Colloquiums and Seminars(2022)

I will give an overview of the mechanics of flyby encounters and how these unbound perturbers can affect the structure of protoplanetary discs. Depending on the density of stellar clusters, stars with gaseous and dusty discs have up to a ~ 30% chance to encounter a flyby event within the first million years. The perturber will excite spiral arms during these flyby events. The spirals have a finite lifetime since the perturber is unbound. I simulate a parabolic encounter interacting with a gaseous protoplanetary disc utilizing three-dimensional smoothed particle hydrodynamical simulations. I find that the spirals can survive even when the perturber has passed periastron and is no longer interacting with the disc. Analyzing the dynamics of these excited spirals can shed light on observations of spirals in protoplanetary discs with no observed companion. *Link to the colloquium: http://meet.google.com/kyg-mjsu-eja

Fuzzy Dark Matter (FDM), consisting of ultralight bosons, is an intriguing alternative to Cold Dark Matter (CDM). Unlike in CDM, FDM halos consist of a central solitonic core, surrounded by an envelope of order unity density fluctuations. The envelope density fluctuations also interact with the soliton causing it to wobble and oscillate. Using novel, high-resolution numerical simulations of an FDM halo, corresponding to a particular boson mass, I will demonstrate that the gravitational potential fluctuations associated with the soliton's wobble, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction and an equilibrium is reached. Similarly, a galaxy undergoes significant size expansion and central density reduction over a Hubble time. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) will be able to constrain the boson mass. After discussing FDM, I will also briefly present my work on the peculiar galaxy NGC 1052-DF2 and show what we can learn about its mass distribution from the dynamical friction-induced orbital decay of its globular clusters. Link to the talk: http://meet.google.com/kyg-mjsu-eja

Critical points are special objects of a density field where the gradient of the field vanishes. They encode the topological information on the underlying density field and typically are the formation sites of large-scale structures of the cosmic web. Hence, the clustering of critical points provides information on the statistical properties of a given density field and the spatial organization of physical structures anchored to these points. In this talk, I will discuss the cosmic evolution of the clustering properties of peak-, filament-, wall-, void-type critical points focusing on both small separations and baryonic acoustic oscillation scales. A qualitative comparison with the corresponding theory for Gaussian random fields will be also discussed. Finally, I will summarize the cosmological implication of our findings and relate them to the cosmic standard rulers. Link to the talk: http://meet.google.com/ezw-onwu-zns

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.

The molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way supermassive black hole (SMBH), SgrA*, resembles the "molecular torus" in AGNs, providing a unique opportunity to study SMBH accretion and nuclear star formation at sub-parsec scales. In recent years, I have been studying the key question of how much of the available gas can actually form stars in the environment around Sgr A*, and how material is being moved around and accreted in this region. The lifetime of the CND has been a long-standing debate over the past decade. The CND can not live longer than 10^5 years if the gas density is under the tidal threshold of SgrA*/nuclear star clusters, thus depleting the source of fuel and star formation. Utilizing the ALMA and various single-dish telescopes, I present CS line maps toward the CND of the Galactic Center. My primary goal is to resolve the compact structures within the CND and the streamers, in order to understand the stability conditions of molecular cores in the vicinity of Sgr A*. My data provide the first homogeneous high-resolution (1.3" = 0.05 pc) observations aiming at resolving density and temperature structures. A stability analysis based on the unmagnetized virial theorem including tidal force shows that 84 (+16/-37) % of the total gas mass (2.5X10^4 Msun) is tidally stable, which accounts for the majority of gas mass. However, turbulence dominates the internal energy and thereby sets the threshold densities 10-100 times higher than the tidal limit at distance >1.5 pc to Sgr A*, and therefore, inhibits the clouds from collapsing to form stars near the SMBH. In this talk, I will also discuss the observed morphology, kinematics of gas, and effects of magnetic fields in the GC. My past, current and future research is a natural extension to 3 main categories fitting the future development of ASIAA - (1) ALMA/JCMT, (2) GLT/EHT, (3) METIS/E-ELT. The Galactic Center is unique and extremely complicated. Therefore to discriminate against different models, eventually we want to utilize multi-wavelength and multi-resolution data to sample the energy of the accretion process at different scales.

Galaxy formation is currently faced with immense datasets, both observational and simulated, with much more on the way. As the simulations continue to improve, I argue it is time to start thinking about how to use these realistic-looking galaxies to develop a predictive theory of galaxy formation. A new generation of semi-analytic models and statistical techniques are needed to simultaneously address the strong degeneracies that persist in the problem of galaxy evolution, and to coherently comprehend the vast quantities of extant and forthcoming data. I will focus on a particular set of puzzles around the turbulent driving in galactic disks - is stellar feedback, local gravitational instabilities, the direct impact of cosmological accretion, or something else responsible? link to this talk: https://meet.google.com/yhy-qkdu-yyi

One of the exciting new frontiers in cosmology and structure formation is the Epoch of Reionization (EoR), when the radiation from the early stars and galaxies ionized almost all gas in the Universe. This epoch is an important evolutionary link between the smooth matter distribution at early times and the highly complex structures seen today. Gaining insights into this epoch has been quite challenging because the current generation of telescopes are only able to probe the tail end of this process. Fortunately, a whole slew of instruments that have been specifically designed to study the high-redshift Universe (JWST, ALMA, HERA, SKA, CCAT-p, COMAP, SPHEREx), are about to come online. This will unleash a flood of observational data that will usher the study of EoR into a new, high-precision era. In this talk, I will introduce the THESAN simulation framework that is designed to efficiently leverage current and upcoming high redshift observations to constrain the physics of reionization. The multi-scale nature of the process is tackled by coupling large volume (~100 Mpc) simulations designed to model the large-scale statistical properties of the intergalactic medium (IGM), with high-resolution (~ 10 pc) simulations that zoom-in on single galaxies which are ideal for predicting the resolved properties of the sources responsible for it. I will briefly discuss applications from the first set of papers, including predictions for high redshift galaxy properties, emergence of the Lyman-alpha forest and back reaction of reionization on galaxy formation. I will then highlight the potential for using line intensity mapping of spectral lines originating from the interstellar medium (ISM) of galaxies and the 21 cm emission from the neutral hydrogen gas in the Universe to constrain galaxy formation and cosmology. I will finish by highlighting how this numerical framework, coupled with accurate observational estimates promises important and potentially transformative changes in our understanding of the primitive Universe. Link to this talk: https://meet.google.com/mcb-zrqu-ncp

One of the main goal of geophysics is to map the structure of the deep Earth, and most particularly of the Earth’s mantle. This region of the Earth extend from about 20 to 2900 km depth and is composed of silicate rocks. In the past few decades seismic tomography models have been able to map lateral changes in seismic velocities within the mantle, providing the best information on the structure of the Earth’s mantle. Global tomographic models have reached a consensus on the large scale structure. The strongest heterogeneities are found in the topmost 300-400 km of the mantle, where they correlate with surface tectonics, and in its lowermost 400-500 km, where the dominant structures are two large low shear-wave velocity provinces (LLSVPs) located beneath Africa and the Pacific. Whether these anomalies originate from purely thermal or a mix of thermal and compositional causes is still debated. Changes in seismic velocity anomalies alone cannot simultaneously resolve the potential thermal and compositional contributions from which they originate, and additional constraints independent from seismic traveltimes are needed to answer these questions. Additional observational constraints may include seismic normal mode, seismic attenuation, the topography of the core-mantle boundary (CMB), tidal tomography, periodic changes in the length-of-the-day, and electromagnetic C-response deduced from long-period variations of the magnetic field. In additional key information can be obtained from experimental and numerical simulations of convection, modelling mantle dynamics. Depending on input parameters, simulations predict possible thermo-chemical structures that can be tested against geophysical observable, in particular seismic tomography maps. In particular, numerical simulations of mantle convection indicate that reservoirs of dense material, modelling LLSVPs, can be maintained for long periods of time provided that the chemical density contrast between these reservoirs and the surrounding mantle is large enough, typically around 1.5-2.0 %. Interestingly, since the laws of physics are invariant in space and time, the tools developed to better understand the Earth’s mantle structure, evolution, and dynamics may be transposed to the study of other bodies. For instance, numerical models of convection may be used to reconstruct the evolution of icy moons and dwarf planets of the solar System and explain structures observed at their surface.

Over the past decade, the so-called Lambda Cold Dark Matter model has been fully established as the standard cosmological model. However, there still remain unresolved issues, including the nature of dark energy or the origin of cosmic acceleration. Toward a deeper understanding of the Universe, more systematic observations, combining multiple probes, e.g., large-scale structure, cosmic microwave backgrounds, and gravitational-wave observations as a recently established new probe, are crucial. In doing so, the exploitation of improved methodologies to extract cosmological information as well as the refinement of the theory in confronting with observations are indispensable. In this talk, I present how I tackled these issues based on analytical approaches. The outcomes of our approaches include the constraint on the modified gravity, total mass of neutrinos, cosmic string, primordial gravitational waves.… To further find a clue beyond the standard cosmological model, I summarize what has to be addressed and scrutinized in the coming decades, together with what I want to pursue at ASIAA. The link to this talk: https://meet.google.com/nzn-dkot-qvp

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. Some of them, the short GRBs, are important sources of gravitational waves originating in mergers of neutron stars or possibly also in mergers of neutron stars with black holes. I will present the detector performance and early science results from GRBAlpha, a 1U CubeSat mission, which is a technological pathfinder to a future constellation of nanosatellites monitoring and localazing GRBs. The localization can be achieved by measuring the time difference between the arrival of the signal at different satellites (synchronized by GPS). GRBAlpha was launched in March 2021 and has been operating already about a year on a 550 km altitude sun-synchronous orbit. The onboard gamma-ray burst detector consists of a 75×75×5mm CsI(Tl) scintillator, read out by a dual-channel multi-pixel (SiPM) photon counter (MPPC) setup. It is sensitive in the ~30-900 keV range. The main goal of GRBAlpha is the in-orbit demonstration of the detector concept, verification of the detector's lifetime, and measurement of the background level on low-Earth orbit, including polar regions and in the South Atlantic Anomaly. GRBAlpha has already detected five GRBs and was even able to detect two GRBs within 8 hours, proving that nanosatellites can be used for routine detection of gamma-ray transients. For one GRB, we were able to obtain a high resolution spectrum and compare it with measurements from the Swift satellite. We find that, due to the variable background, about half of the low-Earth polar orbit is suitable for gamma-ray burst detection. One year after launch, the detector performance is good and the degradation of the SiPM photon counters remains at an acceptable level. The same detector system, but double in size, was launched in January 2022 on VZLUSAT-2 (3U CubeSat). The GRB detectors perform as expected and I will present the early measurements from this mission as well. Our ultimate aim is to develop and launch a constellation of nanosatellites monitoring GRBs and these precursor missions help us to converge to this ambitious goal. The talk is given both in R1203 and online. Link to the Google meet: https://meet.google.com/nmv-neii-mry

Multiple cosmological probes in photometric surveys can measure the cosmic structure. These include cluster abundances and positions, galaxy positions, and weak gravitational lensing shear. In this talk, I will outline a program that combines all these probes to maximize the cosmological science return. Then, using recent results from the Dark Energy Survey as a pathfinder example, I will describe the challenges and opportunities of this program for upcoming large cosmological surveys. I will further present some of the recent progress in tackling these challenges. These include developments of a novel and general sampling scheme that reduces the computational cost of the analysis by more than a factor of 50 and improvements in survey simulations for analysis validation. This talk will be concluded by discussing the prospects of this multi-probe cluster cosmology program in the Rubin Observatory's LSST and CMB-S4 era.

The upcoming cosmological surveys (LSST, Euclid, Roman, SPHEREx, etc) will map the universe with a wide angular and spectral coverage. Most of the studies with large-scale surveys rely on detecting individual sources, and leave the faint, diffuse components in the extragalactic background light (EBL) largely unexplored. To fully exploit the wealth of information from the upcoming cosmological survey datasets, we present two analysis techniques that directly extract cosmological and astrophysical signal from the multi-band large-scale EBL images without individual source detections. First, I will discuss cross correlation for EBL redshift tomography, and show prospects of constraining the EBL by cross-correlating SPHEREx and galaxy surveys. In the second part, I will introduce light cone analysis, a novel technique for analyzing the multi-band EBL images. Without cataloging individual sources, the light cone analysis uses a data-driven approach to simultaneously determine the spectral and spatial distribution of all emitting sources and the underlying large-scale structure traced by them. Our technique does not require any SED assumption and is not subjected to the source confusion as the conventional galaxy redshift surveys. This method will be widely applicable to upcoming cosmological surveys, and can provide complementary information to the galaxy detection approach.

Black holes are generally thought to swallow all forms of matter and energy surrounding them, but it has long been known that they can also shed off some of their mass through a process called superradiance. This phenomenon is only effective if new particles with very low mass exist in Nature, as predicted by several theories beyond the Standard Model of particle physics. The shredded mass would end up forming a large "cloud" around the black hole, creating a system called a "gravitational atom". In this talk, I will discuss the interesting effects that occur when such gravitational atoms are part of binary systems. In particular, I will show that the presence of a massive companion induces resonant transitions between bound states and also triggers transitions from bound to unbound states of the cloud -- a process that we refer to as "ionization" in analogy with the photoelectric effect in atomic physics. I will describe that the backreaction on the binary's orbit leads to characteristic signatures in the emitted gravitational waves.

We present the first images of the supermassive black hole at the center of our Milky Way galaxy, Sagittarius A*. These observations were conducted in 2017 using a global array of 8 radio telescopes operating at a wavelength of 1.3 mm, or a frequency of 230 GHz. The EHT resolves a compact emission region with intra-hour variability. A variety of imaging and modelling techniques agree that the data is best represented by a thick ring of emission with an angular diameter of 51.8 +/- 2.3 μas, making this the largest black hole in the sky in terms of angular size. The ring has some level of azimuthal brightness asymmetry, and a dim central depression. When compared against a large number of numerical simulations, these images are consistent with a 4x10^6 solar mass black hole, well described by the Kerr metric. These results are consistent with those produced by observations of stellar orbits, but constrain the enclosed mass to reside in a region 1000 times smaller. Combined with the EHT observations of M87* we presented in 2019, the Sgr A* images demonstrate consistency with general relativity spanning three orders of magnitude in central mass. Video call link: https://meet.google.com/pmi-ajnn-zvx

Revealing gas kinematics around protostars is crucial to understand the process of star and disk formation. Observational studies have characterized gas motions around protostars; protostars are generally surrounded by infalling envelopes (~1000 au) and rotationally supported disks (~100 au). However, more comprehensive picture of the kinematic structure over a wide spatial range from a core scale (~0.05 pc) to a disk scale is not yet obtained. In this talk, I will present observational studies on protostars, in which I investigated the gas kinematics over the wide spatial scale. Velocity structures on different spatial scales are characterized by measuring the radial dependence of the peak velocity of line emission. The peak velocity approximately follows Keplerian rotation and rotation with a constant specific angular momentum, which is suggestive of the infalling envelope, at radii less than a few thousands au, as was also reported in some other protostellar systems. On the other hand, the peak velocity is proportional to r^~0.6 at larger radii, which resembles the velocity structures of the initial dense cores (J/M~r^1.6). These results give rough size scales of the infalling envelopes of ~1400 to 2900 au around individual protostars, and suggest that the spatial scale possibly increases as the system evolves. I introduced the structure function analysis to probe velocity structures at larger radii, implying that the velocity structures at larger scales originate from turbulent motion. Video call link: https://meet.google.com/ydj-unof-qbg

We present the first image of the Galactic Center black hole, Sgr A*, obtained with the Event Horizon Telescope. The image has an angular diameter of 52 microarcseconds, consistent with predictions of general relativity and the Kerr metric. Comparison with an unprecedented library of GRMHD simulations places strong constraints on the accretion and outflow properties. The result confirms that the gravitational lensing feature observed first in M87 is a universal property of black holes, establishes the consistency of general relativity over three orders of magnitude in mass, and opens the door for future tests of gravity, accretion, and jet formation.

Star formation is the driver of galaxy evolution. Understanding the properties of the cold, star-forming gas in the interstellar medium (ISM) is therefore of crucial importance. In this talk, I will present recent developments in high-resolution (sub-parsec) hydrodynamical simulations of the stellar feedback-regulated ISM and their predictions on chemistry properties and line emissions. I will show that steady-state chemistry strongly over-predicts the abundances of H2 but not CO, leading to a reduced conversion factor (X_CO), especially at low metallicities where the H2 formation time becomes much longer than the dynamical time. On parsec scales, X_CO varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO, and it drops to the Milky Way value once dust shielding kicks in. Finally, I present ongoing simulations that combine ISM chemistry with dust evolution, including SN destruction, AGB yields, and dust growth in dense gas. While dust growth strongly increases the abundance of CO to the observed values via radiation shielding, it has little effect on H2, as the timescales for H2 formation and dust growth are comparable. Video call to this talk: meet.google.com/qnd-xgva-qyr

The Subaru Prime Focus Spectrograph (PFS) is the multiplexed fiber-fed optical/near-IR spectrograph, which will have the first light in early 2024. The PFS collaboration is now planning to perform the cosmological galaxy survey, which will map the large scale structure of the universe in wide redshift range of 0.6 < z < 2.4 via the spectroscopic observation of [OII] emission-line galaxies. The PFS survey will give a significant impact on the fundamental physics, such as the nature of massive neutrinos, dark energy and modified gravity. In this talk I will overview the current status of PFS cosmology project, introduce the simulation suites developed for the PFS, discuss the possible systematics, and forecast the constraining power of PFS. I will also discuss remaining issues (e.g., covariance modeling, likelihood analysis, galaxy distribution) for the analysis of forthcoming real data.

The existence of massive prestellar cores (PSCs) is crucial in turbulent core accretion model. To find the hints of massive prestellar cores, we first performed high resolution simulations of giant molecular cloud (GMC) collisions. We utilise CASA post-processing the surface density maps to generate their ALMA 1.3mm synthetic observation results. We then use dendrogram to identify cores from the simulations and their corresponding synthetic observations. In chemistry aspect, observations have shown that high levels of deuterium fractionation ([N2D+]/[N2H+]>0.1) are often observed in prestellar cores. Therefore, we also carried out chemodynamics simulations of isolated massive prestellar cores. We studied the chemical evolution of species, especially N2H+ and N2D+, in a variety of initial chemical conditions. Our simulation results are compared with observational data, including the core mass functions (CMFs) of clusters, and the N2H+/N2D+ emissions from prestellar cores. We concluded that (1) the clusters do not have strong evidence of collision from the core mass functions, and (2) high depletion factor and high cosmic ray ionisation rate are required for these cores to achieve high level of deuterium fractionation. Video call to this talk: meet.google.com/qnd-xgva-qyr

Recently the Event Horizon Telescope has presented a ring-like emission structure of M87 and our galactic center Sgr A*. In this talk, I would like to present recent progress of the theoretical modeling of black hole accretion flows. Using a large suite of numerical simulations of magnetized accretion flows and self-consistently launched relativistic jets using 3D general relativistic magneto-hydrodynamical simulations and general relativistic radiative transfer calculations in horizon scale, we demonstrate that the EHT images of M87 and Sgr A* are consistent with the expected appearance of a Kerr black hole. From the comparison with two EHT observed black hole shadows, M87 and Sgr A*, I will discuss their similarity and difference.

Type Ia supernovae (SNe Ia) have been used as standardizable candles to discover the accelerating expansion of the universe, leading to the revelation of dark energy and the award of 2011 Nobel Prize in Physics to astronomers. Important as they are, SNe Ia are not fully understood. It is not clear whether they originate from (1) white dwarfs accreting from binary companion stars or (2) mergers of two white dwarfs. To probe the nature of SNe Ia, we search for clues to their previous life in the remains of explosions, called supernova (SN) remnants: if a surviving companion or a dense circumstellar medium from companion’s mass loss is detected, the origin of accreting white dwarf can be affirmed. Over the past decades, no surviving companion has been unambiguously confirmed in the Milky Way. We thus decided to push the boundary to other galaxies. Using images and spectra of stars, we identified possible surviving companions outside the Milky Way for the first time. We reported the first detection of dense circumstellar medium within three extragalactic SN Ia remnants. These studies indicate that the origin of accreting white dwarf for SNe Ia could be more prevalent than people previously thought.

Massive stars of 30 - 80 solar masses eventually collapse to black holes because the neutrino energy cannot drive a strong shock to overcome the ram pressure of infall, so core bounce fails to produce an explosion. But this picture can change with rapidly rotating stars, in which a neutron star (NS) with a period of a few milliseconds may be born. Rotation can amplify the magnetic field of the NS above 1E15 G, creating a magnetar. In this talk, I will discuss the theoretical models of the exotic explosions powered by the magnetars, and their astrophysical applications. Link to the video call: meet.google.com/qnd-xgva-qyr

With the discovery of over 5000 extra-solar planets to date, the formation and evolution of planets and planetary systems is one of the most rapidly developing fields of astrophysics. In the standard ‘bottom-up’ scenario, planets form from planetesimals — km or larger-sized bodies. Planetesimals form from small, mm-cm size pebbles, which themselves form from micro-sized dust grains immersed in gaseous protoplanetary disks around young stars. I will describe several obstacles, but also new possibilities, on the road from dust to planets from recent theoretical modeling of planetesimal formation in modern models of protoplanetary disks. Meeting Link: https://asiaa.my.webex.com/asiaa.my/j.php?MTID=mb018d9210a80051582617bd55cd3e3ce Meeting number: 2552 863 7090 Password: nF85mT7gpfN

Computational simulations are essential tools in astrophysics today and many codes are publicly available. Athena++ is a magnetohydrodynamics simulation code we develop in international collaboration led by Institute for Advanced Study / Princeton University. This code supports adaptive mesh refinement and various physical processes for astrophysical applications, and adopts new concepts to improve the performance and parallel scalability. There is nothing like "One code to solve them all", but we aim to make Athena++ a versatile, high-performance, easy-to-use, one of the best codes in the field. In this presentation, I will explain the design and strategy of Athena++, some scientific results and future plans.

HO Puppis (HO Pup) was considered as a Be-star candidate based on its γ Cassiopeiae-type light curve, but lacked spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral-energy-distribution fit on broadband photometry, the Be-star nature of HO Pup is ruled out. Furthermore, based on the 28,700 photometric data points collected from various time-domain surveys and dedicated intensive-monitoring observations, the light curves of HO Pup closely resemble those of IW And-type stars (as pointed out by Kimura et al.), exhibiting characteristics such as a quasi-standstill phase, brightening, and dips. The light curve of HO Pup displays various variability timescales, including brightening cycles ranging from 23 to 61 days, variations with periods between 3.9 days and 50 minutes during the quasi-standstill phase, and a semiregular ∼14 day period for the dip events. We have also collected time-series spectra (with various spectral resolutions), in which Balmer emission lines and other spectral lines expected for an IW And-type star were detected (even though some of these lines were also expected to be present for Be stars). We detect Bowen fluorescence near the brightening phase, and that can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for four nights, the polarization variation was detected, indicating that HO Pup has significant intrinsic polarization.

Magnetic fields have an important role in the evolution of interstellar medium and star formation. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA) towards L1544—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases characterized by a high central number density and a low central temperature. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding di use, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic fux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the di use CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes.

Strongly magnetized neutron stars occasionally show energetic transient activity in both X-ray and radio bands, such as magnetar flares, faint pulsed radio emission, and bright short-duration radio bursts in reminiscence of cosmological fast radio bursts (FRBs). Magnetar flares are believed to be related to an electron/positron pair plasma produced by a sudden dissipation of magnetic energy in the stellar vicinity. Such a plasma could be either trapped to the stellar surface by strong magnetic fields or launched as a relativistic outflow, which would play important roles in producing/suppressing the simultaneous emission in radio bands. In this talk, I will first present our recent work on (1) how magnetar flare spectra form inside the magnetosphere. Then, I will discuss (2) the potential link between magnetar flares and pulsed radio emission and (3) the plasma properties that are compatible with X-ray and radio bursts co-detected from the Galactic FRB source, a magnetar SGR 1935+2154.

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-redshift relations with the observables including the X-ray count rate, the optical richness, and the weak-lensing mass. As a result, we deliver cosmological constraints that are in excellent agreement (at a level of < 1 sigma) with the results from the Planck mission, the galaxy-galaxy lensing and clustering analysis of the Dark Energy Survey, and the cluster abundance analysis of the SPT-SZ survey. This work not only presents the first fully self-consistent cosmological constraints obtained in a synergy between wide-field X-ray and weak-lensing surveys, but also demonstrates the success of the empirical modeling in X-ray cluster cosmology studies.

The newly launched eRosita X-ray satellite revealed two gigantic bubbles above and below the Galactic center. The "eRosita bubbles" bare a remarkable resemblance to the Fermi bubbles detected in gamma rays, suggesting a common origin. The physical origin of these giant Galactic bubbles has been hotly debated. Using 3D magnetohydrodynamic simulations including relevant cosmic-ray physics, we show that the multi-wavelength observational data of the gamma- ray/X-ray bubbles as well as the microwave haze could be simultaneously explained by a single event of jet activity of Sgr A* about 2.6 million years ago. I will highlight some of the important constraints derived from our simulations and discuss the implications of the results on galaxy-scale AGN feedback in general.

In May 2022, the Event Horizon Telescope has revealed the first image of Sgr A*, the supermassive black hole at the center of our Milky Way Galaxy. Understanding the past activity of Sgr A* has been one of the key questions to address as it will allow us to gain insights into the formation history of the Milky Way. One of the most prominent signature for possible past activity of Sgr A* is the Fermi bubbles, two giant gamma-ray bubbles discovered in 2010. The origin of the Fermi bubbles has been intensely debated; however, in 2020 the newly launched eRosita X-ray satellite has revealed another pair of "eRosita bubbles", providing new constraints on their formation mechanisms. The enormous sizes, symmetries, and properties of the Fermi/eRosita bubbles strongly suggest that they share the same origin. Using cutting-edge numerical simulations including cosmic-ray physics, we show that the Fermi/eRosita bubbles likely originate from past jet activity of Sgr A* about 2.6 million years ago.

Galaxy clusters are known as the largest and most massive gravitationally bound objects in the Universe. X-ray observations discovered a large amount of diffuse, hot X-ray emitting gas with temperature of 10^7-8 Kelvin (so-called intracluster medium; ICM) trapped in deep gravitational potential well of galaxy cluster. Since the total mass of the ICM is estimated as ~ 10^14 solar masses and is a factor of ~5 larger than the total integrated mass of member galaxies in a galaxy cluster, the ICM plays an important role in the thermal evolution of baryons. Cool cores are often found at the center of galaxy clusters. Cool cores are in the form of the dense, relatively cool, metal enriched ICM. The presence of cool cores poses a challenge to our understanding of the thermal evolution of the baryons. SInce the ICM is losing their thermal energy by X-ray radiation, it was expected that runaway cooling takes place and a massive starburst is induced by a cooled ICM. However, X-ray observations indicate that there is no evidence of such runaway cooling. Thus, this is called the cooling flow problem. In this talk, I will briefly introduce the cooling flow problem, and present our recent study finding a ubiquitous presence of gas perturbations in cool cores based on Chandra and ALMA observations. In addition, I will also introduce XRISM, a new generation X-ray observatory, that will be launched in 2023 for exploring the X-ray Universe.

Supernovae ultimately deposit ~10% of their total energy in a population of relativistic cosmic rays that subsequently interact with the interstellar medium (ISM) via magnetic forces. Because these particles lose energy to radiation only slowly compared to the ~90% of the supernova energy that is deposited in the ISM as heat, they are a potentially important feedback mechanism in galaxies despite their comparatively small energy budget. Their effectiveness, however, depends crucially on the poorly-understood plasma processes that couple them to the bulk, neutral ISM. In this talk I introduce a new, physically-motivated model for the coupling between cosmic rays and the neutral, star-forming ISM, and show that it successfully predicts the gamma-ray spectra of resolved nearby galaxies, the galactic IR-gamma correlation, and the cosmological gamma-ray background. I conclude by exploring the implications of this model for the importance of cosmic ray feedback, demonstrating that this mechanism is likely unimportant for rapidly star-forming galaxies either today or in the early universe, but may be critical for local dwarfs and quiescent spirals.

It is now commonly accepted that the formation of the building blocks of our Solar System might have occurred during the early phases of the protoplanetary disk. I will present studies of the self-consistent disk formation starting from the prestellar core collapse. We measure the disk properties, and our findings suggest that 1) the embedded disk structure and dynamics might be (unfortunately) more complicated than we thought and 2) the stellar mass accretion does not necessarily transit through the bulk of the disk as often naturally assumed. The collapse type numerical simulations are computationally challenging and render a wide search of the parameter space unfeasible. During the embedded class 0/1 phase, the close interaction with the envelope should not be neglected when considering the dynamics and evolution of the disk. We proposed a model for self-regulated disk formation, where the magnetic braking is moderated by the non-ideal MHD effects. The resulting disk radius is slowly varying for a wide range of physical parameters, which accounts for the observed small size (<50 au) of young disks. Putting the pieces together, an embedded disk is tightly connected to its envelope, while its global properties are not very sensitive to the prestellar core environment. A simplified view of the young embedded protoplanetary disk might still be possible, while our understanding of the way how a disk is built up within a collapsing envelope awaits further refinement.

I will present recently submitted work (Liao & Cooper, arxiv.org/abs/2209.14166) in which we describe a catalog of 0.6 million galaxy color gradients measured from the DESI Legacy Imaging Survey (0.1 million of which have SDSS spectroscopy, with most of the rest due to be observed by the DESI Bright Galaxy Survey). We made this catalog because we believe forward-modeling of color gradients (for example with cosmological simulations) can be a useful way to constrain the cycle of star formation and feedback and enrichment, complementary to spectroscopic observations. No large representative dataset was available for that purpose, despite the quality and quantity of recent imaging data, so we set out to make one. I will present the trends we see in color gradients across the galaxy color magnitude diagram, and discuss a few interesting things we found along the way. In particular, trends with stellar mass and star formation rate are remarkably clear and consistent with well established ideas about galaxy formation, suggesting they might be readily explained by underlying age, metallicity and dust gradients measured in surveys like MaNGA. However, our comparisons with two MaNGA population synthesis models paint an unexpectedly ambiguous picture, in which (at face value) dust makes a surprisingly important contribution. I will also show preliminary comparisons of our results to the Illustris TNG simulation.

There is growing evidence that Type Ia supernovae (SNe Ia) are likely produced via multiple populations. Understanding how different populations evolve with redshift is critical in determining their precision in measuring the cosmological parameters. Previous studies indicated that SN Ia ejecta velocity is one powerful tool to differentiate between different populations. It was also suspected that the tight correlation between SN ejecta velocity and host-galaxy environment could introduce some evolution effect. In this work, we measure the ejecta velocity from the Si II line of a large sample of SNe Ia. For the first time, we find that the SNe Ia with faster Si II have a significantly different redshift distribution from their slower counterparts, in the sense that their relative fraction decreases with redshift. The trend may suggest a strong evolution of SN Ia ejecta velocity, or imply that the SN Ia demographics (as distinguished by their ejecta velocities) are likely to vary with time.

Dust extinction measurements provide important constraints on the size, composition, shape, and abundance of dust grains and an empirical model to account of the effects of extinction on astrophysical objects. For decades our understanding of dust grains was strongly biased by measurements in our Galaxy and the ultraviolet (UV). The UV bias is due to the extensive spectroscopic observations taken with the IUE satellite revealing the details of the 2175 A bump, far-UV rise, and underlying extinction continuum. I will discuss the results of a dedicated effort to expand our spectroscopic measurements of dust extinction to the far-UV, optical, near-infrared, and mid-infrared wavelength regimes. This work has revealed new optical extinction features, enabled the first combined study of UV and MIR extinction features, shown the possible presence of ice in the diffuse interstellar medium, and revealed an intriguing correlation between UV extinction and molecular hydrogen. Building on these works, a new R(V) dependent extinction relationship at spectroscopic resolution from 912 A to 32 microns has been determined. Moving out of our Galaxy, in progress work shows that the 2175 A bump is rare in an expanded sample of UV extinction curves and M31 and M33 show UV extinction curves quite similar to those seen in our Galaxy. Finally, prospects for future work especially with HST and JWST will be presented. Google meet link

Planets are assembled within disks that orbit around young stars. How these disks evolve from primordial gas and dust into the diverse planetary architecture is not well understood. With the powerful Atacama Large Millimeter/submillimeter Array (ALMA), we are now able to study these planet-forming disks in great detail, which have soon transformed our understanding of the planet formation process. In this talk, I will present recent results from ALMA disk surveys, emphasizing new insights on planet formation revealed from the dust continuum emission. In particular, I will discuss the emergence and cause of disk substructures (e.g., concentric gaps and rings, spiral arms), the implications of these features on disk evolution, and how to utilize them to search for young planets. Link to online talk

n order to obtain a deeper understanding of our Cosmos we gather observations of galaxies via our telescopes and construct theoretical models to reproduce these observations. A particular focus is given on the rate at which stars form within galaxies (labelled as the SFR). The “observed” star formation rate-stellar mass (SFR-M*) relation and the "observed" cosmic star formation rate density (CSFRD) represent both important canvases for our current knowledge of galaxy formation and are both routinely used to constrain cosmological models/simulations. I have been studying galaxy SFRs and galaxy stellar masses from 2015 (Katsianis et al. 2015, 2016, 2017, 2017b) to today (Katsianis et al. 2019, 2020, 2021, 2021, 2022). This investigation had some exciting and unexpected results. In this seminar I present the history of the SFRs within galaxies for the last 13 billion years. I start by presenting some severe limitations for both the simulated galaxy SFRs obtained from state-of-the-art cosmological simulations (Katsianis et al. 2021a, 2021b, EAGLE, IllustrisTNG, Simba, Semi-analytic models) and observations (Katsianis et al 2020, 2021b, UV, IR, radio, SED, Ha, OII indicators). I employ cosmological simulations combined with radiative transfer and demonstrate that the adopted methodology / indicator (e.g. IR, UV, Ha) to obtain the observed galaxy SFRs is bound to heavily affect the final result. I demonstrate that different authors who used different methods, assumptions and indicators to obtain their SFRs indeed reported results in tension by a factor of 4 for the period of 2007 to 2022. These interesting limitations / systematics between different methodologies were discussed extensively at the European Astronomical Society meeting (EAS) 2021 with numerous Polls pointing out a turmoil of opinions in the Field and conflicting qualitative results between different groups. In this seminar I will present these Polls. In addition, state-of-the-art simulations (EAGLE, TNG) are found to suffer from troubling limitations (Zhao et al. 2020, Katsianis et al. 2021) mostly connected to resolution effects and the adopted feedback prescriptions which have to be reconsidered. After demonstrating these limitations for both observations and simulations, I keep trying to find a way to address them. I demonstrate that the observed star formation rate density can be described by only two parameters (against 4, Madau et al. 2014) and a function that resembles a Γ distribution, like numerous other physical processes in Nature (from Economy to Biology) while it has a plateau from z ~ 1-4 and not a peak at z ~ 2. I demonstrate that a simple formalism relying on dark matter halo growth can emerge using this finding. The later can help significantly our efforts with cosmological simulations and Semi-analytic models which currently seem to lose legitimacy according to the Polls. Last, using up to date data and insights for different SFR indicators I present evidence that the observed CSFRD is dictated by episodic spikes and does not have a smooth evolution as previously thought.

Broad line regions of AGN disks are observed to have super solar metallicity even at redshift 7. Moreover, it does not evolve with time. These properties provide supporting evidence for in situ star formation in AGN disks. Based on the analogous theories of planet formation in protostellar disks, I construct a Stellar Evolution and Pollution in AGN Disks (SEPAD) model to account for these observed properties and their implications on the merging of compact remnants as sources of gravitational waves.

Turbulence is essential to many fundamental processes in protoplanetary disks, including angular momentum transport, dust evolution, and planet migration. I will focus on two instabilities that can drive turbulent motions in outer disks. In the first part of this talk, a series of global 3D non-ideal MHD simulations via Athena++ code will be presented. The outer disk is found to be weakly MRI turbulent, and annular substructures arise due to magnetic flux concentration. The weak MRI turbulence permits the growth of the vertical shear instability. In the second part of this talk, the saturation of the vertical shear instability mediated via a parametric instability will be covered. Once the parametric instability prevails, it is anticipated that the vertical shear instability is far more incoherent than extant published simulations suggest. Finally, I will touch on a new topic I recently started to work on - debris disk gas evolution. Link to online talk

Molecular gas is the fuel for star formation in galaxies. Using observations from the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM), that spatially resolve (1-30 pc) individual molecular clouds across the Hubble sequence, I will reveal a clear dependence of the nature of the molecular interstellar medium of galaxies on Hubble type, and present a simple diagnostic of cloud formation. In particular, I will highlight the shortcomings of the usual virial approach to clouds as self-gravitating objects, and stress the importance of the external galactic potential and in-plane shear to regulate the dynamical states of clouds. I also introduce a simple but powerful cloud-cloud collision formalism that accounts for the cloud properties in both nearby and high-redshift systems. Finally, I discuss the impact of these different mechanisms on the star formation efficiency of the clouds and thus the quenching of star formation, particularly in galaxy nuclei and spheroids (morphological quenching).

The origin of the molecular outflows associated with protostellar objects is under debate. They can be associated to one of the following scenarios: as driven by fast stellar winds (so-called "entrainment scenarios") or as actual disk winds. I will present a time-dependent model of the interaction between a stellar wind and a rotating cloud envelope in gravitational collapse. Together with this, I will present a comparison of the model with ALMA observations of the molecular emission lines of the molecular outflow associated with Orion Source I. Finally, I will present the sensitive molecular line emission and 1.3 mm continuum observations made with the Submillimeter Array (SMA) of the bipolar outflow associated with the young star located in the Bok globule known as CB 26.

Stars like our Sun are forming everywhere in our home galaxy, the Milky Way. ALMA, which stands for Atacama Large Millimeter/submillimeter Array, is currently the largest radio interferometry array on Earth. With its unprecedented sensitivity and resolution, we have mapped a few young star-forming regions in great detail, making a few breakthrough discoveries in the study of star formation. In this talk, I will first briefly introduce the current theory of star and planet formation with some ALMA results. Then, I will present our ALMA results of star formation in detail. In particular, I will report our results of accretion disks and jets around the forming stars and discuss their formation mechanisms. The accretion disks are expected to evolve later into protoplanetary disks in which planets are formed.

Magnetic fields play a fundamental role on the gas dynamics and evolution of protoplanetary disks (PPDs), yet astronomical observations are yet to provide useful constraints on magnetic fields. As direct analog of PPDs, the solar nebula provides an alternative to understand disk magnetism, where information about nebular magnetic fields can be recorded in specific minerals and preserved in certain meteorites. I will review the basic principle and our current understandings on meteorite paleomagnetism of the solar nebula. Direct measurements have been achieved in a handful of samples, and general results show consistency with theoretical expectations of nebular field strength and our present understanding of PPDs evolution. However, new data start to reveal some tension, which call for new developments in the field.

The wavelength range between roughly 30 and 300 micron offers an unique window on the Universe, which has not been explored to its full potential depth due to technological and engineering limitations. This so-called “THz gap” between JWST in the mid-IR, and ALMA in the sub-mm domain, is very rich of key diagnostic lines, characteristic spectral features, and peaks in dust emission by cold and dusty objects, which can only be observed from space. The main astronomical themes address some of the most compelling questions about galaxy evolution over cosmic time and the water trail from molecular clouds to planet forming systems. In order to obtain a natural sky background limited view on this wavelength domain, a sufficiently large, and deeply cooled space telescope is required, eliminating the thermal background emission of the optics. A number of mission concepts like SAFIR, and more recently SPICA and the GEP, have been proposed and studied, but did not make it into actual implementation so far. The major challenge is to cool a large aperture telescope and instruments to about 4K, and to meet the demanding detector requirements, which demand very low NEP (of order 10-20 to 10-19 W/√Hz), sub-K cryo-coolers, and large format focal plane arrays with low-power high-multiplex advantage readout electronics. In my talk I will offer a technology push perspective on further development of such space missions. I will briefly review the core science objectives and drivers defining the strong astronomical pull and desire to have such telescope in space at some day in the future. In the context of the Astro2020 decadal survey by the USA, there are currently a number of ongoing initiatives to develop so-called far-IR probe missions. I will provide a short overview and introduction of the mission concepts known to date, and their underlying instrumental concepts, and then discuss the technological needs matching state-of-the-art enabling technology and ongoing developments at SRON. In my talk I will particularly focus on SRON’s MKID detector technology, highlighting some of our recent breakthroughs, and activities in the area of advanced optical instrumentation employing dispersive optics, including our collaboration with ASIAA.

Pulsars and Fast Radio Bursts are intrinsically fascinating objects at radio wavelengths, especialy below 1 GHz where most of the action appears to happen. While instruments like CHIME and ASKAP are efficient survey instruments, telescopes like th Giant Metrewave Radio Telescope are better suited for targeted follow-up observations because of their ability to track the sky. The GMRT's flexibility is unmatched: newer instrumentation and modes of observing keep pushing the boundaries of its capabilities to newer territory. I will describe the results from our ongoing CHIME/FRB monitoring programme. In addition, GMRT is an excellent instrument to study pulsars. I will also describe novel ways of using the GMRT for scintillation studies using pulsars. With a new VLBI/baseband mode now enabled for the GMRT, it is well and truly poised to be a game-changer in low frequency radio astronomy. Google meet link

Multidimensionality necessary for a successful explosion in theories of core-collapse supernova explosions has suggested that core-collapse supernovae are aspherical. Whereas, observations of nearby supernova remnants have also revealed aspherical natures independently. However, how the explosions are connected to their supernova remnants has been unclear due to the lack of multi-dimensional modeling of such a long-term evolution. Therefore, we are conducting a project to link the explosions to their supernova remnants via three-dimensional hydrodynamical simulations. It is expected that several important properties, such as ones on the explosion mechanism, the progenitor star, and the compact object, can be extracted by comparisons of the three-dimensional models with observations. In the talk, I review several interesting observations and the results of our recent works on SN 1987A and Cas A. An attempt to model the chemical reactions in the ejecta of SN 1987A is also introduced, which is motivated by the recent ALMA observations of molecules and dust.