of the populations within the Kuiper Belt. This talk presents the findings from an analysis of 23 sq. deg. DECam fields situated along the invariable plane. Our analysis involves characterizing the efficiency and false-positive rates of our moving-object detection pipeline, enabling us to construct a Bayesian signal probability for each identified source. By employing this procedure, we can treat all detected Kuiper Belt Objects (KBOs) statistically, while simultaneously accounting for efficiency and false positives. As a result of our analysis, we found around 2300 Kuiper Belt Object detections with a signal-to-noise ratio (S/N) greater than 6.5. Leveraging these objects, we compute the luminosity function of the Kuiper Belt as a whole, including the Cold Classical (CC) population. Additionally, we investigate the absolute magnitude (H) distribution of the CCs, finding consistency with an exponentially tapered power-law, as predicted by streaming instability models of planetesimal formation. We plan to extend our search through the Jovian Trojans, the other primitive minor planet population of the solar system in the near future.

Unusual time: 2-3 pm

Quantum computation is a novel way of information processing that allows, for certain classes of problems, exponential speedup over classical computation. In addition to the standard circuit picture, various models of quantum computation exist, such as the adiabatic, circuit, and measurement-based models, and they operate very differently and may suit different physical realizations. I will give a pedagogical introduction to quantum computation so as to give some idea why quantum computers seem powerful and yet it is not easy to design quantum algorithms that outperform classical ones. Quantum computers can be used to simulate other quantum systems, and this potentially opens up many potential applications of quantum computers, including quantum many-body problems and quantum chemistry,  in addition to other quantum algorithmic pursuits. I will also discuss the opportunities that  cloud-based quantum computers offer. This is an exciting era where many quantum information and quantum computing tasks can be at least tested and technological applications may soon arrive.

 

Unusual time: 2-3 pm, Venu: Natural Science Building Complex B413

Unusual time: 3-6 pm, Venue: Natural Science Building Complex B101

Unusual time: 3-6 pm, Venue: Natural Science Building Complex B101

The best way to understand planet formation is to study planets while they are still forming. Planetary systems that host hot jupiters (HJs) are especially intriguing because of HJs’ large sizes and peculiar short orbital periods. Moreover, the formation history of hot jupiters may inhibit the development of habitable worlds. Searching for HJs around pre-main sequence stars using the radial velocity (RV) method, however, is challenging because of the strong stellar activity of the host star. Large cool spots (as large as 80% of the visible disk) can overwhelm and mimic RVs induced by planets. The Gaussian Process Regression (GPR) technique has been used widely to isolate the RV signal produced by the stellar activity; still, prior knowledge of the stellar rotation period, spot(s) lifetime, and spot(s) variability period are essential for GPR to model the activity signal properly. Knowing these stellar properties is even more critical for applying GPR to young stellar systems where the RV signal from the extreme stellar activity can be several times larger than the RV signal of the planet. 

Here, I will show the recent development and highlights from our Young Exoplanets Spectroscopic Survey (YESS) team. This includes the RV pipeline for the IGRINS spectrograph, the re-discover of the sub-stellar companion to DITau, a new method using the OH/Fe equivalent width ratio to trace the spot variability of T Tauri stars, and a simulation tool that can reproduce the observed periodicity in the OH/Fe equivalent width ratio and the radial velocity.

Time: 2-4 pm, Venue: M212 (Math Building)
General relativity is a physical theory on gravitation that is based on Riemannian/Lorentzian geometry or, more precisely, on the physics and geometry of the Einstein equations. From the linearized equations, Einstein got the idea of gravitational waves. However, the nature of gravitational radiation was only clarified half a century later by the work of physicists Bondi, Sachs, Penrose etc. I will introduce the theory, Bondi-Sachs formalism, behinds it.

LIGO’s successful detection of gravitational waves has revitalized the theoretical understanding of the angular momentum carried away by gravitational radiation. An infinite-dimensional “supertranslation ambiguity” has presented an essential difficulty for studying angular momentum at null infinity. In this talk, we will present a definition of angular momentum in general relativity that is supertranslation invariant. The new definition of angular momentum is derived from the limit of the Chen-Wang-Yau quasilocal angular momentum. The talk is based on joint work with Po-Ning Chen, Jordan Keller, Mu-Tao Wang and Shing-Tung Yau.

Unusual time: 2:20 pm
The lunar impact flash observing camera “DELPHINUS” onboard the 6U spacecraft EQUULEUS, developed in collaboration with Nihon University, the University of Electro-Communications, the University of Tokyo, JAXA, and major manufacturers, has been launched on November 16, 2022, as a one of 10 piggyback 6U satellites onboard the NASA Artemis 1 SLS, and successfully detached from the SLS rocket. After the checkout operation and first light imaging, the camera successfully took images of the far side of the moon during the lunar flyby, and comet ZTF (C/2022 E3) etc. In this talk, I’ll also present ground-based observations of the lunar impact flashes in Japan and laboratory experiments of hyper-velocity impact flash, ~7kms, using JAXA’s 2-stage light gas gun.

Unusual time: 2:20 pm
I describe new tools and techniques I’ve built to solve different problems in gravitational dynamics.  First, I describe our popular Rebound code, MERCURIUS, designed to solve for the evolution, including formation and long-term dynamics, of planetary systems.  Unlike its predecessor, MERCURY, it is numerically symplectic and time-reversible.  I also describe current work on a code, TRACE, which significantly improves on the accuracy and speed of MERCURIUS.

I then tackle the problem of the stability of the Solar System.  Although great progress has been made in the last decades towards an understanding of chaos and stability of the Solar System due to the development of modern computers, I show that all studies I found are affected by numerical chaos, which causes artificial Solar System chaos and instability.  The physical mechanism behind Mercury’s orbital instability has been traditionally described by a diffusive process in a secular frequency, but our current work shows a subdiffusive process fits simulated data far better.

I describe new tools for the potential discovery of interstellar objects (IOs).  The tools solve the linking problem, in which independent observations are linked together as a potential object of interest, using crude approximations.  These objects are then validated using state of the art orbit fitters.  I describe the validation tests I’ve done with this pipeline, which promises to be able to exploit the vast Solar System data expected from Rubin/LSST.

I next describe a suite of tools, including powerful new Kepler solvers and new symplectic integrators and their tangent equations, called NBODYGRADIENT.  Unlike other popular methods, we can solve planetary systems with arbitrary geometries and orbits including moons.  We have implemented these tools to solve the transit timing variation problem, and derive the properties and possible compositions of TRAPPIST-1 planets.

https://web.ntnu.edu.tw/~chenlw/all_colloq/abs/20230425.txt

The protoplanetary disk around MWC 758 is a source with complex substructure including an inner cavity, spiral arms, two large dust clumps, and a dust ring with eccentricity ~0.1. This project consists of two parts: (1) studying the gas kinematics of 13CO and C18O to constrain the gas eccentricity, and (2) analyzing 1.3 continuum data to measure the proper motion of the two dust clumps. In the first part, we obtained non-Keplerian velocity deviations by subtracting an ideal Keplerian model from the observed velocity pattern. We examined several possibilities that may cause the velocity deviations and found that eccentric orbital motion with eccentricity 0.1+-0.04 best explains our results. The gas eccentricity is consistent with that of dust, indicating strong dust-gas coupling. For the second part, we found that the dust clumps approximately follow the Keplerian flow, which may suggest that the clumps are dust-trapping vortices. Deviations from Keplerian speed may be due to other mechanisms such as spiral motion or radial flows. We discuss the implications of these results on dust and gas properties in potentially planet-forming disks.

The majority of stars born in dense stellar clusters are part of binary star systems. Circumbinary discs of gas and dust commonly surround binary star systems and are responsible for accreting material onto the binary. The gas flow dynamics from the circumbinary disc onto the binary components have significant implications for planet formation scenarios in binary systems. Misalignments between the circumbinary disc and the binary orbital plane are commonly observed. A misaligned circumbinary disc undergoes nodal precession. For a low initial inclination, the precession is around the binary angular momentum vector, while for a sufficiently high initial inclination, the precession is around the eccentricity vector. Dissipation causes the disc to evolve to align coplanar to the binary orbital plane or perpendicular (i.e., polar) to the binary orbital plane. I present 3-dimensional hydrodynamical simulations and linear theory on the evolution of highly misaligned circumbinary discs. I show that polar-aligned circumbinary discs are favorable environments for forming polar circumbinary (P-type) planets. Moreover, misaligned and polar circumbinary material flows around each binary component, forming misaligned and polar circumstellar discs. These circumstellar discs undergo long-lived Kozai-Lidov oscillations that may prompt the formation of giant circumstellar (S-type) planets in binary star systems. The evolution of protoplanetary discs in and around binary star systems bears important implications for planet formation.

Unusual time: 4 pm, Venue: General Hall B1 Auditorium
Over the last few years, a set of new results from pulsar timing has not only introduced some of the most precise tests of general relativity (GR) carried out to date, but have also introduced  much tighter constraints on violations of the strong equivalence principle (SEP), a fundamental physical principle that is embodied by GR. This was done via a direct verification of the universality of free fall for a pulsar in a triple star system and with tests of the nature of gravitational waves, in particular a search for dipolar gravitational wave emission in a variety of binary pulsars. No deviations from the SEP have been detected in our experiments. These results introduce some of the most stringent tests of GR, which introduce the tightest constraints on several classes of alternative theories of gravity and complement recent results from the ground-based gravitational wave detectors.

Starless cores are the potential sites for star and planet formation. Although we know gravity plays the main role during evolution, the details, in particular the timescale, are not yet well understood. The lifetime suggested by different scenarios could vary by more than a factor of ten. With time-dependent chemical analysis, measuring the chemical timescale of the cores allows us to infer possible evolutionary scenarios. We determine the density, temperature, and molecular abundance profiles of two nearby low-mass starless cores, L1512 and L1498, with dust extinction measurements from near-infrared observations and non-local thermal equilibrium radiative transfer with single-dish radio observations (H2D+ as well as deuterated N-bearing tracers using IRAM30m, JCMT, and GBT). Then we perform chemical modeling of the two targets to measure their chemical timescales using deuterium fractionation as a chemical clock. We find that L1512 is chemically evolved while L1498 is chemically young. This might imply that the magnetic field is stronger in L1512 than in L1498. Consequently, ambipolar diffusion may have slowed the contraction of L1512 or even halted it to the present state. We aim to extend this method to the dense starless substructures recently discovered by the ALMASOP project. In these substructures, even N2D+ is depleted such that H2D+ and D2H+ would be the best gas-phase tracers to probe their physical properties. We will present our preliminary H2D+ detections toward these starless substructures.