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中央研究院天文及天文物理研究所
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Activity > Colloquium

Colloquium (2018)

ASIAA Colloquium is usually held on Wednesdays at 2:20-3:20 pm in Room 1203 of the Astronomy-Mathematics Building, NTU. All scientists are welcome to attend.

The ASIAA-NTU joint colloquium series aims to bring to the physics/astronomy/cosmology community in ASIAA/NTU world renown researchers who will talk about the forefront development of physical sciences.

Contact: Colloquium Committee (talks_replace2@_asiaa.sinica.edu.tw)

NEXT Special Seminar: 2018-01-18 Thu 14:20~15:20 [R1203]
Speaker:Haifeng Yang
Topic:Origins of (sub)millimeter disk polarization
Abstract:Polarized (sub)millimeter emission from dust grains has long been established as a reliable way to probe magnetic fields in molecular clouds, based on the magnetic alignment of dust grains. At the same time, from theoretical studies, magnetic field is very important in the star formation and the evolution of protoplanetary disks, through either magnetorotational instability or magnetic-driven winds. The application of this method to the first resolved polarization observation in the protoplanetary disk HL Tau, however, yields rather unphysical magnetic field configuration and thus fails badly. This leaves an outstanding question: what are the origins of (sub)millimeter disk polarization? I will discuss the favored alternative mechanism, the “self-scattering” of dust grains, which successfully explains a lot of ALMA polarization observations to date. This brings both good news and bad news to the community. On the dark side, it’s harder to probe magnetic field with (sub)millimeter disk polarization, although we still found suggestive evidence in the IRAS 4A1 disk at long wavelength. On the bright side, polarization coming from “self-scattering” strongly depends on the properties of dust grains, including grain sizes, the optical depth, and thickness of dust disks. These allow us to study many important things, such as grain growth and dust settling.
NEXT Colloquium: 2018-01-23 Tue 14:20~15:20 [R1203]
Speaker:Olivia Jones
Topic:Dust Production from Evolved Stars in the Local Group
Abstract:Infrared observations of nearby galaxies and the Milky Way show that there are two main sources of ISM dust: the winds of evolved stars and supernovae ejecta. However, the total dust contribution from evolved stars relative to supernovae, and how it changes with metallicity, is less certain. Infrared photometric and spectroscopic Spitzer Surveys of the Large and Small Magellanic Clouds (LMC, SMC): Surveying the Agents of Galaxy Evolution (SAGE) resulted in the discovery of thousands of evolved stars. Here, I will describe how the composition and quantity of dust produced by these stars depends on metallicity. I will also discuss how the mid-IR stellar populations of the Magellanic Clouds can be used as a template for potential observations with JWST, and how we have applied this to our observing programs of Local Group galaxies and SN1987A with JWST.
No. Time/Place Speaker Topic / Abstract
download PDF: download talk PDF file
12018-05-09 Wed
14:20~15:20
R1203
Vivien Chen
[NTHU]
22018-04-25 Wed
14:20~15:20
R1203
Neal Katz
[UMass]
TBA
Abstract

TBA

32018-04-18 Wed
14:20~15:20
R1203
Dan Whalen
[ICG]
42018-04-11 Wed
14:20~15:20
R1203
Yuri Fujii
[Nagoya University]
52018-03-28 Wed
14:20~15:20
R1203
Fernando Olguin
[NTHU]
62018-03-21 Wed
14:20~15:20
R1203
Yukari Ohtani
[NAOJ]
72018-03-13 Tue
14:20~15:20
R1203
Kashiyama Kazumi
[Tokyo U.]
82018-03-12 Mon
14:20~15:20
R1203
Nario Kuno (Tsukuba Univ.)
[ASIAA]
*Special Seminar*
TBD
92018-03-07 Wed
14:20~15:20
R1203
Chiaki Hikage
[Kavli IPMU]
102018-03-01 Thu
14:20~15:20
R1203
Guey-Lin Lin
[NCTU]
112018-02-13 Tue
14:20~15:20
R1203
Chi-Ting Chiang
[Stony Brook University]
*Special Seminar*
Simulating structure formation in different environments and the application
Abstract

The observables of the large-scale structure such as galaxy number density generally depends on the density environment (of a few hundred Mpc). The dependence can traditionally be studied by performing gigantic cosmological N-body simulations and measuring the observables in different density environments. Alternatively, we perform the so-called "separate universe simulations", in which the effect of the environment is absorbed into the change of the cosmological parameters. For example, an overdense region is equivalent to a universe with positive curvature, hence the structure formation changes accordingly compared to the region without overdensity. In this talk, I will introduce the "separate universe mapping", and present how the power spectrum and halo mass function change in different density environments, which are equivalent to the squeezed bispectrum and the halo bias, respectively. I will then discuss the extension of this approach to inclusion of additional fluids such as massive neutrinos. This allows us to probe the novel scale-dependence of halo bias and squeezed bispectrum caused by different evolutions of the background overdensities of cold dark matter and the additional fluid. Finally, I will present one application of the separate universe simulations to predict the squeezed bispectrum formed by small-scale Lyman-alpha forest power spectrum and large-scale lensing convergence, and compare with the measurement from BOSS Lyman-alpha forest and Planck lensing map.

122018-02-12 Mon
14:20~15:20
R1203
Yasuhiro Hasegawa
[JPL]
*Special Seminar*
Planet Formation in Star-Forming Regions: from the Solar System to Other Worlds
Abstract

Planet formation is one of the fundamental ingredients in (exo)planetary sciences. Its importance is currently boosted up due to the recent significant progress of astronomical observations, theoretical modeling, and lab experiments. The most remarkable example may be the rapid accumulation of observed extrasolar planets and the characterization of their atmospheres. These studies are interesting in the sense that they can fill out a gap in research between the solar and extrasolar planetary systems. In this talk, I will present all the key results of my latest work. These include theoretical modeling of protoplanetary disks, out of which planetary systems are born, semi-analytical modeling of chondrule formation and the origin of asteroids, and a comprehensive analysis of solid accretion onto planets and its implications for the composition of observed exoplanets. In these studies, the outcome of the recent observations and lab experiments is utilized in order to derive new insights into the formation mechanisms of planets including the solar system. The combination of these attempts may enable us to move towards a comprehensive understanding of planet formation, covering a full size range from mm-sized, tiny dust grains to planetesimals and up to fully formed planets.

132018-02-05 Mon
14:20~15:20
R1203
Richard Crutcher
[University of Illinois]
*Special Seminar*
Magnetic Fields and Star Formation: Observations and Implications
Abstract

Stars are a fundamental unit of the universe, and their formation is a fundamental astrophysical process. The role of magnetic fields in star formation remains controversial today. I will discuss several molecular cloud and star formation theoretical ideas that have very different roles for magnetic fields and describe techniques for observing magnetic fields in regions of star formation. The talk will focus on the Zeeman effect, the only technique for directly observing magnetic field strengths. I will describe specific tests of vario Stars are a fundamental unit of the universe, and their formation is a fundamental astrophysical process. The role of magnetic fields in star formation remains controversial today. I will discuss several molecular cloud and star formation theoretical ideas that have very different roles for magnetic fields and describe techniques for observing magnetic fields in regions of star formation. The talk will focus on the Zeeman effect, the only technique for directly observing magnetic field strengths. I will describe specific tests of various theories of the role of magnetic fields in star formation and the application of observational results in those tests. Finally, I will describe briefly a star formation scenario that meets all of the observational tests, and mention future observational opportunities. us theories of the role of magnetic fields in star formation and the application of observational results in those tests. Finally, I will describe briefly a star formation scenario that meets all of the observational tests, and mention future observational opportunities.

142018-02-01 Thu
14:20~15:20
R1203
Nanda Kumar
[Universidade do Porto]
*Special Seminar*
High-mass star formation: new observational insights
Abstract

While the hunt for disks around high-mass stars continues in the ALMA era, other interesting predictions of high-mass star formation theories remain untested. Dense accretion flows leading to the formation of the highest mass stars can become Jeans unstable, producing a near-equal mass binary. This can explain the near-equal mass binarity well-known in field massive stars. I will show high-angular resolution observations (VLT and ALMA) of two such systems in formation. Young high-mass stars are also expected to be bloated, with radii up to 400Rsun, because of high internal entropy. I will present new infrared observations supporting this hypothesis. Finally, I will present a new systematic search and discovery of infrared variability in a large sample of young massive stars.

152018-01-31 Wed
14:20~15:20
R1203
James Wurster
[Exeter]
162018-01-23 Tue
14:20~15:20
R1203
Olivia Jones
[University of Edinburgh]
Dust Production from Evolved Stars in the Local Group
Abstract

Infrared observations of nearby galaxies and the Milky Way show that there are two main sources of ISM dust: the winds of evolved stars and supernovae ejecta. However, the total dust contribution from evolved stars relative to supernovae, and how it changes with metallicity, is less certain. Infrared photometric and spectroscopic Spitzer Surveys of the Large and Small Magellanic Clouds (LMC, SMC): Surveying the Agents of Galaxy Evolution (SAGE) resulted in the discovery of thousands of evolved stars. Here, I will describe how the composition and quantity of dust produced by these stars depends on metallicity. I will also discuss how the mid-IR stellar populations of the Magellanic Clouds can be used as a template for potential observations with JWST, and how we have applied this to our observing programs of Local Group galaxies and SN1987A with JWST.

172018-01-18 Thu
14:20~15:20
R1203
Haifeng Yang
[University of Virginia]
*Special Seminar*
Origins of (sub)millimeter disk polarization
Abstract

Polarized (sub)millimeter emission from dust grains has long been established as a reliable way to probe magnetic fields in molecular clouds, based on the magnetic alignment of dust grains. At the same time, from theoretical studies, magnetic field is very important in the star formation and the evolution of protoplanetary disks, through either magnetorotational instability or magnetic-driven winds. The application of this method to the first resolved polarization observation in the protoplanetary disk HL Tau, however, yields rather unphysical magnetic field configuration and thus fails badly. This leaves an outstanding question: what are the origins of (sub)millimeter disk polarization? I will discuss the favored alternative mechanism, the “self-scattering” of dust grains, which successfully explains a lot of ALMA polarization observations to date. This brings both good news and bad news to the community. On the dark side, it’s harder to probe magnetic field with (sub)millimeter disk polarization, although we still found suggestive evidence in the IRAS 4A1 disk at long wavelength. On the bright side, polarization coming from “self-scattering” strongly depends on the properties of dust grains, including grain sizes, the optical depth, and thickness of dust disks. These allow us to study many important things, such as grain growth and dust settling.

182018-01-17 Wed
14:20~15:20
R1203
Andreas Schulze
[NAOJ]
New constraints on the black hole spin in radio-loud quasars
Abstract

One of the major unsolved questions on the understanding of the AGN population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests radio-loud quasars to have higher black hole spin. However, the measurement of black hole spin remains extremely challenging. We here present results comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet SDSS quasars at 0.3 1.5 in the radio-loud sample, suggesting that the radio-loud quasar population has on average higher black hole spin that the radio-quiet population. This provides observational support for the black hole spin paradigm.

192018-01-12 Fri
14:20~15:20
R1203
Somnath Dutta
[S. N. Bose National Centre for Basic Sciences]
*Special Seminar*
Stellar population and star formation histories of star-forming regions
Abstract

The interaction between massive ionizing sources (O and early B-type) with their natal molecular environment plays a constructive role of new generation star formation via. various mechanisms e.g., Radiation Driven Implosion, collect & collapse process etc. The aim of this colloquium is to understand some aspects of interaction of high and low mass stars with their environment on the basis of observational evidences of massive ionizing sources, young stellar contents and molecular/ionized gas properties. The young T Tauri stars show various types of complex profiles on the infrared emission due to disk around them, strong emission lines on their spectra, strong flux variation due to spot signatures. The characterization of those phenomena could be directly addressed to the properties of these young stars, and also helps to pick up the young population in a young cluster against the field population. Thus, multi- wavelength studies of star-forming regions (SFRs) provide census of YSOs, their fundamental parameters e.g. masses, ages, effective temperatures, circumstellar disks around them etc. From such parameter space, broad pictures emerge on the young star-forming regions like star- formation history, star-formation efficiency, timescales etc. We studied the stellar contents and star formation activities of three distant Galactic SFRs (e.g. NGC 2282, Sh2-149 complex, Cygnus OB7) using deep optical, near-Infrared, mid-Infrared and radio continuum observations. We estimated the stellar content of NGC 2282, a young cluster in the Monoceros constellation, using deep optical BVI and IPHAS photometry along with IR data from UKIDSS and Spitzer- IRAC. With the aim of investigating star formation mechanism, we studied the Galactic H II region Sh2-149 and associated molecular clouds with the help of optical spectra, CFHT- WIRCAM, FCRAO 12CO(1-0), JCMT 13CO(3-2). Adiitionally, we performed CCD I-band time series photometry of a young cluster NGC 2282 and Lynds 1003 cloud in Cygnus OB7 to identify and characterize the variability of pre main-sequence stars.

202018-01-11 Thu
14:20~15:20
R1203
Upasana Das
[University of Colorado]
*Special Seminar*
Modeling Cosmic Extremes: Hypermagnetized White Dwarfs and Accretion Disks
Abstract

In this talk, I aim to establish the important role played by ultra-strong magnetic fields to explain diverse astrophysical phenomena. First, I will talk about my doctoral thesis work on hypermagnetized white dwarfs. Such white dwarfs have strong interior magnetic fields greater than 4.4 x 10^{13} G, which introduce quantum mechanical effects, and also provide additional pressure support against gravity, yielding highly super-Chandrasekhar white dwarfs in the range 1.7-3.4 Msun. I arrived at this conclusion by systematically progressing from a simple, spherically symmetric, Newtonian model to a rigorous, non-spherical, general relativistic model. Hypermagnetized white dwarfs have several important astrophysical implications, the most compelling being their role as the plausible progenitors of peculiar, highly over-luminous type Ia supernovae (SNeIa). These SNeIa violate conventional Chandrasekhar-mass explosion scenarios and seem to invoke white dwarf progenitors having mass greater than 2 Msun. Finally, I will talk about my current work on modeling the global properties of hypermagnetized accretion disks. Local magnetohydrodynamic simulations of accretion flows have shown that in the presence of a sufficiently strong but subthermal (plasma-\beta = Pgas/Pmag > 1) vertical magnetic flux, a large-scale, suprathermal toroidal field (plasma-\beta < 1) is generated, which becomes the dominant source of pressure support in the disk. I term such magnetically dominated disks as hypermagnetized accretion disks, which can resolve several observational shortcomings of the standard, geometrically thin, accretion disk model. I have performed a global, linear stability analysis of hypermagnetized accretion disks, whose results partly confirm the predictions of a local model. However, it also reveals important differences that highlight the necessity of a global treatment to accurately capture the geometric curvature terms, which are otherwise neglected in weak-field studies of accretion flows.

212018-01-10 Wed
14:20~15:20
R1203
Takaya Nozawa
[NAOJ]
Consensus and issues on dust formation in supernovae
Abstract

The role of core-collapse supernovae (SNe) as sources of cosmic dust is the key to understanding the dust enrichment history in the universe. Far-infrared and submillimeter observations with Herschel and ALMA have revealed the presence of cool dust above 0.1 Msun in the ejecta of SNe, which is in good agreement with masses of newly formed dust predicted by theoretical studies. However, there remain two unsettled issues: formation time and survivability of dust. Observationally, there seems a growing pieces of evidence that dust mass gradually increases with time over 20 years. However, we see from a simple argument that this is not easy to be realized. On the other hand, the survivability of dust against destruction by the reverse shock, which is critical to know the final amount of dust ejection form SNe, is also hard to be evaluated observationally. In particular, it depends on the initial size distribution of dust, which is not constrained very well from infrared observations. In this talk, I will introduce some recent approaches that address these issues, and attempt to discuss how we can tackle them from now on.

222018-01-09 Tue
14:20~15:20
R1203
Tom Broadhurst
[University of the Basque Country]
*Special Seminar*
Fundamental Physics from the First Detections of Cosmological Neutrinos, Axions and Gravitational Waves
Abstract

I will present the first direct evidence that the ubiquitous Dark Matter includes a small fraction of massive neutrinos (0.11eV summed over 3 flavours), with the rest composed of light axions (10^{-22}eV) in a Bose-Einstein state, motivated by String Theory. We have made the first cosmological simulations in this context, demonstrating the vast network of structure in the Universe is a beautiful interference pattern, with a standing solitonic wave at the center of every galaxy, representing the ground state. We have detected such a dark soliton in new detailed measurements of stars and gas at the center of our Galaxy. Furthermore, the LIGO motivated "primordial black hole" interpretation of the Dark Matter is excluded by our new Hubble Space Telescope discovery of individual stars detected through the huge intervening columns of dark matter in galaxy clusters. Instead, these gravitational wave sources can be better interpreted as cosmologically distant, lower mass colliding black holes of stellar origin, that are lensed by intervening galaxies.

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