學術演講及專題研討 (2020)

Supernovae are among the most influential events in every astrophysical scale. New wide-field and high-cadence transient surveys enable us to watch supernovae from the moment of explosion. Paired with rapid and continuous monitoring facilities, these observations reveal unprecedented features that bridge our understanding of their progenitor systems to explosion mechanisms. From the discovery to follow-up, the Global Supernova Project is a world-wide collaboration of +150 supernova observers and theorists facilitated with the Las Cumbres Observatory and various other ground and space telescopes. In this talk, I will highlight the recent advancements in core-collapse supernova observation, especially in the context of the Global Supernova Project.

It is widely accepted that relativistic jets in AGNs are accelerated by strong magnetic fields. The magnetohydrodynamic (MHD) models of jet acceleration predict that (i) jet is gradually collimated by the pressure of an external medium confining the jet and (ii) jet collimation and acceleration occur simultaneously. Indeed, systematic collimation has been revealed in the jet of M87 inside the Bondi radius by many recent very long baseline interferometry observations. However, both the nature of the external confining medium and the presence of gradual jet acceleration in the jet collimation zone have not been well constrained by observations. In the first part of my talk, I briefly introduce our recent study of Faraday rotation in the M87 jet, where information on the external medium is imprinted. We found that the magnitude of the Faraday rotation measure systematically decreases with increasing distance from the black hole in the jet collimation zone. Our data is consistent with a picture that substantial winds, non-relativistic un-collimated gas outflows launched from hot accretion flows, confine the jet, resulting in the observed jet collimation. In the second part, I present the results of our recent kinematic analysis of the high-cadence monitoring data observed with the East Asian VLBI Network (EAVN) and of the Very Long Baseline Array (VLBA) archival data. We found that the jet is gradually accelerated from non-relativistic to relativistic speeds over a broad distance range that coincides with the jet collimation zone, which is in good agreement with the prediction of the MHD models.

Exomoons remain amongst the most elusive targets in observational astronomy. Nevertheless, these worlds stand to provide an unprecedented window into the formation and evolution of planetary systems. If the Solar System is any guide, we can expect exomoons will be geologically active and diverse, with the potential for hosting volatiles, atmospheres, and even life. Moreover, a thorough understanding of the population and occurrence rates of exomoons will help to place our own Solar System in a galactic context, speaking to the commonality of our own history. And though there are a variety of known pathways for moon formation, the discovery of exomoons may yet reveal heretofore unanticipated system architectures that defy easy explanation, thereby enriching our theoretical understanding of system formation. In this talk I will present my dissertation research, focusing first on a population study of exomoons in the Kepler data. I will then highlight my work related to the HST observation of Kepler-1625b, potentially the first transiting exomoon discovery. Finally, I will discuss my ongoing efforts to detect candidate exomoon signals in the Kepler data through deep learning, and argue that both targeted and survey observations have a role to play in finding exomoons going forward.

A 1 kJ pulsed-power system was built for studying various topics related to high-energy-density plasma, a regime with pressure greater than 1 MBar in general. Astrophysics and space sciences can be studied experimentally using the pulsed-power system due to the magnetohydrodynamic scaling. The pulsed-power system consists of twenty 1 μF capacitors, two rail-gap switches, two parallel plate transmission lines, and a cylindrical vacuum chamber orientated vertically. Two capacitors are first connected in series forming a brick. Five bricks are connected in parallel forming a wing. Finally, two wings are connected in parallel forming the whole capacitor bank, i.e., 5 μF in total. The system is charged to 20 kV. When it is discharged, a peak current of 110±20 kA with a rise time of 1.51±0.06 μs, i.e., a power of ∼ 700 MW, is provided. It is the pulsed current that will be used to drive different loads for different experiments. In particular but not restricted, supersonic plasma jets are generated via imploding conical-wire arrays made of tungsten wires using the pulsed-power system. The supersonic plasma jets will be used to simulate solar winds in the laboratory. When the generated supersonic plasma jet flows around an obstacle with or without surrounding plasma, either the Martian bow shock or wake cavity behind the moon will be studied based on the hydrodynamic similarity between the solar system and the laboratory. Besides generating plasma jets using conical-wire arrays, we are also developing a new scheme of generating plasma jets by compressing argon plasma plume, which will be generated by a plasma gun, using conical-theta pinches. Therefore, experiments can be conducted with higher repetition rates. To diagnose different experiments, a suite of ultrafast x-ray imaging systems such as time-integrated pinhole camera with an exposure time of 1 μs, a streak camera with a temporal resolution of 15 ps, and a framing camera with a temporal resolution of ns are being built by ourselves. Interferometry using a q-switch laser with the single longitudinal mode is planned to be used for plasma density measurements. Collective thomson scattering will be used for measuring local electron temperatures, ion temperatures, electron densities, and ion densities of the plasma. The laser pulse will be compressed using stimulated brillouin scattering (SBS) in water so that a temporal resolution of sub- ns can be achieved. Although all diagnostics are being developed, we have started imploding conical-wire arrays. Time-integrated images in visible light will be shown. This work was supported by the Ministry of Science and Technology (MOST), Taiwan, under Award Number 105-2112-M-006-014-MY3.

The multiwavelength analysis is one of the useful tools to understand the dust properties in protoplanetary disks. I study the synthetic ALMA multiband analysis to find the best three ALMA band combination for deriving the accurate dust temperature Tdust, optical depth τν, and dust opacity power-law index β (κν ∝ ν^β) using the ALMA archival data of the TW Hya disk. Through this approach, I can directly derive the three unknowns (Tdust, τν, β) without any assumptions on some of the dust properties. The synthetic multiband analysis results show that the Band [10,6,3] set is the best combination for minimizing the uncertainties of the dust properties derived from ALMA observations. The band combinations with Band 9 or 10 and with the large frequency intervals between the bands in the set give us stronger constraints on the dust properties. Also, large β and low Tdust are preferable for reducing the uncertainties of the dust properties. Some of those conditions are interpreted that the combination of optically thick and thin bands are required for obtaining stronger constraints on the dust properties. To examine the consistency of the synthetic analysis results, I apply the multiband analysis to ALMA archival data of the TW Hya disk at Band 4, 6, 7, and 9. I confirm that the trend of the synthetic model is consistent with the derivations from the real data. Additionally, I examine my synthetic analysis results with ALMA Band 1 which has been developing in ASIAA. The results show that the ALMA band combinations with Band 1 will improve the constraints on the dust properties.

Redshifts of an astronomical body measured at multiple epochs (e.g., separated by 10 years) are different due to the cosmic expansion. This so-called Sandage-Loeb test offers a direct measurement of the expansion rate of the Universe. However, acceleration in the motion of Solar System with respect to the cosmic microwave background also changes redshifts measured at multiple epochs. If not accounted for, it yields a biased cosmological inference. To address this, we calculate the acceleration of Solar System with respect to the Local Group of galaxies to quantify the change in the measured redshift due to local motion. Our study is motivated by the recent determination of the mass of Large Magellanic Cloud (LMC), which indicates a significant fraction of the Milky Way mass. We find that the acceleration towards the Galactic Center dominates, which gives a redshift change of 7 cm/s in 10 years, while the accelerations due to LMC and M31 cannot be ignored depending on lines of sight. We create all-sky maps of the expected change in redshift and the corresponding uncertainty, which can be used to correct for this effect.

RIFT is the abbreviation of Robotic Imagers For Transients, which will be the first robotic multiple-telescope observatory dedicated to the study of multi-messenger transients in Taiwan. The project has been approved by MOST and will be funded through the Young Scholar Fellowship Program, hopefully from 2020. In this talk, I will present the configuration of RIFT, our five-year plan, and the science that RIFT can do.

The astrophysical reach of current and future ground-based gravitational-wave detectors is mostly limited by the quantum noise, induced by vacuum fluctuations entering the detector output port. The replacement of this ordinary vacuum field with a squeezed vacuum one has proven to be an effective strategy to mitigate such quantum noise and it is currently used in advanced detectors. However, current squeezing cannot improve the noise in the whole spectrum, because of the Heisenberg uncertainty principle: when shot noise at high frequencies is reduced, radiation pressure at low frequencies is increased. A broadband quantum noise reduction is possible by using a more complex squeezing source, obtained reflecting the squeezed vacuum off a Fabry-Perot cavity, known as filter cavity. In this talk, I will report our recent implementation of squeezed vacuum states at 1064 nm. With a bow-tie optical parametric oscillator (OPO) cavity, and our home-made balanced homodyne detectors, noise reduction up to 10dB below the vacuum is measured. Applications of our squeezer to the gravitational wave detection will be reported, for the first demonstration of a frequency dependent squeezed vacuum source able to reduce quantum noise of advanced gravitational-wave detectors in their whole observation bandwidth. The experiment uses a suspended 300 m long filter cavity at National Astronomical Observatory of Japan (NAOJ), similar to the one planned for KAGRA, Advanced Virgo and Advanced LIGO, and capable to impress a rotation of the squeezing ellipse below 100 Hz.

Advanced LIGO-Virgo have detected tens of stellar mass compact binary mergers, including binary black holes, binary neutron stars, and potentially neutron star-black hole mergers. These binary merger detections carried plenty of information about the binaries and the Universe. In this talk I will focus on a few topics we learned from the gravitational-wave detections: the electromagnetic counterparts of binary mergers, the neutron star equation-of-state, and the expansion rate of the Universe. I will first summarize current status of the field and the future projections. I will then discuss future plans to expand and improve the study.

Galaxies are believed to form in dark matter halos. In principle, we should be able to constrain the physical parameters of some dark-matter candidates by studying the properties of galactic halos. On the other hand, galaxy formation is believed to connect to the formation of the supermassive black hole located at the center of the galaxy; therefore, the formation and properties of active galactic nuclei (AGNs) should also be related to the dark matter halos. In this talk, I would like to show our results related to the studies of galaxies and AGNs and discuss possible effects on the connection between the dark matter halos and the supermassive black holes. I will also discuss some constraints about the properties of dark matter obtained from our investigations.

Planet formation may start during the embedded phase of star formation. In this scenario, the chemistry of embedded disks may directly determine the chemical composition of the forming planets. In recent years, observations discover several embedded protostars that have developed complex chemistry at the disk-forming region. However, only a few observations attempt to constrain the occurrence of complex molecules at embedded protostars and their relationships to star formation processes. I will present the first result of the Perseus ALMA Chemistry Survey (PEACHES), which aims to unbiasedly survey the chemistry toward 47 embedded protostars with a spatial resolution comparable to the size of disk-forming region. In PEACHES, we identify a variety of molecules and their isotopologues, including CCH, c-C3H2, SO, SO2, CH3OH, CH3CN, CH3OCHO, CH3OCH3, and C2H5OH. I will discuss the detection statistics of these molecules with respect to the physical properties of these protostars, such as their evolutionary stages and disk properties. I will also discuss the correlations of these complex molecules and the comparison with the chemistry of the protostars at other regions and environments. The occurrence rate of different complex molecules learned from the PEACHES survey will provide a primer for constraining chemical evolution during the star formation.

Energetic feedback from stars and supermassive black holes (SMBHs) is key ingredient in the formation and evolution of galaxies and clusters, as shown by state-of-the-art cosmological simulations. However, predictive powers of these simulations are limited by the important but often neglected microphysics -- physical processes that are unresolvable and not captured by purely hydrodynamic simulations. One of such examples is cosmic rays (CRs). In this talk, I will discuss how CRs could influence feedback processes across different mass scales, including AGN feedback in galaxy clusters, Fermi bubbles within our Milky Way Galaxy, and galactic winds in dwarf galaxies. In particular, I will discuss how the detailed microscopic processes of CR transport could have dramatic impacts on the macroscopic properties of galaxies and clusters.

With insufficient masses to sustain core hydrogen fusion, substellar objects continue to cool and fade after birth. Those heavier than 13 jupiter masses, called brown dwarfs, manage to ignite deuterium or lithium in the cores, thereby maintaining hydrostatic equilibrium for a short period of time. Those less massive than this do not undertake any nuclear reaction whatsoever in their lives and evolve like planets. So far a few thousand brown dwarfs and planetary-mass objects are known, almost exclusively found in the Galactic field in the solar neighborhood, i.e., they are already aged. Searching for the youngest substellar objects by spectroscopy is hampered by their faintness and often confusion with foreground or background contaminations. We describe our efforts to identify substellar candidates in nearby star-forming regions of 1 to 3 Myr old, when brown dwarfs are being formed or in their infancy. Our sample of substellar populations in star clusters, with known ages and distances, provides stringent constraints to confront theoretical modeling of ultracool atmospheres, and of chromospheric activity. We also present how these least-massive members as the most vulnerable members in stellar dynamics get ejected, leading to eventual disintegration of star clusters.

At high redshifts, protogalaxies were forming their first generations of stars in intense bursts. During their active phases, these starburst galaxies would presumably be environments rich in energetic cosmic rays due to the presence of massive stars and their remnants. Stellar remnants can supply seed particles and generate the shocks (via supernova explosions and other violent events) needed to accelerate the seeds to very high energies. I will present an overview of my current research activities, which consider the effects these energetic cosmic rays can have on the early Universe, particularly on the formation and evolution of the galaxies therein. I will outline how these energetic particles can interact with protogalactic environments, and discuss the impacts they may have on the evolution and star-formation histories of their hosts, how their astrophysical effects can manifest themselves on galactic, sub-galactic and super-galactic scales, and how their signatures might be observed in the CTA era.

The Greenland Telescope (GLT) project is to investigate black holes and their immediate vicinity with imaging the shadows of super massive black holes (SMBHs) using the submillimeter Very Long Baseline Interferometry (submm VLBI) technique together with other submm telescopes all over the world. US National Science Foundation (NSF) awarded a 12 m ALMA North America prototype antenna to the group led by the ASIAA and the Smithsonian Astrophysical Observatory (SAO). The telescope was retrofitted for the extreme cold environment of Greenland, and assembled at the US Thule Airforce Base, located at the northwestern region of Greenland. We are currently commissioning and operating the telescope at there, and preparing for the traverse to the Summit of Greenland. In this talk, I will present the history and the current status of the GLT, and describe our future plan.

Solar flare is one of the most explosive phenomena in the solar system that large amount of energy (1032~1033 ergs) releases within 102-103 seconds, which can accelerate electrons up to few hundreds of MeV and ions up to several tens of GeV. One of the most important issues in solar physics is to investigate the energy release and particle acceleration processes. The flare-accelerated particles generated in the corona can propagate upward along the open magnetic fields into the interplanetary space and move along the closed magnetic fields downward to the lower solar atmosphere. In general, the chromospheric hard X-ray sources occur in the impulsive phase of solar flares and are thought to be the footpoints of newly reconnected magnetic fields mapped to the primary energy release site. The radio emissions are the important diagnostic tool to trace and study the dynamics of flare-accelerated particles. In this talk, I will first introduce the solar flare characteristics in multi-wavelength observations. Then I will present the research works on the estimation of reconnection electric field, asymmetry of chromospheric hard X-ray emissions, fine structures seen in a radio dynamic spectrogram, as well as the pre-flare Type III radio bursts and their solar associations.

The Zwicky Transient Facility (ZTF) is an on-going time-domain sky survey with an aim to detect various classes of transients. ZTF utilizes a wide-field 48-inch telescope (the P48 Telescope), located at the Palomar Observatory, and a dedicated mosaic CCD to survey the sky in gri filters. The telescope and CCD combination gives ZTF a stunning 47 degree-squared field-of-view, with a depth of ~20.5mag in the r-band. National Central University has joined the ZTF consortium under the TANGO collaboration, with research topics focused on asteroids (or Solar System small bodies) and variable stars. In the first part of this talk I will give an overview of the ZTF project. In the second part of the talk, I will present our published and on-going work on Be stars and RR Lyrae using the ZTF data.

HO Puppis (HO Pup) was considered as a Be star candidate due to its γ Cassiopeiae-type light curve, but lacks spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral energy distribution (SED) fitting on broadband photometry, the Be star nature of HO Pup is ruled out. In contrast, the SED of HO Pup can be fit well with a hot and sub-luminous star or, possibly, a hot subdwarf. 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 resemble well to the IW And-type stars, exhibiting characteristics such as standstill phase, outbursts and dip events. The light curves of HO Pup display various variability time scales, including outbursts cycles range from 23 to 61 days, variations with periods of 3.9 days and 50 minutes during the standstill phase, and a semi-regular ~14 days for the dip events. We have also collected time-series spectra (with various spectral resolutions), at which Balmer emission lines and other expected spectral lines for an IW And-type star were detected, even though some of these lines were also expected to be present on Be stars. Nevertheless, detection of Bowen fluoresces near the outburst phase can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for 4 nights, the polarization variation was detected, indicating that HO Pup has intrinsic polarization.

Fast radio bursts (FRBs) are millisecond transients of unknown origin(s) occurring at cosmological distances. Here we, for the first time, show time-integrated-luminosity functions and volumetric occurrence rates of non-repeating and repeating FRBs against redshift. The time-integrated-luminosity functions of non-repeating FRBs do not show any significant redshift evolution. The volumetric occurrence rates are almost constant during the past ~10 Gyr. The nearly-constant rate is consistent with a flat trend of cosmic stellar-mass density traced by old stellar populations. Our findings indicate that the occurrence rate of non-repeating FRBs follows the stellar-mass evolution of long-living objects with ~Gyr time scales, favouring e.g. white dwarfs, neutron stars, and black holes, as likely progenitors of non-repeating FRBs. In contrast, the occurrence rates of repeating FRBs may increase towards higher redshifts in a similar way to the cosmic star formation-rate density or black hole accretion-rate density if the slope of their luminosity function does not evolve with redshift. Short-living objects with <~Myr time scales associated with young stellar populations (or their remnants, e.g., supernova remnants, young pulsars, and magnetars) or active galactic nuclei might be favoured as progenitor candidates of repeating FRBs.

I will talk about science goals and current status of three projects: the Compton Spectrometer and Imager (COSI), the Compton Polarimeter (Compol), and the Gamma-ray Transient Monitor (GTM). These are all for building instruments to detect photons in the MeV energy range, which is relatively unexplored in the whole electromagnetic spectrum because of difficulties in detection techniques. COSI is a general-purpose, next-generation Compton telescope based on high-purity germanium detectors. It is led by Space Sciences Lab, UC Berkeley, and has been selected by NASA SMEX program for phase A study. Compol is a small Compton polarimeter based on scintillator detectors to fly on a CubeSat for observing polarization of soft gamma rays from Cyg X-1.Compol is still looking for CubeSat opportunities. GTM is, similar to Compol, based on scintillator detectors. It is mainly for GRB monitoring and may also detect bright transients from other sources. It has been selected by NSPO to fly on Formosat 8B, planned to launch in 2024. Feasibility study for GTM to fly in a lunar orbit is underway.

Observations of the cosmic microwave background over the last two decades hint that all structure in our Universe, including us, arose from quantum fluctuations at its birth. One key piece in confirming this picture, however, remains missing. In this talk, I will describe how observations of gravitational waves from the very beginning of our Universe can provide this missing piece. I will also talk about how we can use these primordial gravitational waves to probe the constituents of our Universe at its moment of birth, and how they might provide possibly the only evidence for quantum gravity.

Searching for clues on supernovae (SNe) from their supernova remnants (SNRs) is like a crime scene investigation (CSI). This talk is based on Type Ia SNR work by my students and postdoc in Taiwan. Type Ia SNe have been used as standardizable candles to discover the accelerated expansion of the Universe; however, we still are not certain how Type Ia SNe exploded exactly. Two popular scenarios have been suggested, one involves a white dwarf (WD) having accreted too much material from a normal-star companion, and the other involves merger of two WDs. In the former scenario, the stellar companion survives the SN explosion, while in the latter scenario no stellar remnant is left. We have been studying the Type Ia SNRs in our neighboring galaxy the Large Magellanic Cloud (LMC), using Hubble Space Telescope images to search for surviving companions of their SN progenitors and dense circumstellar material ejected by the SN progenitors. I will report our findings and suggest possible explosion mechanisms. I will also report on our search for young Type Ia SNRs in the M33 galaxy. The goal of this research is to understand the explosion mechanisms of Type Ia SNe.

Star formation in galaxies is determined by the efficiency of the conversion of gas to stars and stellar feedback that shapes the reservoir of gas for future star formation. In this talk, I will introduce the PHANGS survey (Physics at High ANgular resolution in Nearby GalaxieS), a large multi-wavelength project to survey nearby galaxies at ~100-pc scale in molecular gas, star-forming regions, and stellar clusters, aiming to study the link between the small-scale physics of gas and star formation and galaxy properties and evolution.
I will also show you the preliminary results that probe the global property dependency of the relative distribution of molecular gas and star formation in nearby galaxies. We employ a simple approach to quantify the distributions of regions with different evolutionary stages of star formation: from quiescent molecular gas, to star-forming gas, and to regions of massive star formation only. At the best-match resolution of 150 pc, the distribution of regions with different evolutionary stages show a dependence on the stellar mass and morphology of galaxies, both globally and locally. Galactic dynamics (e.g., bar and spiral arms) also add to the complexity of the star formation process. Our results highlight the potential importance of galaxy properties for the gas-star formation cycle, and imply a global-property-dependent Kennicutt–Schmidt relation. However, any trends between galaxy properties and molecular gas and HII region distributions is visible only when the observed spatial scale is smaller than ~500 pc, indicating a critical resolution requirement to resolve the dependency.

Lensed quasars are powerful tools to study various topics in astrophysics and cosmology, such as quasar host properties, dark matter substructure, and the measurement of the Hubble constant, H_0. To make the use of lensed quasar more efficient and competitive, it is essential to enlarge the sample size of lensed quasars. I will present a lensed quasar finding algorithm based on its variability. Specifically, this variability-based lens finding algorithm detects lensed quasar with the difference images from cadenced surveys. I will show the application of this algorithm on the Hyper Suprime-Cam (HSC) transient survey and the resulting lensed quasar candidates. With the prospects of applying this variability-based algorithm on future wide-field cadenced imaging surveys, such as LSST, we expect to skillfully discover hundreds to thousands of new lensed quasars.

The interaction between a supermassive black hole (SMBH) and the surrounding material is of primary importance in astrophysics. The detection of the molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way SMBH, SgrA*, resembles the "molecular torus" in AGNs, provides an unique opportunity to study SMBH accretion at sub-parsec scales. However, the CND is transient if its gas density is under the tidal threshold of SgrA*/nuclear star clusters, thus depleting the source of fuel. In our new CS line maps, we find several dense gas streamers appear to carry gas directly toward the nuclear region and might be captured by the central potential. These streamers show a signature of “infalling” motion with progressively higher velocities as the gas approaches the CND and finally end up co-rotating with the CND. Furthermore, utilizing the JCMT 850 um polarization data, we also find that the magnetic field appears dynamically significant toward the CND and also onwards to the inward ionized flows. Our results might suggest a possible mechanism of gas feeding to the CND. I will also talk about the observed morphology, kinematics of gas, and effects of magnetic fields in the GC. I will present our newest ALMA CS line maps in this region and discuss the gravitational stability of molecular cloud cores near Sgr A*.. As the nearest observable Galactic nucleus, this feeding process may have implications for understanding the processes in extragalactic nuclei.

Compact objects are natural laboratories to test fundamental physics in extreme conditions. Our knowledge of them grows dramatically with the improvement of new theoretical models, observational instruments, analysis techniques, and computing power. For example, the simultaneous observations made with NICER and ground-based radio telescope allow us to detect the X-ray enhancement during the giant radio pulse in Crab pulsar. Furthermore, advanced and automated algorithms are needed in exploring hidden physics and searching for new objects as new data formats are born and a huge amount of data is accumulated. I will introduce my recent works on developing and applications of time-frequency analysis algorithms, including a systematic search for ultraluminous X-ray pulsars that challenges the current understandings of magnetospheric accretion onto neutron stars, and a study of time-frequency properties of gravitational wave signals from both coalescence of compact objects and core-collapse supernovae. Finally, I will introduce an X-ray CubeSat project that aims for continuous gravitational waves from compact objects, follow-up observations of transient events, and non-stationary phenomena around super-massive black holes.

Asteroids are believed to have the so-called "rubble-pile" structure (i.e., gravitational aggregates) and several evidences point to this kind of loose interior structure, including (a) smaller bulk density smaller compared to the composition materials, (b) asteroid families, (c) boulders on asteroid surface, and (d) the 2-hr spin-rate limit. However, the rubble-pile structure has been challenged recently by the discovery of super-fast rotators (SFRs), a group of asteroids supposed to be destroyed by a spinning of < 2 hr (i.e., centrifugal force > self gravity). In order to understand SFRs itself and asteroid interior structure, I used wide-field telescopes (e.g., PTF, ZTF, PS1, and Subaru/HSC) to collect numerous asteroid light curves and tried to find and study SFRs. In addition to asteroids, I also try to find the spin-rate limits for Hildas and Jupiter Trojans. This helps to understand if the rubble-pile structure can be applied to other asteroid populations as well.

Stars like our Sun are forming everywhere in our home galaxy, the Milky Way. ALMA stands for Atacama Large Millimeter/Submillimeter Array and 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. I will also report the detection of more than 10 organic molecules including prebiotic molecules in one of the accretion disks. These molecules could be passed down to protoplanetary disks and thus incorporated into the planets to be formed later.

Detection of gravitational waves from a nearby core-collapse supernova could be the next milestone of gravitational-wave astronomy and multimessenger astrophysics. In this presentation, I will discuss the numerical challenges in modeling for these systems that involved detailed both micro- and macro-physics and present a few recent full 3D simulations with neutrino transport. In particular, I will focus on the dynamics of stellar-mass black hole formation and the effects of rotation on the explosion engines and unique gravitational wave features that might be detected with the current gravitational wave detectors, e.g., Advanced LIGO, Virgo, and KAGRA.

The Event Horizon Telescope (EHT) recently released the first horizon-scale images of the black hole in M87. Combined with other astronomical data, these images constrain the mass and spin of the black hole as well as the accretion rate and magnetic flux trapped on the black hole. An important question for EHT is how well key parameters such as spin and trapped magnetic flux can be extracted from present and future EHT data alone. Here we explore parameter extraction using a neural network trained on high resolution synthetic images drawn from state-of-the-art simulations. We find that the neural network is able to recover spin and flux with high accuracy. We are particularly interested in interpreting the neural network output and understanding which features are used to identify, e.g., black hole spin. Using feature maps, we find that the network keys on low surface brightness features in particular.

The cluster mass is one of the fundamental properties of galaxy clusters. In optical surveys, several measures can be taken to estimate the cluster mass, including, for example, utilizing the velocity dispersion of the cluster galaxies, or the weak gravitational lensing. On the other hand, deep learning provides an unique way to predict the cluster mass by approximating the underlying complicated mappings from inputs (images or catalogs) to outputs. In this talk, I will present a semi-supervised machine learning model which estimates the cluster mass directly from the SDSS ugriz-band images. I will also show the visualization of the model which can be used to interpret the network output.