專題演講 2020

Since a significant fraction of stars are in multiple systems, binaries are now a critical component in modern astronomy. Close binaries are the origin of many exotic astronomical events in the Universe, including stellar mergers, type Ia supernovae, and gravitational wave events. On the other extreme, wide binaries are easily disrupted by gravitational perturbations, making them a unique tool to probe the Galactic structures. However, the formation and evolution of close and wide binaries remain an unsolved problem, especially because the age of binaries is difficult to measure. In this talk, I will show how we can learn about the age evolution of close and wide binaries from the Galactic kinematics using the Gaia data. For main-sequence contact binaries, I will discuss when and how they are born and when they merge. By combining Gaia with LAMOST, I will show that the wide binary fraction strongly depends on the metallicity, shedding light on their formation processes. In the end, I will demonstrate how these binaries can help us to understand the mass-radius relation of white dwarfs and planet formation.

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.
https://arxiv.org/abs/2007.00794

When a stellar mass object is captured into an inspiral orbit of a supermassive blackhole having mass around 10^6-10^9 M_0 then the gravitational waves emitted are called extreme mass ratio inspirals(EMRIs). These inspirals are slow and gradual and hence the frequency of EMRIs ranges between 10^(-3) to 1 Hz. Since the inspiralling object is very small compared to the central object their dynamics are dominated by the spacetime metric of the central blackhole as a result the gravitational waves from EMRIs can encode the spacetime structure of a central object with a high accuracy. In this talk, I will discuss the properties of EMRIs and the basic concept of the gravitational self force(GSF) in the context of EMRI waveform modeling.

Flare activity is an important phenomena caused by the chromosphere activity in late-type stars. We collected the light curve data from Kepler telescope and spectral data from ground-based telescopes for detecting flare events and measuring the chromospheric active levels of late-type stars with exoplanets or in binary systems. Our result agrees with the previous studies that slow rotators are less active than fast rotators and the stars with exoplanets tend to have less flare activity and lower chromospheric activity levels. M-type eclipsing binaries (EBs) show less flare events with large flares (energy release 10^34 ergs) than the hyperflaring M dwarfs, but their cumulative flare frequency factor is 10 times higher than the flaring single M dwarfs. The flare timing also shows that the secondary stars might be the major flare producers in some EB systems. Flare activity is also important to the habitability of the exoplanets. Stars with less frequent superflares provide better environments for life development, especially for the M-type stars, whose longevity and abundance make them good environments for habitable exoplanets. Kepler-442 is a K-type main-sequence star with an earth size exoplanet (2.3 M_earth) in the habitable zone. Flare events are detected on this star, which is different from our result that exoplanet systems rarely show flare activity.

Planets orbiting low-mass stars are prime systems for atmospheric characterization due to their low star-to-planet brightness contrast, high transit frequency, and prolonged main sequence stellar lifetimes. However, small stars such as M-dwarfs are highly magnetically active stemming from strong interactions between the stellar photosphere and corona. As a result, the atmospheres of attendant rocky planets in these systems may suffer severe atmospheric escape and chemical modulations by strong X-ray and UV activity. Previous 1D models found that although a single large flare does not substantially impact the ozone layers, repeated secular flaring with the inclusion of highly energetic particles could rapidly destroy the ozone columns of planets with initially Earth-like atmospheres. In this talk, I present our results in studying the effects of repeated stellar flares on planets orbiting G-, K-, and M-stars using state-of-the-art 3D models. First, I will briefly present key conclusions from my previous work simulating the atmospheres of rocky planets. Second, I will describe our methodology of this study by coupling an M-dwarf flare model to the 3D atmosphere model. Specifically, I will discuss how we compute realistic flare intensities, frequencies, and durations with a flare toolkit with data drawn from Hubble Space Telescope observational campaigns. I will then present new results in that we find that recurring stellar flares drive planetary atmospheres around K- and M-dwarfs into new chemical equilibria that substantially deviate from their initial pre-flare regimes. This stems from increased M-dwarf proton fluences, changes in the latitudinal extensions of energetic particle deposition due to absence of planetary magnetic fields, and transport via large-scale circulation and wave breaking. Using a newly published radiative transfer model, I will show that chemical compounds such as nitrous oxide (N2O) and nitric acid (HNO3) can be prominently observed throughout the entirety of the planet’s orbit in active stellar systems, making these “beacons of life” highly amenable to detection by the James Webb Space Telescope and next-generation instruments.

Understanding how galaxies form and evolve has been one of the most important topics in astronomy. In the past two decades, astronomers have realized that gas flow processes, gas accretion and outflows, play a crucial role in galaxy evolution. The processes regulate the amount of gas in and out of galaxies and leave their signatures in gas around galaxies. In this talk, I will introduce absorption line spectroscopy, a powerful technique to probe gas around galaxies and to reveal the gas flow processes. I will show that by applying novel statistical techniques to massive spectroscopic and photometric datasets from sky surveys, one can obtain unprecedented measurements of the gas properties, including the metal abundance, gas density, and its distribution around galaxies. Finally, I will discuss how these gas properties can place strong constraints on the models of galaxy evolution.

Comet is the most ancient object in our solar system. Ice-rich cometary nuclei are believed as remnants of planetesimals forming billion years ago. The great success of the European Space Agency Rosetta rendezvous mission opened a new vision on the nature of comet 67P/Churyumov-Gerasimenko. In-situ observations from multiple instruments onboard the Rosetta spacecraft revealing unprecedented high-resolution images of the cometary surface, distribution of several chemical species in the coma environment, and other incredible scientific results we had never seen before. In this talk, I will briefly introduce what we had learned from the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) instrument, such as the H2O column density measurement, constraint the physical properties of the coma environment, and estimate the Ortho-to-Para ratio (OPR) of water.

Dark matter (DM) is roughly 80% of the matter in the universe and not made up of any known particle. It interacts with other particles weakly or not at all except by gravity. Direct detection experiments aim to observe low-energy recoils of nuclei induced by interactions with DM particles, especially for the most popular candidate weakly interacting massive particles (WIMPs) with mass from few GeV to 1 TeV. To expand a direct detector’s coverage of low-mass WIMPs, or more generically light dark matter (LDM), different types of interactions have been explored and allow conventional technologies to have sensitivity to DM masses that would otherwise be inaccessible. One such mechanism is inelastic scattering where an additional particle can be created in the collision, resulting in a wide parameter space of energy-momentum transfer. In this talk, I’d like to present my recent works on the ionization signals from DM-electron scattering and Migdal effects of a nuclear recoil.

Kepler Mission have found that earth-sized planets or super-Earths are extremely common around solar-like stars. How the initially sub-micron sized interstellar dust particles coagulates to form kilometer-sized and/or further bigger solid bodies (e.g., planetesimals, protoplanets), is a key open question which is ultimately related to our own origin. Experimentally, this topic is challenging since there is no way we can physically probe the interstellar dust particles. In this talk, I will introduce how we attempt this problem using the high angular resolution interferometric observations at 1-10 millimeter bands, towards extrasolar protoplanetary disks.

About a half of the elements heavier than iron in the universe are made by the so-called rapid neutron-capture process (r-process) in explosive astrophysical environments. The observation of the kilonova emission associated with the binary neutron star merger event GW170817 provided the first evidence that some of the r-process elements were being made in the ejecta of the mergers. However, a number of related challenges remain. For instance, are mergers the only r-process sources or are some rare types of supernovae needed to account for the metalicity evolution of the r-process elements? what and how much heavy elements are made in mergers? On the other hand, the detection brings hopes that future observations may further shed lights on some of the yet-resolved issues not only in astrophysics, but also in nuclear and neutrino physics. In this talk, I will touch on a few selected topics above and discuss the related on-going efforts.

I will talk about my “journey” from star-forming clouds, protoplanetary disks, chaotic planet-forming regions, hot Jupiters, and finally back to the Solar System based on the stories in my very recent works. Through this journey, the audience will enjoy the power of interstellar shocks, potential young planets embedded in disks, violent giant impacts among planets, anomalously bloated exoplanets, and even the hot debris just hidden in the glory of the solar corona that we see during a solar eclipse!

Dark matter (DM) composes one-fifth of the Universe but its particle essence is still undetermined. In addition to the current terrestrial detectors to measure the signal from the direct interaction between DM and the Standard Model particles, compact stars such as neutron stars (NS) may shed the light on DM searches. Once DM accumulating inside the NS due to energy loss after scattering with the baryons in the star, the captured DM could annihilate and acts as a heating source and changes the star’s temperature. On the other hand, even DM does not annihilate, a huge amount of DM gathering in the star could also trigger gravitational instability and collapse into a black hole. This destroys the possibility of finding NS with age older than a few Gyrs. I’ll briefly review these issues and examine the implication for DM search in the talk.

Recent observations have revealed that circumbinary disks that are misaligned to the binary orbit could be common in the universe. Dissipation in the disc causes it to move either towards coplanar alignment or polar alignment. In the polar configuration, the disc is perpendicular to the binary orbit with the disc angular momentu vector aligned to the binary eccentricity vector. Since planets form inside disks, circumbinary planets may also form misaligned to the binary orbit. We explore the dynamical evolution and stability of misaligned circumbinary planets. We find that around eccentric binaries, the most stable orbits are those that are close to a polar alignment. Our results have implications for circumbinary planet formation and evolution and will be helpful for understanding future circumbinary planet observations.

Weak gravitational lensing (WL) is the phenomenon of image distortion by gravitational light deflection. Images of background sources are distorted by foreground matter structures. By measuring the distortion to percentage levels, the state-of-the-art studies have found WL as a powerful tool to constrain cosmology. The upcoming Stage-IV surveys such as LSST and Euclid are expected to provide high-quality constraints comparable to Planck.
In this talk, I will provide an overview on up-to-date WL studies and surveys. This will include scientific motivations, an introduction to theoretical basis, observational challenges, state-of-the-art results, and future perspectives. The talk will be accessible for non-cosmologists.

Infall drives the mass growth of protostars as well as the structural and chemical complexities. The rich spectra of molecules best probe the kinematics and chemistry at the star-forming regions. Using ALMA, we detect direct evidence of infall, the red-shifted absorption against the central continuum source, in the collapsing protostellar envelopes, such as B335 and BHR 71. A simple inside-out rotating infalling envelope can reproduce the infall signatures. However, the synthetic spectra disagree with the observations at off-center positions, hinting a faster rotation at the inner 50 au region compared to our envelope model. Serendipitously, we also identified emissions of complex organic molecules (COMs), revealing the “hot corinos” nature of BHR 71. 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. 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.