Title: Interstellar scintillation as micro-arcsecond scale probes of compact radio AGNs
The variability of compact AGNs on timescales of hours and days observed at cm-wavelengths is predominantly caused by scattering in the ionized interstellar medium (ISM) of our Galaxy. With the ISM as an AU-scale interferometer, interstellar scintillation (ISS) provides an exquisite probe of the micro-arcsecond scale structure of AGNs. I present results from the Micro-arcsecond Scintillation-Induced Variability (MASIV) Survey of ~500 compact AGNs and its follow-up observations.
I will discuss the dependence of ISS on intrinsic AGN properties, including their gamma-ray loudness, radio spectral indices, optical spectral classification, redshift, and intrinsic variability. I will show how we have used ISS to probe the source size-redshift relation of compact AGNs, and place strong constraints on the turbulent properties of the intervening intergalactic medium.
Future surveys of ISS with highly-sensitive instruments such as the SKA will potentially probe the micro-arcsecond structure of faint (~100 muJy to 10 mJy) AGNs, thereby complementing studies at comparable angular resolutions with Space-VLBI and mm-VLBI which are limited only to the brightest AGNs.
Title: Exploring the 5:1 Neptune Resonance: Dynamics, Population, and Origin
Abstract: The long-term evolution of objects in the outer n:1 resonances with Neptune provide clues to the evolutionary history of the Solar System. Based on 4 objects with semi-major axes near the 5:1 Neptune resonance, we estimate a substantial and previously unrecognized population of objects, perhaps more significant than the population in the 3:2 (Plutino) resonance. Understanding the characteristics and trapping history for objects in these populations is critical for constraining the dynamical history of the solar system. The 4 objects detected in the Canada-France Ecliptic Plane Survey (CFEPS) were classified using dynamical integrations. Three are resonant, and the fourth appears to be a resonance diffusion object, part of a population which exited the resonance through chaotic diffusion. The dynamical behavior of the known objects, suggests that thetrapping mechanism for the 5:1 resonance is resonance sticking from the scattering objects. This is consistent with the measured surface colors of the objects. However, our investigations of Solar System evolution models show that they do not emplace a sufficiently large population into this resonance, and the source of this large population remains unexplained.
Title: Circumstellar Spirals/Shells/Arcs: the Messages from Binary Stars
A growing consensus has been developing in the past few decades that binarity is key in providing an understanding of the morphological diversities of the circumstellar envelopes (CSEs) surroudning stars in the Asymptotic Giant Branch (AGB) to Planetary Nebula (PN) phase. However, despite extensive efforts to detect companions of AGB stars and the central stars of PNe, the number of detected binaries in particular with their orbital properties derived are still small. As a consequence, the possible roles of binaries in the shaping of PN and in the CSEs of AGB stars have yet to be clarified.
On the other hand, recurrent (ring/spiral/arc) patterns are often found in the CSEs of AGB stars and the outer halos of pre-PNe and PNe. Such patterns provide a fossil record and can be used to trace the temporal history of the mass loss dynamics during the AGB phase. In this regard, recent molecular line observations using radio interferometric facilities such as ALMA and JVLA have revealed the spatio-kinematics of such patterns. Numerical simulations of binary interactions producing spiral-shells have been extensively developed and are now becoming increasingly sophisticated, revealing new probes for extracting the stellar and orbital properties from these patterns.
I will review the recent theoretical and observational investigations on the circumstellar spiral-shell patterns and discuss their implications in linking binary properties to the asymmetric ejection events in the post-AGB phase.
Planet formation is rapidly developing field in astronomy. We are in an era of not only regular detections of extra-Solar planets, but also the planet formation process itself. Recent observations of protoplantary disks reveal stunningly detailed sub-structures such as gaps, rings, spirals and lopsided asymmetries. Understanding the origin of these structures, for example due to unseen planets or dynamical instabilities, can place constraints on the physical conditions for planet formation. I will discuss some works on hydrodynamic processes important to protoplanetary disk/planet evolution and in explaining observations. These include gravitational instabilities, vortex formation, and the vertical shear instability for generating turbulence in disks. I will also present a new effort to study dusty protoplanetary disks through a set of modified fluid dynamic equations.
Speaker: Dr. Hsi-Wei Yen (European Southern Observatory, Germany)
Title: Formation and Evolution of Protoplanetary Disks
Protoplanetary disks are sites of planet formation. It is essential to study how protoplanetary disks form in dense cores and evolve, to understand the environment of planet formation. In this presentation, I will introduce our observational studies on protoplanetary disks at different evolutionary stages, from the formation and growth of protoplanetary disks around deeply embedded young protostars (Class 0 stage), to the gas dynamics of the material surrounding protoplanetary disks around more evolved protostars (Class I stage), and to the properties of a larger sample of protoplanetary disks around young stellar objects after the main accretion phase (Class II stage).
(1) Speaker: Prof. Wenwu Tian (Calgary University, Canada & National Astronomical Observatories of China)
Time: 12:10pm, 3/14 (Tue), 2017
Title: Supernova remnants and the origin of cosmic rays
I will first give a short introduction to Supernova Remnant (SNR), then focus on a recent hotspot of SNR research: Studying the origin of cosmic rays by TeV gamma-ray survey in the Galactic plane. TeV SNRs show great promise to increase our understanding of cosmic rays. By neutral hydrogen (HI) 21 cm continuum and HI line observations to some TeV SNRs, we have measured their kinematic distances which help improving our understanding of cosmic rays' origin.
(2) Speaker: Dr. Jeng-Lun Chiu (Space Sciences Lab, UC Berkeley, USA)
Time: 12:10pm, 3/17 (Fri), 2017
Title: The Compton Spectrometer and Imager (COSI) Project
The Compton Spectrometer and Imager (COSI) project is an effort to develop the next generation Compton telescope of higher sensitivity. COSI is currently a balloon-borne telescope project. The heart of COSI is an array of 12 cross-strip germanium detectors, each with 15mm x 80mm x 80mm dimension and full 3D position resolution of less than 2 mm^3. COSI performs Compton spectroscopic imaging in the 0.2-10 MeV gamma-ray band with a field of view about 50 degrees across and capability of polarization measurement. It is also well suitable for monitoring transient events. Several COSI balloon flights have been conducted. The most recent flight was launched from Wanaka, New Zealand, in May 2016 with a super-pressure balloon flying for 47 days. During this flight, COSI discovered GRB160530A and detected several sources, including the 511-keV emission from the galactic center, the Crab, Cen A, and Cyg X-1. The COSI collaboration is now working for the next flight in spring 2019, to launch again from Wanaka, New Zealand, for a 100-day flight. I will report the current status of the COSI project. COSI is a join effort of several institutions in Taiwan, US and France.
Speaker: Prof. Paul M. Ricker (University of Illinois, USA)
Title: There Is Life after Stellar Middle Age
The discovery of how stars function and evolve counts among the twentiethcentury's greatest scientific achievements. In the twenty-first century the problems that still challenge us are intrinsically multidimensional and multiscale: star formation, supernovae, stellar convection and mass loss, and common envelope evolution, among others. Given the extreme conditions and large spatial and temporal ranges of stellar evolution, our theoretical understanding of this subject rests heavily on numerical simulations containing a mixture of modeling and first-principles calculation. I will discuss the application of these techniques to the evolution of common envelope systems and explain the potential significance of advances in this area for a wide range of astrophysical problems.
Speaker: Dr. Hsiang-Yi Yang (University of Maryland, USA)
Time: 10:30 am, Feb 17, 2017 (Fri)
Venue: R620, Physics Building
Title: The Microphysics of AGN Feedback
Feedback from active galactic nuclei (AGN) is one of the most important processes governing the formation and evolution of galaxies and galaxy clusters. It is believed to be responsible for inhibiting the formation of massive galaxies and for solving the long-standing cooling-flow problem in galaxy clusters.
A lot of understanding of AGN feedback has been gained using hydrodynamic simulations; however, some of the relevant physical processes are unresolvable or not captured by pure hydrodynamics, such as plasma effects and cosmic-ray (CR) physics.
In this talk, I will present how we use simulations that incorporate this microphysics to understand how AGN jets feedback on galactic and cluster scales. Specifically, I will discuss the roles of thermal conduction and CRs in addition to purely hydrodynamic models. I will also talk about how we could use multi-messenger observations of the Fermi bubbles as a nearby laboratory for studying AGN feedback. Finally, I will conclude with open questions and future prospects of applying simulations beyond hydrodynamics to various interesting astrophysical systems.
Speaker: Dr. I-Chun Shih (GEPI, Paris Observatory, France)
Gaia Data Release 1 and what to expect in the future releases
Gaia mission aims to provide the most precise stellar atlas in the human history so far. Its result will touch many fields of astronomical science and everyone can access them openly. This talk will introduce the nature of the mission, the content of the Data Release 1 and its limitations. More importantly, how can you prepare yourself with the data (the present and the future) for your research.
First Hydrostatic Cores (FHCs) are the transient phase between prestellar cores and Class 0 protostars. It is a key to understand the earliest stage of star formation. Recently, some potential candidates of FHCs have been suggested, but not enough to enable statistical property studies. In the talk, I will summarize the papers which propose candidates of FHCs. I will also talk about my recently work on searching FHCs candidates in Perseus molecular cloud.
Planet formation in AB Auriga: imaging inner gaseous spirals inside the dust cavity
Studies of planet forming disks have rapidly advanced in the last few years thanks to high contrast high angular resolution images obtained in the optical/near infrared and in the mm/submm regime. In the meantime, several exoplanets have been found by direct imaging in nearby debris disks. Studying planet formation in younger systems where the molecular gas and dust material are not yet fully dissipated is a clue to understand how a planetary system forms and evolves while the disk dissipates.
In order to trace the structures induced by embedded objects, such as companion or planets, I observed the 12CO 2-1 transition at 0.1 angular resolution in the AB Aurigae system using ALMA. The 12CO 2-1 gas exhibits two spiral-like features within 0.7" (radius, r, 90AU) away from the star. These spirals are trailing and non-self-gravitating. Two possible locations of the embedded objects are suggested in order to explain the observed features in AB Aurigae system.
Subaru observations of dark Gamma-Ray Burst host galaxy
Study of host galaxies of Long Gamma Ray Burst (GRB) is a key to understanding the origin of GRB. We focus on the metallicity environment of optically 'dark' GRBs, which is a missing population due to the faintness of their afterglows. We found that the host galaxy of GRB 070306 and 080325 are among the highest metallicity in current GRB host samples in contrast to the theoretical low-metallicity requirement. I will discuss the metallicity environment and origin of dark GRBs in comparison with the so-called fundamental metallicity relation of star-forming galaxies.
Speaker: Dr. Hsing-Wen Lin (National Central University)
The Neptune Trojans and the Vertical Moving Trans-Neptunian Objects
Many primitive bodies exist in the vast regions of the solar system beyond Jupiter, of which the largest population is the trans-Neptunian objects (TNOs). The details of the orbital distribution of the TNOs preserve information about the evolution of the solar system. In this talk, I will introduce our two recent works, which are related to TNO studies: 1. The Neptune Trojans: dynamically very hot or not that hot? and 2. The vertical moving TNOs: is there a perpendicular Kuiper Belt?
Currently 17 Neptune Trojans (NTs), the asteroids in co-orbital 1:1 mean motion resonance with Neptune, have been found. The very high and wide orbital inclination distribution of NTs implies an unusual formation mechanism of NTs; the NT have to be captured from a pre-excited planetesimal disk. Recently, we used Pan-STARRS 1 (PS1) survey data to study the orbital distribution of NTs and found that the inclination distribution is rather low compare with the results of previous studies. The new result may suggest that NTs can be captured from a planetesimal disk without pre-excited.
In the PS1 survey data, we also identified an unusual TNOs, 2011 KT13 (nicknamed Niku). This object has an 110 degree orbital inclination; it has a retrograde and almost vertical moving orbit. Together with other five high-inclination TNOs, the six vertical moving objects appear to occupy a common orbital plane. We found that this alignment is statistical significant; the probability occurring by chance is only 0.016%. Our simple simulations including the hypothetical Planet Nine fail to maintain the common orbital plane, therefore, an unknown mechanism is required to explain the observed clustering.