Frontier of Astrophysics Forum

1 Jul 2024, 09:00 → 5 Jul 2024, 22:35 Asia/Shanghai

Tsung-Dao Lee Institute/N4F-N400 - meeting room (Tsung-Dao Lee Institute)

Note: Talk time includes Q&A: 

50'=40+10, 40=30+10, 35'=25'+10', 15'=10+5'

Monday 1 July

09:30 → 10:10 Welcome & Introduction

Welcome & Introduction to TDLI; Self-introduction for visitors and local participants

10:10 → 10:40 Coffee break

10:40 → 11:30 The scattering transform and its cosmological applications

Extracting information from stochastic fields is a ubiquitous task in science.  However, from cosmology to biology, it tends to be done either through a power spectrum analysis, which is often too limited, or through the use of convolutional neural networks, which require large training sets and lack interpretability.

I will present a powerful statistical tool called the “scattering transform”, which stands nicely between the two extremes, and recent updates to extend this idea.  I will use various examples in cosmology and beyond, including its recent application to HSC weak lensing data, to demonstrate its efficiency, interpretability, and advantage over traditional statistics. 

Bio:
Sihao Cheng is a postdoc member of the Institute for Advanced Study and a visiting fellow of the Perimeter Institute.  He obtained his Ph.D. from Johns Hopkins University and Bachelor degree from Peking University.  He uses innovative and interdisciplinary ideas to analyze survey data and acquire new physical understandings. His work led to the discovery of special stars powered by gravitational energy while they are crystallizing and the cosmological applications of a new statistic that borrows ideas from deep learning. His interest spans from cosmology to (exo)planets.

Speaker: Sihao Cheng (IAS)

11:30 → 14:00 Lunch

14:00 → 14:50 Understanding the earliest stages of protoplanetary disk evolution and planet formation

Abstract:
Embedded, Class 0/I protoplanetary disks represent the earliest stages of protoplanetary disk evolution and set the initial condition for planet formation. In this talk, I will construct a clear and quantitative physical picture of Class 0/I disks through a synergy of simulation, theory, and observation. I will first present radiation non-ideal MHD simulations of disk formation and early evolution, which demonstrate that Class 0/I disks are mainly self-regulated by gravitational instability (GI). Using semi-analytic calculations of dust coagulation, I will argue that the strong turbulence driven by GI prohibits planet formation in Class 0/I but promotes planet formation at later times (Class II). I will test these insights against recent ALMA and VLA observations of Class 0/I disks, highlighting our theory's ability to not only fit but also predict observation. Finally, I will introduce a few recent and ongoing explorations in understanding later stages of disk evolution and planet formation.

Bio:
Wenrui Xu is a Flatiron Research Fellow in the Center for Computational Astrophysics at the Flatiron Institute. He obtained his bachelor’s degree from Cornell University and his Ph.D. from Princeton University. Wenrui's main research interests include theory and simulation of protoplanetary disks, planet formation, and dynamics.

Speaker: Wenrui Xu (CCA)

14:50 → 15:25 Dynamical origin of young stars around Sgr A* SMBH at the Galactic Center

Speaker: Doug Lin (University of California)

15:25 → 15:40 A future Jupiter confirmed by JWST and Gaia

The Sun’s evolution shapes the fate of our solar system. In this analogy, inner rocky planets are likely to be consumed as the Sun transitions into a red giant, while outer gas giants, akin to future Jupiters, will persist until the Sun transforms into a white dwarf. Evidence of these future Jupiters is inferred from the metal-rich atmospheres of white dwarfs, believed to be influenced by material disturbances from these distant gas giants. Initial indications of their presence arise from direct imaging of giant planet candidates around a metal-polluted white dwarf, WD 1202-232, courtesy of JWST’s Mid-Infrared Instrument (MIRI). Here, by combining Gaia astrometry and MIRI imaging data, we validate this candidate as the first future Jupiter, also marking it as the coolest thus far. It has a mass of 2.0±0.6 MJup and a semi-major axis of about 7 au. Our analysis further scrutinizes their atmospheres and metal pollution mechanisms via robust reanalysis of MIRI data and orbital simulations, merged with stellar evolution models.

Speaker: Fabo Feng (TDLI)

15:40 → 15:55 Nightside temperature of hot Jupiters: the role of clouds

Clouds play a significant role in shaping planetary atmospheres. Observationally, clouds tend to mute spectral features, obscuring our inference of fundamental parameters of planetary atmospheres such as abundances and structure measurements. On the other hand, clouds in hot Jupiters' atmospheres, due to their strong radiative effects in high-temperature atmospheres and their coupling with atmospheric flows, facilitate a new window to probe climate dynamics in a regime that has never been encountered in our solar system planets. For both reasons, understanding clouds in a self-consistent dynamical framework is vital. This presentation will update our latest findings of cloud dynamics and their consequences on atmospheric heat transport and observational signatures of spectrum based on general circulation models of hot Jupiters.

Speaker: Xianyu Tan (TDLI)

17:45 → 18:45 Reception

Light dinner at N400, TDLI

Tuesday 2 July

09:30 → 10:20 Origins and Diversity of Giant Planets

Assuming planetary systems are independent and identically distributed (iid), their population-level properties can be summarized into distributions via hierarchical Bayesian inference frameworks. I will present the application of this technique to uncover the stellar obliquity distribution of hot Jupiter systems and the eccentricity distribution of warm Jupiters. I will then discuss how population-level properties shed light on the origins of close-in giant planets.

Bio：Jiayin Dong is currently a Flatiron Research Fellow at the Center for Computational Astrophysics. Her research interests include exoplanets, planet formation, and astrostatistics. Jiayin uses a combination of observational and theoretical approaches to uncover the physical processes that drive diversity in exoplanets. She will join the University of Illinois at Urbana-Champaign as an Assistant Professor of Astronomy in Fall 2025.

Speaker: Jiayin Dong (Flatiron Institute)

10:20 → 10:40 Coffee break

10:40 → 11:30 Damping Obliquities of Hot Jupiter Hosts by Resonance Locking

Abstract: When orbiting hotter stars, hot Jupiters are often highly inclined relative to their host star equator planes. By contrast, hot Jupiters orbiting cooler stars are more aligned. Prior attempts to explain this correlation between stellar obliquity and effective temperature have proven problematic. We show how resonance locking—the coupling of the planet's orbit to a stellar gravity mode (g-mode)—can solve this mystery. Cooler stars with their radiative cores are more likely to be found with g-mode frequencies increased substantially by core hydrogen burning. Strong frequency evolution in resonance lock drives strong tidal evolution; locking to an axisymmetric g-mode damps semimajor axes, eccentricities, and, as we show for the first time, obliquities. Around cooler stars, hot Jupiters evolve into spin–orbit alignment and may avoid engulfment. Hotter stars lack radiative cores and therefore preserve congenital spin–orbit misalignments. We focus on resonance locks with axisymmetric modes, supplementing our technical results with simple physical interpretations, and show that nonaxisymmetric modes also damp obliquity. Outstanding issues regarding the dissipation of tidally excited modes and the disabling of resonance locks are discussed quantitatively.

Bio: I am a theoretical astrophysicist, who's research investigates the ways in which planets form.  I am a 51 Pegasi b Postdoctoral Fellow at the University of California Berkeley, working with Prof. Eugene Chiang.  Before I arrived at Berkeley, I was a Postdoctoral Fellow at the Canadian Institute for Theoretical Astrophysics at the University of Toronto.  I received my PhD from Cornell University, under the supervision of Prof. Dong Lai.

Speaker: J. J. Zanazzi (UC Berkeley)

11:30 → 14:00 Lunch

14:00 → 14:35 Exoplanet & Iodine

Speaker: Paul Butler (EPL, Carnegie Institution for Science)

14:35 → 15:15 Formation and Evolution of Massive Stars in AGN Disks

Study of stellar objects embedded in AGN accretion disks around massive Black Holes have been motivated by i) the disk of stars that formed in-situ in the galactic center; ii) the redshift independence of metallicity in AGN disks, which also suggest in-situ pollution; iii) massive LIGO-Virgo gravitational wave sources that possibly evolved in a gas rich environment, and iv) quasi-periodic eruptions connected to star-disk collisions. In this talk, I will introduce some recent progress on star formation and capture in AGN disks, as well as structure and lifetime of embedded massive stars and their co-evolution with the chemical abundance of AGN disks. In relation to the AGN channel for LIGO-Virgo events, I will discuss how tidal interaction in hierarchical systems influences the accretion, migration and mutual interaction of disk-embedded compact objects, and therefore their merger properties. I will emphasize that with the environment of AGN disks (featuring strong turbulence, radiation pressure prevalence, dynamical crowded population of stellar mass objects) being much more extreme than protoplanetary disks, established theories from planet formation must be transplanted into the AGN context with caution.

Yi-Xian Chen is a PhD candidate at the Department of Astrophysical Sciences, Princeton University, who obtained his BS in Physics at Tsinghua University in 2021. His research interests include planet formation and evolution, protoplanetary disks, and applying analogous framework to study properties of stellar objects embedded in AGN disks. His current focus is on using radiation hydrodynamic simulations to study the formation and evolution of massive stars in AGN disks.

Speaker: Yixian Chen (Princeton University)

15:15 → 15:30 Terrestrial planet formation compatible with super-Earth formation

The formation of the radially concentrated terrestrial planets in the Solar System is best explained by planet formation from a ring of solids at 1 au with convergent/suppressed migration. In contrast, the formation of close-in super-Earths necessitates migration when originating from rings in the 1-au region. We investigate whether the formation of planets with different orbits and masses can be realized with the same formation model.

Speaker: Masahiro Ogihara (TDLI)

15:30 → 15:45 Dynamics of planetesimals in the presence of a cold Jupiter

The presence of an early-formed cold giant planet can have significant influence on the formation of inner planets. In this talk, I will briefly share our previous work investigating the orbital evolution of planetesimals in the inner disk in the presence of nebula gas and a (proto-) cold Jupiter. We explore the dependence of the relative velocities of the planetesimals on the mass, eccentricity, and location of the planet. We find that, for small planetesimals (~10km), gas drag plays a dominant role and forces a size-dependent alignment of orbits (apse alignment). Such an effect keeps the relative velocity low for similar-size bodies but raises the relative velocities for planetesimals with higher mass ratio. For massive planetesimals (~100km), the interplay of viscous stirring, gas damping, and secular perturbations from the planet makes the random velocities almost unperturbed and the planetesimals can grow in a runaway fashion. These results imply that the presence of a cold Jupiter does not impede the formation of inner rocky planets through planetesimal accretion under certain conditions.

Speaker: Kangrou Guo (TDLI)

Wednesday 3 July

09:30 → 10:20 Zwicky Transient Facility and the Bright Transient Survey

Abstract: The Zwicky Transient Facility (ZTF) stands as one of the most scientifically prolific time-domain surveys. As one of its core science experiments, the Bright Transient Survey (BTS) performs a flux-limited spectroscopic census of supernovae and other extragalactic transients. This talk will discuss the overall survey design of ZTF, focusing on the detection and follow-up observation of extragalactic transients. The operational strategies of BTS and its preliminary science results, including the luminosity function, supernova rates, and their association with host galaxies, will be highlighted.

Biography: Yu-Jing Qin is a postdoctoral researcher at Caltech. He completed his Ph.D. at the University of Arizona in 2022. His research mainly focuses on the population statistics of transients, the connection between transients and their host galaxies, and the progenitors of supernovae. He has also worked on the dynamics of galaxies prior to his Ph.D.

Speaker: Yu-Jing Qin (California Institute of Technology)

10:20 → 10:40 Coffee break

10:40 → 11:30 Inferring the evolution pathways and the explosion mechanism of core-collapse supernova through late-phase spectroscopy

Abstract: Core-collapse supernovae (CCSNe) are considered as the final explosions of massive stars, following the depletion of the nuclear products in their cores. These catastrophe events are diverse in observation especially the chemical composition in the expelled material (ejecta), which implies varied mass-loss histories preceding the explosion. Despite over a century of discoveries, the mechanism responsible for the diversity in CCSNe, and its potential connection with the still-unresolved core-collapse process, is still a topic of active debate. In this talk, I will introduce the application of late-phase (nebular) spectroscopy of CCSNe to reveal these longstanding mysteries. Beginning with the fundamental concepts of CCSNe, I will provide an overview of the physical quantities that can be inferred from nebular spectroscopy. Next, I will demonstrate how the statistics analysis of nebular spectroscopy can be employed to constrain the properties of the progenitor, the dynamics of the ejecta, and their mutual relations. This investigation suggests massive stars leads to more aspheric and energetic explosions. Finally, I will introduce my future research plan, which aims to connect the diverse pre-SN activities discovered by recent transient surveys with the properties of their progenitors.

Bio: Dr. Qiliang Fang is a JSPS postdoctoral fellow at the National Astronomical Observatory of Japan. He received his B.Sc. from Peking University in 2017 and his Ph.D. from Kyoto University in 2023. Dr. Fang's research primarily focuses on exploring the mass-loss history and explosion mechanisms of core-collapse supernovae. He is also interested in hydrodynamic and radiative transfer modeling of transients, aiming to reveal the origin of their observational diversity.

Speaker: Qiliang Fang (University of Tokyo)

11:40 → 14:00 Lunch

14:00 → 14:40 Perturbations of Black Holes beyond General Relativity

Abstract: Anticipating the launch of several next-generation gravitational wave (GW) detectors in the 2030s, we will be able to more precisely measure spacetime ripples from binary black hole (BH) mergers in much larger parameter space. The forthcoming data requires more accurate modeling of GW emissions not only in General Relativity (GR) but also in theories beyond GR and diverse astrophysical environments. BH perturbation theory is one crucial approach for making these predictions. In this talk, I will present a novel formalism, based on Teukolsky's seminal work in the 1970s, to study perturbations of BHs with arbitrary spin in beyond-GR theories and complicated astrophysical environments. I will first prescribe the derivation of a modified Teukolsky equation in these complicated settings. I will then focus on the application of this formalism in studying BH ringdown, including analyzing isospectrality breaking of quasinormal modes (QNMs) and computing the QNM spectrum in specific beyond-GR theories.

Bio: Dongjun Li (李东骏) is a postdoctoral researcher in Prof. Yanbei Chen's relativity group at Caltech. In August, he will join Prof. Nico Yunes' gravity group at UIUC as a postdoctoral associate. He earned his Ph.D. from Caltech in 2024 and holds a bachelor's degree from Cornell University. His research focuses on modeling gravitational wave emissions in theories beyond Einstein's general relativity and in complex astrophysical environments using analytical and semi-analytical methods, with a particular focus on black hole perturbation theory. Additionally, he has been involved in leveraging gravitational wave detectors to test quantum gravity in flat spacetime.

Speaker: Dongjun Li (California Institute of Technology)

14:40 → 14:55 Gamma-ray Astronomy with the Large High Altitude Air Shower Observatory (LHAASO)

The Large High Altitude Air Shower Observatory (LHAASO), located in China at 4410m above sea level, is a hybrid extensive air shower detector array. Utilizing different detection techniques, LHAASO monitors the northern gamma-ray sky from a few hundreds of GeV to beyond PeV energies. LHAASO has been fully operational since July 2021. Recently, the first LHAASO catalog published 90 gamma-ray sources. 43 of them are PeVatron candidates with gamma-ray emission above 100 TeV. Among these sources, a giant gamma-ray bubble with energy up to 2 PeV is detected in the Cygnus region, indicating a super-PeVatron cosmic-ray accelerator that is likely associated with young massive star cluster Cygnus OB2. These findings confirmed the existence of PeV cosmic-ray accelerators in our Galaxy and opened up an era of ultra-high-energy gamma-ray astronomy. On Oct 9, 2022, the brightest gamma-ray burst ever seen in history happened in the field-of-view of LHAASO. Tens of thousands of very-high-energy gamma-ray photons were recored. I will briefly summarize recent highlights from LHAASO and relevant analysis topics being done at TDLI.

Speaker: Hao Zhou (TDLI)

14:55 → 15:10 Particle acceleration to ultra-high energies at extragalactic jet termination shocks

Extragalactic plasma jets are some of the few astrophysical environments able to confine ultra-high-energy cosmic rays, but whether they are capable of accelerating these particles is unknown. In this work, we revisit particle acceleration at relativistic magnetized shocks beyond the local uniform field approximation, by considering the global transverse structure of the jet. Using large two-dimensional particle-in-cell simulations of a relativistic electron-ion plasma jet, we show that the termination shock forming at the interface with the ambient medium accelerates particles up to the confinement limit. The radial structure of the jet magnetic field leads to a relativistic velocity shear that excites a von Kármán vortex street in the downstream medium trailing behind an over-pressured bubble filled with cosmic rays. Particles are efficiently accelerated at each crossing of the shear flow boundary layers. These findings support the idea that extragalactic plasma jets may be capable of producing ultra-high-energy cosmic rays. This extreme particle acceleration mechanism may also apply to microquasar jets.

Speaker: Gwenael Giacinti (TDLI)

15:10 → 15:25 Asymmetric cosmic-ray diffusion around sources: "Mirage" sources and large offsets

We show that a large asymmetric halo may be misidentified as multiple "mirage" sources, and that asymmetric diffusion could lead to a very large offset between the injection site and the identified halo. We add background noise into the region and try to identify the sources. We utilize the concept of asymmetric diffusion to elucidate several observed sources that were previously challenging to interpret. Our model offers intuitive explanations for these observations and has the potential to help identify a broad range of sources in the future.

Speaker: Yiwei Bao (TDLI)

Thursday 4 July

09:30 → 10:20 Dynamical tides of neutron star via general relativistic hydrodynamics

Abstract: 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 will present our recent general-relativistic hydrodynamical simulations that explore the resonance phenomena in a single TOV star, where the tidal fields are imposed via spacetime boundary conditions. Our preliminary and exploratory simulations demonstrate a clear manifestation of a resonant fundamental mode.

Bio: Sizheng Ma earned Ph.D. from Caltech in 2023 and is currently a postdoctoral researcher at the Perimeter Institute. His work focuses on general relativity and gravitational waves, covering areas such as black hole perturbation theory, numerical relativity, mathematical relativity, modified gravity, the data analysis of LIGO gravitational wave data, as well as some interfaces between classical and quantum gravity.

Speaker: Sizheng Ma (Perimeter Institute)

10:20 → 10:40 Coffee break

10:40 → 11:30 Effects of subhalos on interpreting highly magnified sources on lensing caustics

Abstract:  The extreme magnification near gravitational lensing caustics of galaxy cluster lenses allows the study of individual stars or compact stellar associations at cosmological distances.  We study how the presence of sub-galactic subhalos, a robust prediction of cold dark matter, can alter the property of caustics and hence change the interpretation of highly magnified sources that lie atop these caustics. First, we consider a lens populated with subhalos sampled from a realistic mass function calibrated to $N$-body simulations. Then, we compare a semi-analytical method and an adaptive ray-shooting method to quantify the property of the caustics. Finally, as a case study, we investigate Earendel, a redshift $z = 6.2$ candidate of magnified single star or multiple star system which appears to be atop the critical curve in the Sunrise Arc. We demonstrate that the source size constraint ($\lesssim 0.3\, \mathrm{pc}$) previously derived from the macro lens models is relaxed by a factor of a few to ten when subhalos are accounted for, therefore allowing the possibility of a compact star cluster. The subhalos could introduce an astrometric perturbation that is $\lesssim 0.5''$, which does not contradict observation. These conclusions are robust to changes in the subhalo population. Subhalos therefore should be seriously accounted for when interpreting the astrophysical nature of similar sources.

Bio:  I am now a postdoc at the Berkeley Center for Cosmological Physics, and before that I obtained my PhD in Physics from the Johns Hopkins University.  My main interests lie in gravitational lensing and gravitational waves.

Speaker: Lingyuan Ji (UC Berkeley)

11:30 → 14:00 Lunch

14:00 → 14:40 Unveiling Galaxy Formation and Cosmology through Integration of Cosmological Spectroscopic and Photometric Surveys

Abstract:
Over the past two decades, cosmological spectroscopic and photometric surveys have substantially advanced our understanding of galaxy formation and cosmology. Nevertheless, these two types of surveys possess unique advantages and disadvantages, often aligned with distinct scientific objectives. Combining them can yield a wealth of information. I present a series of investigations utilizing both Stage-III spectroscopic and photometric surveys to deliver novel measurements and results across various areas. These include precise measurements of the galaxy stellar mass function and galaxy-halo connection down to very low mass ranges, the first measurement of baryon acoustic oscillations from galaxy-ellipticity correlations, and the use of magnification for measuring gravitational lensing as an independent and complementary method to shear. With the forthcoming Stage-IV cosmological surveys, these measurements can be significantly enhanced, addressing fundamental questions about the nature of dark matter and dark energy, as well as the physics of galaxy formation.

Speaker: Kun Xu (SJTU)

14:40 → 14:55 EM counterparts of extreme mass ratio inspirals

Quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts with recurrence times about a few hours to a few weeks from galactic nuclei. Though the debates on the origin of QPEs have not completely settled down, more and more analyses favor the interpretation that QPEs are the result of collisions between a stellar mass object (a stellar mass black hole or a main sequence star) and an accretion disk around a supermassive black hole (SMBH) in galactic nuclei. If this interpretation is correct, QPEs will be invaluable in probing the orbits of stellar mass objects in the vicinity of SMBHs, and further inferring the formation of extreme mass ratio inspirals (EMRIs), one of the major targets of spaceborne gravitational wave missions.

Speaker: Zhen Pan (TDLI)

14:55 → 15:10 Understanding intergalactic magnetic fields: Decay laws of nonhelical magnetically-dominated MHD turbulence

The absence of GeV halos around TeV blazars implies a lower bound of the intergalactic magnetic fields which could be a result of decaying primordial fields from the early Universe. However, the inferred decay rate is slower than what theorists have expected. A refined theory of decaying MHD turbulence by Hosking & Schekochihin resolves this problem. In this talk, I will explain the role played by the "Hosking integral" from which the turbulence decay rate is derived. Our high-resolution simulations supports Hosking's theory that (i) the Hoksing integral is an ideal invariant of MHD, and (ii) the decay is governed by the reconnection time scale rather than the Alfvén time scale, justifying its relevance in understanding primordial magnetic fields.

Speaker: Hongzhe Zhou (TDLI)

15:10 → 15:25 Numerical Modeling and Constraint of the Shadow of the Supermassive Black Holes

Recently the Event Horizon Telescope has presented a ring-like emission structure of M87 at 1.3mm with unprecedented angular resolutions. These images show the asymmetric ring. To understand the physical conditions of black holes and accretion flows, we construct a large library of models based on general relativistic magnetohydrodynamic simulations and synthetic images produced by general relativistic radiation transfer calculation. We compare the observational data with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. In this talk, I would like to present the recent progress of theoretical modeling by numerical simulations of magnetized accretion flows onto a black hole and a new approach based on a machine learning.

Speaker: Yosuke Mizuno (TDLI)

18:00 → 20:00 Banquet

Friday 5 July

09:30 → 10:00 Journal club

10:00 → 10:35 Lensed extreme starburst systems at Cosmic Noon: can young star clusters retain massive star ejecta?

Speaker: Liang Dai (UC Berkeley)

10:35 → 11:00 Coffee break

11:00 → 11:35 Galaxy formation and cosmology

Will talk about modeling diffuse Lyman-alpha emission around galaxies.

Speaker: Zheng Zheng (University of Utah)

11:35 → 14:00 Lunch

14:00 → 17:30 Coffee & free discussion