Academia Sinica Institute of Astronomy & Astrophysics
中央研究院天文及天文物理研究所
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Activity > Colloquium

Colloquiums and Seminars(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. Seminars on more specialized topics are also held on a regular basis.

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 Colloquium: 2018-06-20 Wed 14:20~15:20 [R1203]
Speaker:Atsushi Taruya
Topic:Nonlinear structure formation in cold dark matter cosmology: shell crossing and multi-stream flows
Abstract:Invisible matter component called cold dark matter (CDM) is now widely accepted in cosmology as an essential building block of the cosmic structure formation. One important feature of CDM is that the phase-space velocity distribution was virtually null at very early time. This leads to, at later time, several interesting phenomena in nonlinear stage of gravitational dynamics: shell-crossing and multi-stream flow. In this talk, I will focus on these two phenomena, and discuss a way to describe quantitatively their phase-space structures, which is compared with state-of-the-art Vlasov-Poisson and cosmological N-body simulations.
No. Time/Place Speaker Topic / Abstract
download PDF: download talk PDF file
12018-12-12 Wed
14:20~15:20
R1203
Hirofumi Noda
[Tohoku University]
*Colloquium*
22018-12-05 Wed
14:20~15:20
R1203
Matthew Payne
[CfA]
*Colloquium*
32018-11-14 Wed
14:20~15:20
R1203
Min-Kai Lin
[ASIAA]
*Colloquium*
Dust dynamics in protoplanetary disks
42018-11-07 Wed
14:20~15:20
R1203
Alex Heger
[Monash]
*Colloquium*
52018-10-31 Wed
14:20~15:20
R1203
Leung, Shing Chi
[IPMU]
*Colloquium*
62018-10-16 Tue
14:20~15:20
R1203
Jordi Miralda-Escude
[University of Barcelona]
*Colloquium*
72018-10-03 Wed
14:20~15:20
R1203
Misao Sasaki
[IPMU/YITP/NTU]
*Colloquium*
82018-09-26 Wed
14:20~15:20
R1203
Oliver Just
[Riken]
*Colloquium*
92018-09-19 Wed
14:20~15:20
R1203
Kuo-Chuan Pan
[NTHU]
*Colloquium*
102018-09-12 Wed
14:20~15:20
R1203
Suzanne Madden
[CEA/Saclay]
*Colloquium*
Dwarf galaxies, their ISM and star formation properties
112018-08-29 Wed
14:20~15:20
R1203
Hirai, Yutaka
[RIKEN]
*Colloquium*
122018-08-15 Wed
14:20~15:20
R1203
Mike Alexandersen
[ASIAA]
*Colloquium*
132018-08-08 Wed
14:20~15:20
R1203
Hsien Shang
[ASIAA]
*Colloquium*
142018-08-01 Wed
14:20~15:20
R1203
Rob Wittenmyer
[University of Southern Queensland]
*Colloquium*
152018-07-26 Thu
14:20~15:20
R1203
Tom Broadhurst
[University of the Basque Country]
*Seminar*
162018-07-25 Wed
14:20~15:20
R1203
Yongzhong Qian
[University of Minnesota]
*Colloquium*
Our Solar System: What Triggered Its Formation?
172018-07-23 Mon
14:20~15:20
R1203
Matthieu Gounelle
[Muséum National d’Histoire Naturelle]
*Colloquium*
182018-07-12 Thu
14:20~15:20
R1203
Takuji Yamashita
[Ehime University]
*Seminar*
192018-07-11 Wed
14:20~15:20
R1203
Wen-Pin Hsieh
[ASIES]
*Colloquium*
202018-07-04 Wed
14:20~15:20
R1203
Marco Antonio Munoz Gutierrez
[ASIAA]
*Colloquium*
212018-06-27 Wed
14:20~15:20
R1203
Luis Zapata
[UNAM]
*Colloquium*
The ALMA view of the explosive outflow in Orion KL: Relics from a violent protostellar merger?
Abstract

In this talk, I will show the recent results obtained in deep ALMA observations of the explosive outflow located in the heart of the Orion Nebula, the Orion Kleinmann-Low Nebula (Orion KL). These observations revealed over a hundred arcsecond wide and tens of arcseconds long high-velocity 12CO (J=2−1) streamers that approximately point to a central region where a young stellar massive system disintegrated about 500 yrs. The kinematics and morphology of the molecular streamers confirmed the explosive nature of the outflow in Orion KL. The energetics of the explosive outflow require the formation of a binary with an AU-scale or smaller semi-major axis. This event may have led to stellar merger which powered the explosion in the gas. Finally, I will show the latest efforts to reveal more cases where possible mergers events could led explosive outflows like the one in Orion KL.

222018-06-20 Wed
14:20~15:20
R1203
Atsushi Taruya
[YITP, Kyoto University]
*Colloquium*
Nonlinear structure formation in cold dark matter cosmology: shell crossing and multi-stream flows
Abstract

Invisible matter component called cold dark matter (CDM) is now widely accepted in cosmology as an essential building block of the cosmic structure formation. One important feature of CDM is that the phase-space velocity distribution was virtually null at very early time. This leads to, at later time, several interesting phenomena in nonlinear stage of gravitational dynamics: shell-crossing and multi-stream flow. In this talk, I will focus on these two phenomena, and discuss a way to describe quantitatively their phase-space structures, which is compared with state-of-the-art Vlasov-Poisson and cosmological N-body simulations.

232018-06-19 Tue
14:20~15:20
CCMS R104
Atsushi Taruya
[YITP, Kyoto University]
*ASIAA/NTU Joint Colloquium*
Relativistic effects on observed large-scale structure of the Universe
Abstract

The large-scale structure of the Universe observed via galaxy redshift surveys contains valuable cosmological information, and it has been playing a major role to improve our understanding of the Universe. However, ​the observed large-scale structure ​​appears distorted due to the observed ​​systematics in ​​the redshift determination, known as redshift-space distortions (RSD). Recently, the measurement of RSD is renewed with great interest as a probe of gravity on cosmological scales. In this talk, after reviewing the 'standard' RSD caused by the peculiar velocity of galaxies, I w​ill ​discuss yet another distortion arising from general relativistic effects. ​U​​nique feature of the​​ n​ew ​distortion e​ffects ​is demonstrated b​​​​ased on the simulated catalog taking a​ proper ​account of the relativistic effects,​ ​and detectability and implications to cosmology will be also discussed.

242018-06-13 Wed
14:20~15:20
R1203
Shingo Hirano
[Kyushu University]
*Colloquium*
Formation of first stars with ten to one million solar-masses
Abstract

First stars (Population III stars) play vital roles in the early cosmic evolution by initiating cosmic reionization and chemical enrichment of the intergalactic medium. The characteristic mass of first stars and its initial mass function are thus essential to understand the formation and evolution of the first galaxy, one of the important observational targets by the next-generation telescopes. The numerous theoretical/numerical works suggest various paths of first star formation and final masses depending on the star-forming environment properties. A large cosmological sample of first star formation shows a wide range of stellar masses from 10 to 1000 Msun depending on the properties of the star-forming gas cloud. Under a quite limited condition, very massive first stars can form with 1e5-1e6 Msun and leave a promising seed for the formation of observed high-z quasars. Furthermore, a recent study shows the formation of the first star clusters and massive star binaries. The eventual formation of the remnant black holes will leave a close binary of massive black holes, which can be a progenitor of strong gravitational wave sources. This talk will introduce these latest results and prospects on the formation theory of first stars.

252018-06-06 Wed
14:20~15:20
R1203
Vivien Chen
[NTHU]
*Colloquium*
Filamentary Accretion Flows in the IRDC M17 SWex
Abstract

Although filamentary structures are ubiquitous in molecular clouds, basic observational constraints are needed to clarify the role of filaments in the mass assembly process. Using ALMA Band 3, we have performed full-synthesis imaging of the N2H+ (1-0) emission in the remarkable IRDC complexes, M17 SWex, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. We derive gas kinematics by fitting the hyperfine components of N2H+ spectra. The mass accretion rates are in the range of 10^-5 to 10^-4 Msun/yr. The line widths suggest a transonic nature of dense gas in the filaments. A comparison study of viral parameters from filaments, clumps, to cores is made. The observation results all together are consistent with the hierarchical gravitational collapse scenario.

262018-05-23 Wed
14:20~15:20
R1203
Jonathan Marshall
[ASIAA]
*Colloquium*
Debris dust as a tracer of planetary system architectures
Abstract

Debris discs are tenuous rings of rock and ice around main sequence stars left over from planet formation processes. Their presence, most commonly inferred through the detection of excess emission at infrared wavelengths, is a signpost for the presence of a planetary system around the host star. At millimetre wavelengths, the dust emission is believed to closely trace the unseen belt of dust-producing planetesimals (asteroids and comets). Resolved imaging of debris discs at millimetre wavelengths reveals structures, e.g. azimuthal asymmetries, that can be linked to perturbation of the planetesimal belt by an unseen companion. Here I will present modelling of a sample of debris discs that have been observed at infrared and millimetre wavelengths in order to reveal the size scales of these systems as a function of the host stars' luminosities, and then used that as a ruler to interpret the emission from unresolved systems in order to identify potentially perturbed systems.

272018-05-16 Wed
11:00~12:00
R1203
Roger Chevalier
[University of Virginia]
*Seminar*
Supernova Remnant in Molecular Clouds
Abstract

A class of supernova remnants show evidence for interaction with molecular clouds. A first approximation to a molecular cloud is a low density medium with embedded clumps. If the shock wave becomes radiative in the lower density medium and the resulting dense shell interacts with clumps, high pressures can be produced, as are observed. These remnants are observed to be sources of high energy gamma-ray emission, both at GeV (with Fermi) and TeV (with Cherenkov telescopes) energies. The emission appears to be associated with slow shock waves moving into clumps. The particles being accelerated are likely to be from the preshock population of cosmic rays, as opposed to injected from the thermal particles. Simulations of cloud interactions allowing for the cloud structure resulting from supersonic turbulence are underway. Observed remnants show some evidence for features resulting from such an interaction.

282018-05-16 Wed
14:20~15:20
R1203
Gabriel Torrealba
[ASIAA]
*Colloquium*
Understanding Dark Matter through Ultra-Faint dwarf galaxies
Abstract

The standard model of cosmology states that the dark matter present in the Universe consists of particles that mainly interact gravitationally. This model is extremely successful in explaining the large scale structure: from the Cosmic Microwave Background to how galaxies distribute in the Universe. In this scenario, the dark matter dominated formation of structure is hierarchical: small structure form first, and then acts as building blocks to form bigger structure. One of the key predictions of this scenario, or any scenario where formation of structure is hierarchical, is the presence of hundreds to thousands of small dark matter halos orbiting galaxies like the Milky Way. However, due to their dark nature and small scales, these have proven difficult to find. Nevertheless, while most of the smallest of the dark halos will remain dark forever, some will be lit by small and dim galaxies, providing a window of opportunity to probe the nature of dark matter and the formation of structure at the smallest scales. In this contribution, I will present some results of a new quest for the search of the dark halos around the Milky Way, via the detection of small satellite galaxies. By improving the detection algorithms and by the use of new deep wide surveys, I have pushed the detection limits yielding several new discoveries, including the discovery of the enigmatic galaxy Crater 2. Even when being the fourth largest in the vicinity of the Milky Way, Crater 2 remained hidden until very recently due to its extreme properties. These same properties may be in tension with our current models of the Universe posing interesting new challenges to the standard model of cosmology.

292018-05-15 Tue
14:20~15:20
CCMS R104
Roger A. Chevalie
[Univ. ov Virginia]
*ASIAA/NTU Joint Colloquium*
Superluminous supernovae
Abstract

Unbiased supernova searches led to the recognition of the class of superluminous supernovae (SLSN) about one decade ago. These events can be a factor 100 more luminous than ordinary supernovae. Some SLSNe show narrow hydrogen lines and are likely powered by interaction with a dense circumstellar medium. In one case, the progenitor star lost several solar masses of gas in the 30 years leading to the explosion; the reason for the extreme mass loss is not understood. Other SLSNe do not show narrow lines, but show broad lines and are stripped of hydrogen. These features are difficult to explain in an interaction scenario. Another possibility is radioactive power, but the required mass of 56Ni can be larger than allowed by the light curve and the expectations of 56Ni synthesis. Power from a magnetar (highly magnetized pulsar) with a millisecond period can roughly explain light curves and spectra.

302018-05-14 Mon
14:20~15:20
R1203
Ken Osato
[Univ. of Tokyo]
*Seminar*
Strong orientation dependence of surface mass density profiles of dark haloes at large scales
Abstract

We study the dependence of surface mass density profiles, which can be directly measured by weak gravitational lensing, on the orientation of haloes with respect to the line-of-sight direction, using a suite of N-body simulations. We find that, when major axes of haloes are aligned with the line-of-sight direction, surface mass density profiles have higher amplitudes than those averaged over all halo orientations, over all scales from 0.1 to 100 Mpc/h we studied. While the orientation dependence at small scales is ascribed to the halo triaxiality, our results indicate even stronger orientation dependence in the so-called two-halo regime, up to 100 Mpc/h. The orientation dependence for the two-halo term is well approximated by a multiplicative shift of the amplitude and therefore a shift in the halo bias parameter value. The halo bias from the two-halo term can be overestimated or underestimated by up to ~ 30% depending on the viewing angle, which translates into the bias in estimated halo masses by up to a factor of two from halo bias measurements. The orientation dependence at large scales originates from the anisotropic halo-matter correlation function, which has an elliptical shape with the axis ratio of ~ 0.55 up to 100 Mpc/h. We discuss potential impacts of halo orientation bias on other observables such as optically selected cluster samples and a clustering analysis of large-scale structure tracers.

312018-05-12 Sat
14:00~15:00
0109 Auditorium
Gordon Baym
[UIUC]
*Colloquium*
Neutron stars: from hadrons to quarks
Abstract

Our understanding of neutron stars is advancing remarkably. A new windows into neutron star behavior and structure has been the recent direct detection of gravitational and contemporaneous electromagnetic radiation from merging neutron stars. The identification of several two solar mass neutron stars challenges our current understanding of how dense matter can be sufficiently stiff to support such a mass against gravitational collapse. Programs underway to determine simultaneously the mass and radius of neutron stars constrain and inform theories of neutron star interiors. At the same time, an emerging understanding in quantum chromodynamics (QCD) of how nuclear matter can evolve into deconfined quark matter at high baryon densities is leading to advances in understanding the equation of state of the matter under the extreme conditions in neutron star interiors. This talk will review these recent developments and describe the consistent picture of neutron star interiors that is coming into focus.

322018-05-09 Wed
14:20~15:20
R1203
Miikka Väisälä
[ASIAA]
*Colloquium*
Exploring the observational implications of supernova-driven turbulence with radiative transfer
Abstract

A sizable work has been performed by examining the magnetic dynamo process of supernova driven turbulence using kiloparsec scale MHD simulations. These simulations were able to generate a self-consistent magnetic field structure for the diffuse interstellar medium. In the recent work we have been able find a way of comparing these supernova results with the Planck large-scale polarization observations by using the radiative transfer tool SOC. This was achieved by putting the observational point of view inside the kiloparsec scale computational volume, creating a set synthetic all-sky maps from the MHD model. From this these maps, we computed their polarization fractions and polarization angle dispersion functions, averaged them into combined 2D-histograms, which were used for comparison with the Planck results. We found that our SN-turbulence simulation is able to produce comparable behavior to Planck, but is limited by its spatial resolution and horizontal extent. We also note that polarization does not directly correlate in any way with the supernova shocks. Instead, they are connected with the small-scale magnetic fluctuations generated by the SN-driven dynamo.

332018-05-02 Wed
14:20~15:20
R1203
Wesley Fraser
[Queen's University, Belfast]
*Colloquium*
Icy compositions, explosions, and the Grand Smack
Abstract

It is well accepted that the giant planets originated in a more compact, stable, and multi-resonant configuration due to interactions with the gas in the protoplanetary disk. Following the dispersal of the gas, the planets migrated to their current locations through 1) interactions with the planetesimal disk and 2) a dynamical instability during which two or more of the planets experienced mutual close encounters. The latter are necessary to account for the current orbital configuration of the planets. The exact dynamical mechanism which triggered the instability between the giants remains an open question. The currently favoured mechanism is the resonant crossing of one of the ice giants and Saturn. In this talk, I will introduce the state of the art in our understanding of this dynamical scenario. I will highlight successful aspects of the model. Namely, how it does extremely well in reproducing the orbital structure of the Solar System, and in particular, the Kuiper Belt. I will discuss how dynamicists are progressing from using just the observed dynamical structures of the Kuiper Belt, but are now including compositional constraints in their models. In particular, I will focus on the Colours of the Outer Solar System Origins survey, or Col-OSSOS, and what Col-OSSOS revealed about the migrations of the gas-giants, and the compositional structure of the protoplanetary disc itself. Despite accounting for many of the properties of the current Solar System, the favoured dynamical scenario has two problematic requirements. Firstly, it demands an extremely compact initial resonant configuration. Secondly, it invokes the presence of a 5th gas-giant in the early Solar System, along with the implicit condition that the 5th planet is ejected! In the last part of my talk, I will present a new idea, the Grand Smack, in which the instability amongst the gas-giants is triggered by a large impact onto one of the 4 planets. Specifically, we demonstrate that the impact responsible for producing Uranus’s large obliquity is also sufficient to cause the system to go dynamically unstable. Our model can broadly account for all the major features of the Solar System’s dynamical architecture, including the relative locations of the gas-giants and their orbital excitations, the formation of the Kuiper Belt, and the preservation of the initial low dynamical excitation of the asteroid belt and the terrestrial planets. Critically, compared to models that invoke other instability mechanisms, a broader range of initial orbital configurations is able to reproduce the known Solar System. Our model’s strength is that it explains the overall architecture of the Solar System as the result of an event we know has happened - the tilting impact on to Uranus, rather than invoking the presence of a 5th planet.

342018-04-25 Wed
14:20~15:20
R1203
Neal Katz
[UMass]
*Colloquium*
Letting the Data Speak for Themselves: What Observations Tell Us About Galaxy Formation
Abstract

We infer the star formation rates in dark matter halos at different redshifts from halo merger histories expected in a Lambda CDM cosmology using Bayesian inference and Bayes ratios to restrict the model complexity, constrained to match the observed stellar mass/luminosity functions of galaxies at different redshifts and the local cluster galaxy luminosity function, which has a steeper faint end than that of field galaxies. The only other assumptions that we make are that the star formation rate of central galaxies depends on the halo mass and redshift and that when a galaxy becomes a satellite its star formation rate is quenched exponentially and it can eventually merge with the central galaxy on a dynamical friction timescale. We find that 1) the star formation in the central galaxies of high mass halos (>10e12) has to be boosted at high redshift beyond what is expected from a simple scaling of the dynamical time; 2) below z=2 the star formation in halos below 1e11 must be quenched, which is not directly expected in standard stellar feedback models and is most easily explained by some form of preheating, and implies that there is a significant old stellar population in present-day dwarf galaxies with M_star < 1e8 and steep slopes for the high redshift stellar mass and star formation rate functions 3) the stellar mass of galaxies assembles in one of three ways depending on halo mass: > 1e12 the galaxies assemble through mergers and should hence have a spheroidal morphology and between 1e11 and 1e12 (e.g MW) it assembles slowly and at z>2 has less than 5% of its mass in place, which has extreme observational consequences.

352018-04-19 Thu
11:00~12:00
R1203
Dan Whalen
[ICG]
*Seminar*
The First Supernovae in the Universe
Abstract

Primordial stars formed about 200 Myr after the Big Bang, ending the cosmic dark ages. They were the first great nucleosynthetic engines of the universe and may be the origins of the supermassive black holes found in most massive galaxies today. In spite of their importance to the evolution of the early universe not much is known for certain about the properties of Pop III stars. But with the advent of JWST, WFIRST and the 30 m telescopes it may soon be possible to directly observe their supernovae in the NIR and thus unambiguously constrain the properties of the first stars. I will present radiation hydrodynamical calculations of the light curves of the first SNe in the universe and discuss strategies for their detection. I will also describe how some may already have been captured in surveys of galaxy cluster lenses such as CLASH, Frontier Fields and GLASS.

362018-04-18 Wed
14:20~15:20
R1203
Daniel Whalen
[ICG]
*Colloquium*
The Formation of the First Quasars in the Universe
Abstract

Supermassive black holes (SMBHs) are found at the centers of most massive galaxies today. But over 120 quasars have now been discovered at z > 6, including ULAS J1342, an 800 million solar mass BH at z = 7.5, and ULAS J1120, a two billion solar mass BH at z = 7.1. We have developed new radiation hydrodynamical simulations of SMBH evolution in cosmological environments with the Enzo AMR cosmology code in which X-rays from the BH are fully coupled to nonequilibrium primordial gas chemistry and hydrodynamics. These models include ionizing UV radiation, winds and SNe due to star formation in the host galaxy, all of which are known to regulate the growth of the BH and the rise of the stellar populations themselves. Our models reproduce for the first time all the observed properties of J1120: BH mass, radius of its H II region, and the star formation rate, metallicity and dynamical mass of the central 1.5 kpc of its host galaxy. I will discuss these models as well as detection limits for the first quasars in the NIR and 21 cm as a function of redshift in connection with future AGN surveys at 5 < z < 15 by Euclid, JWST, the E-ELT and SKA.

372018-04-17 Tue
14:20~15:20
R1203
Tilman Hartwig
[IPMU]
*Seminar*
Stellar Archaeology as a Time Machine to the First Stars
Abstract

Understanding the nature of the first stars is a major challenge of modern cosmology. Despite their importance for the formation of subsequent stars and galaxies, their mass distribution is not well constrained due to a lack of direct observations. Extremely metal poor stars in the Milky Way allow to constrain the mass of their progenitor supernovae and thereby provide precious information about the first generation of stars ("Pop III"). I will present a new diagnostic to reliably distinguish mono-enriched from multi-enriched metal-poor stars, based on the individual and combined chemical yields of Pop III supernovae. Milky Way satellites are the most promising regions to find second generation stars that were enriched by only one previous supernovae. We apply our new diagnostic to recently observed stars from the TOPoS survey and I will show that our results help to optimise upcoming Galactic archaeology surveys to detect more of these precious stellar fossils, which allow to better constrain the mass function of the first stars.

382018-04-11 Wed
14:20~15:20
R1203
Yuri Fujii
[Nagoya University]
*Colloquium*
Formation of Regular Moons in Weakly Accreting Circumplanetary Disks
Abstract

During the formation phase of gas giants, circumplanetary gaseous disk form around the planets. Circumplanetary disks are important not only for mass supply to gas giants but also for formation of regular satellites. Because of the comparatively small size-scale of the sub-disk, quick magnetic diffusion prevents the magnetorotational instability (MRI) from being well-developed at ionization levels that would allow MRI in the parent protoplanetary disk. In the absence of significant angular momentum transport, continuous mass supply from the parental protoplanetary disk leads to the formation of a massive circumplanetary disk. We have developed an evolutionary model for this scenario and have estimated the orbital evolution of satellites within the disk. In a certain temperature range, we find that inward migration of a satellite can be stopped by a disk structure due to the opacity transitions. We also find that the second and third migrating satellites can be captured in mean motion resonances. In this way, a compact system in Laplace resonance, which are similar to inner three bodies of Galilean satellites, can be formed in our disk models.

392018-03-29 Thu
13:20~14:20
R1203
Russ Taylor
[IDIA]
*Seminar*
SKA Developments in South Africa
Abstract

In 2012 the international SKA Organisation decided to site the SKA-mid frequency telescope in Africa, hosted by an eight-country African partnership. SKA-mid will be an array of thousands of parabolic antennas spread out over baselines of thousands of kilometres. As a first step toward SKA-mid, South Africa is constructing MeerKAT, a 64-element array of 13.5-m offset parabolic antennas. MeerKAT is a precursor of the SKA mid-frequency dish array, and following several years of operation as a South African telescope will be incorporated into the SKA phase 1 facility. Construction of MeerKAT is well advanced at the African SKA central site on the South African Karoo plateau. The array is scheduled to be operational in mid 2018. The MeerKAT science program will consist of key-science, legacy-style, Large Survey Projects, plus open time available for new proposals. The Large Survey Projects are direct pathfinder to key science programs being planned for the SKA. I will present an update on the MeerKAT progress and specifications, the key science programs. The completion of MeerKAT foreshadows one of the most significant data challenges of the coming decade and the beginning of an era of big data in African astronomy. The new South African Inter-University Institute for Data Intensive Astronomy has been set up to build capacity to meet the data challenge of the SKA in Africa.

402018-03-28 Wed
14:20~15:20
R1203
Fernando Olguin
[NTHU]
*Colloquium*
Determining the physical properties of the massive YSO AFGL 2591
Abstract

The distribution of matter around massive proto-stars tells us which are the physical processes dominating the formation of massive stars. These physical processes can in turn help us to determine whether massive stars are formed as an upscale version of their low-mass counterparts. We can limit the effects of some physical by studying massive young stellar objects (MYSOs) which are where the ionising nature of the radiation from the massive star has not started to change the circumstellar environment. One of the most observed MYSOs is AFGL 2591, which has similar properties to those found in other MYSOs (e.g. jets/outflows) and it is relatively isolated. In this presentation, I will show the latest results of our multi-wavelength dust continuum study of the density and temperature structure of this source and molecular line emission study of its envelope kinematics. In particular, I will show how our radiative transfer modelling explains resolved Herschel 70 micron data, and evidence of rotation in the inner envelope as mapped by methyl cyanide emission.

412018-03-21 Wed
14:20~15:20
R1203
Yukari Ohtani
[NAOJ]
*Colloquium*
X-ray emission of shock breakout in a wind
Abstract

Supernova shock breakout is an electromagnetic phenomenon in which observable features are highly sensitive to the motion and the shape of the shock front. Therefore it would be important to study the emission properties for obtaining information on the mechanisms of a stellar explosion and stellar evolution shortly before the explosion. The previous studies suggest that the shape of the observed light curve of XRO 080109/SN 2008D implies the asphericity of shock propagation (Suzuki & Shigeyama 2010a, Couch et al. 2011). Furthermore, the power-law feature of the observed X-ray spectrum can be reproduced by bulk-Compton scattering in the shock with a radial velocity of > 30% of the speed of light (Suzuki & Shigeyama 2010b). Though the studies have revealed the importance of the observational properties, there have been no studies which reproduce the shapes of the observed light curve and spectrum with a model, taking bulk-Compton scattering into account. In this study, we investigate the relationship between the properties of the emission and the motion of the shock, by using a toy model in which ejecta works as a piston in a spherically symmetric CSM. By using a Monte-Carlo method, we found that both the observed light curve and spectrum of XRO 080109 can be reproduced by a shock with a velocity of 60% of the speed of light and a circumstellar matter with a mass loss rate of 5.e-4 solar mass per year.

422018-03-15 Thu
11:00~12:00
R1203
Hsi-Yu Schive
[UIUC]
*Seminar*
Wave Dark Matter Predictions from GPU-accelerated Adaptive Mesh Refinement Simulations
Abstract

The conventional particle interpretation of cold dark matter (CDM) still lacks laboratory support and struggles to explain the basic properties of dwarf galaxies. This tension motivates wave dark matter (ψDM) composed of extremely light bosons (m ψ ~10^(-22) eV), which suppresses structure below the kpc scale by the uncertainty principle but retains the large-scale structure predicted by CDM. In the first part of this talk, I will present the first cosmological ψDM simulations that achieve an unprecedented high resolution capable of resolving dwarf galaxies. These simulations reveal that every ψDM halo has a prominent soliton core surrounded by fluctuating density granules. These predictions compare favorably with the observations of galaxy formation, the Lyman-alpha forest and reionization, and also help explain gravitational lensing flux anomalies. The second part of the talk focuses on GAMER, a GPU-accelerated adaptive mesh refinement (AMR) code. A rich set of physics modules is incorporated and which outperforms other widely-adopted AMR codes by one to two orders of magnitude. The code scales well to thousands of GPUs and achieves a uniform resolution as high as 10,240^3 cells. I will present several ongoing astrophysical projects with GAMER that require substantially higher resolution than previously feasible, including turbulence cascade in galaxy cluster mergers, star formation in isolated disk galaxies, supermassive black hole accretion, and ψDM simulations.

432018-03-13 Tue
14:20~15:20
R1203
Kashiyama Kazumi
[Tokyo U.]
*Colloquium*
Fast radio bursts and the young neutron star model
Abstract

A growing number of fast radio bursts (FRB) has been detected but the source is still unknown. One of the leading models is the young neutron star model, in which a pulsar (or magnetar) of < 100 yrs-old surrounded by a wind nebula and supernova remnant are considered and connections between FRBs and luminous pulsar-driven supernovae are predicted. I will discuss multi-messenger/multi-wavelength approaches for testing the scenario.

442018-03-12 Mon
11:00~12:00
R1203
Andrew Cooper
[University of Durham]
*Seminar*
Galactic Archaeology: Simulations and Surveys
Abstract

Many of the facilities poised to dominate optical and infra-red astronomy over the next 20 years have put galactic archaeology (in a nutshell, high-redshift science with low-redshift data) at the core of their scientific programmes, including Gaia, PFS, HSC, LSST, MOONS, JWST, WFIRST, ELT, MSE and TMT. They promise surveys of galactic stellar halos, tidal streams and dwarf satellites with ultra-deep photometry (exemplified by outstanding deep imaging from HSC) and fine-grained spectroscopy of stars in the Local Group (the domain of PFS). These data will provide stringent new probes of the evolving relationship between star formation and the growth of cosmic structure at the heart of the LCDM model, but only if we can successfully combine Milky Way and extragalactic data in a sufficiently detailed theoretical framework, integrated with results from other wavelengths and epochs. In this talk I’ll outline my work to develop efficient cosmological simulations (called STINGS) that bridge the gap between all these exciting observations and the big picture questions of galaxy formation theory. I’ll show quantitative insights these simulations give into how the hierarchical disruption of satellites contributes to galactic structure across scales ranging from the Milky Way to the most massive clusters, and highlight some of the pressing problems and priorities for this field. I’ll also give an update on the DESI Milky Way Survey, an ambitious wide-area spectroscopy programme targeting our own Galaxy’s stellar halo that will be highly complementary to the Galactic Archaeology goals of PFS.

452018-03-07 Wed
11:00~12:00
R1203
Kenneth Wong
[NAOJ]
*Seminar*
Studying the Dark Universe with Gravitational Lenses
Abstract

Strong gravitational lensing is a powerful probe of the mass distribution in the Universe. Lensing is sensitive to the total mass distribution along the line of sight, making it a unique probe of dark matter in lensing galaxies, and useful for studying resolved properties of the magnified background sources. I briefly discuss applications of lensing magnification to recent ALMA observations of the lens SDP.9. Strong lensing is also valuable for cosmology through lensed quasars, which can be monitored to measure the "time delay" between the multiple images. This can be used to measure the "time-delay distance", which is primarily sensitive to the Hubble constant (H0). This method of measuring H0 is independent of type Ia supernovae and CMB observations, and may shed light on the growing H0 discrepancy between local universe and CMB measurements. I discuss the H0 Lenses In COSMOGRAIL’s Wellspring (H0LiCOW) project, which has measured H0 to ~3.8% precision for a flat Lambda CDM cosmology from three time-delay lenses. Our results are in moderate tension with the latest Planck results for a similar cosmology, hinting at possible new physics beyond the standard LCDM model and highlighting the importance of this independent probe. To improve this measurement, as well as leverage the power of strong lensing to study galaxy structure and magnified background sources, we require deep wide-area imaging surveys to build up a statistical sample of lenses, which are quite rare. The Hyper-Suprime Cam survey is an ongoing multiband imaging survey using the Subaru Telescope that will cover 1400 deg^2 of the sky to a depth of r~26. I present the current work of the HSC SSP strong lensing working group, which is focused on searching for new lenses and using these systems for studies of galaxy structure and cosmology. The search methods and science cases being developed now for surveys such as the HSC SSP are necessary to prepare for the upcoming revolution in strong lensing from LSST and Euclid, which will discover orders of magnitude more lenses than are currently known.

462018-03-07 Wed
14:20~15:20
R1203
Chiaki Hikage
[Kavli IPMU]
*Colloquium*
Cosmology with Hyper-Suprime Cam cosmic shear analysis
Abstract

The coherent pattern of distant galaxy images distorted by gravitational lensing of large-scale structure, i.e., cosmic shear, is a powerful probe of matter distributions in the Universe. Cosmic shear is a unique cosmological probe and is especially sensitive to the combination of the matter density parameter Ωm and the amplitude parameter of matter fluctuations σ8, i.e., S8(α) ≡ σ8(Ωm/0.3)^α with α ∼ 0.5. Recent cosmic shear measurements such as DES and KiDS prefers 2-3σ lower value of S8 than Planck CMB data in ΛCDM model. The tension may indicate some physics beyond ΛCDM model such as dynamical dark energy or modified gravity. Hyper-Suprime Cam (HSC) Subaru Strategic Program is a wide-field imaging surveys using a gigantic imaging camera on the 8.2-meter Subaru telescope. The unique property of the HSC is the combination of its depth (i~26 at 5σ) and excellent image quality (i-band seeing∼0.58”), which enables us to measure cosmic shear signals up to higher redshifts with lower shape noises than KiDS and DES. We measure cosmic shear power spectra with HSC survey first-year shear catalog covering 137 sq. deg. of the sky. The high effective number density of 16.5 per sq. arcmin after the photometric redshift cut of 0.3 ≤ z ≤ 1.5 allows a high significance measurement of cosmic shear signals with the total signal-to-noise ratio of 16 in the multipole range 300 ≤ l ≤ 1900. I present the current status of the HSC cosmic shear analysis.

472018-03-05 Mon
11:00~12:00
R1203
Takashi Okamoto
[Hokkaido University]
*Seminar*
Simulations of galaxy formation --The roles of feedback
Abstract

Cosmological hydrodynamic simulations are a powerful tool for studying non-linear formation processes of galaxies. Studies using cosmological galaxy formation simulations have revealed that "feedback", which is the process that gives energy and/or momentum to gas from stars and active galactic nuclei, plays crucial roles in galaxy formation. I explain how different feedback processes operate on different mass scales of galaxies. The simulations are also useful for observers as a tool to plan future observations. I briefly introduce how we collaborated with observers for ALMA observations of high-redshift emission line galaxies. Finally, I show some problems that current simulations have.

482018-03-02 Fri
14:20~15:20
R1203
Taiki Ogihara
[Tohoku University]
*Seminar*
Triple-ridge structure of relativistic jets driven by black holes
Abstract

The driving and mass injection mechanisms of Active Galactic Nucleus jets are long-standing problems. M87 is the galaxy that has the second largest angular-size black hole and a relativistic jet, which allows us to test the theoretical models of these mechanisms. The edge brightened structure has been observed with VLBI in the downstream at least up to ~10^4 Rg from the black hole. This structure can be explained by the synchrotron emission from the steady axisymmetric electromagnetically-dominated jet (Takahashi et al. 2018). They reveal that the magnetic field lines must penetrate the black hole, not the accretion disk, and the black hole must rotate rapidly. Recent observations have shown a new feature, fork-like triple-ridge structure (Asada, Nakamura & Pu 2016; Hada 2017). We show that the above model with different electron density distributions can produce the triple-ridge structure. Such study will constrain the electron injection mechanism and will be required to improve the emission models around the black hole which would be observed by the Event Horizon Telescope.

492018-03-01 Thu
11:00~12:00
R1203
Ya-Wen Tang
[ASIAA]
*Seminar*
From filaments to circumstellar disks: on the star and planet formation at parsec and 100s AU scales
Abstract

The details of what physical processes between turbulence, gravity, or magnetic field regulate the mass transfer from the large scales (several parsecs) of clouds and filaments onto the small scales of clumps and cores is still highly debated. The origin of such a debate resides in our (past) inability to track the magnetic (B) field properties over such a large range of scales and densities that are involved in the star formation process. In this talk, I will report the pioneer study of the correlation of the B field and velocity gradient, and the interplay between turbulence, gravity and B field within star forming filaments G34.43. Different fragmentations within clumps are compared. For the second part of the talk, I will present the approaches to look for embedded planets within disks using the indirect evidences obtained from SUBARU, VLT, ALMA and future JWST.

502018-03-01 Thu
14:20~15:20
R1203
Guey-Lin Lin
[NCTU]
*Colloquium*
Astrophysical neutrinos and new physics
Abstract

Through many years of data taking, the IceCube observatory located at South Pole observed few tens of high energy astrophysical neutrinos. The flavor composition of these neutrinos could change during propagation as a result of neutrino oscillation and possible new flavor transition mechanism. Hence a precise measurement on the flavor composition of astrophysical neutrinos arriving on Earth could test new physics beyond Standard Model. In this talk, we shall review recent developments and future prospects of this research field.

512018-02-26 Mon
11:00~12:00
R1203
Hsi-Wei Yen
[ESO]
*Seminar*
Observational study of evolution from formation of protostellar disks to formation of planets
Abstract

Protoplanetary disks, the sites of planet formation, are often observed around young stellar objects. However, it remains unclear as to how and when such disks form during the process of star formation, and whether they possess forming planets or not. Formation and evolution of disks are closely related to angular momentum transfer from their parental dense cores to the vicinity of protostars under the influence of the magnetic field. In this presentation, I will introduce my project aiming to link the analyses of the gas kinematics and the magnetic field in protostellar sources in order to understand the role of the magnetic field in disk formation and evolution. In addition, the gas kinematics in protoplanetary disks can exhibit signatures of dynamical interaction between planets and disks, and thus it can be a signpost of forming planets. In this presentation, I will also introduce my project of searching for planets embedded in protoplanetary disks by detecting signatures induced by disk-planet interaction.

522018-02-14 Wed
14:20~15:20
R1203
Paulo Freire
[Max-Planck-Institut für Radioastronomie]
*Colloquium*
Discovery of five new, extreme double neutron star systems
Abstract

Since the discovery of the first double neutron star system (also, the first binary pulsar), PSR B1913+16 (the famous Hulse-Taylor binary pulsar), such systems have been extremely valuable for tests of general relativity and the study of gravitational waves, decades before the LIGO detection. They have also been a great tool for studies of stellar evolution. In this talk I will present two of the most important systems known (the Hulse-Taylor and the ``Double Pulsar" system), but then emphasize the extraordinary systems discovered in the last 3 years, with a discussion of their enormous potential for advancing our knowledge of fundamental physics and stellar evolution. I then discuss some of the trends emerging from this study, particularly the correlation between the mass of the second-formed NS and the orbital eccentricity of the systems, and what this tells us about supernova astrophysics.

532018-02-13 Tue
14:20~15:20
R1203
Chi-Ting Chiang
[Stony Brook University]
*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.

542018-02-12 Mon
14:20~15:20
R1203
Yasuhiro Hasegawa
[JPL]
*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.

552018-02-05 Mon
14:20~15:20
R1203
Richard Crutcher
[University of Illinois]
*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.

562018-02-01 Thu
14:20~15:20
R1203
Nanda Kumar
[Universidade do Porto]
*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.

572018-01-31 Wed
14:20~15:20
R1203
James Wurster
[Exeter]
*Colloquium*
The effect of non-ideal magnetohydrodynamics on star formation
Abstract

Until recently, numerical simulations of low-mass star formation have been unable to produce large discs around a forming protostar. This contradicts observations. With the inclusion of non-ideal magnetohydrodynamics (MHD), large discs are now being formed in numerical simulations, indicating the necessity of non-ideal MHD. If the inclusion of non-ideal MHD can self-consistently re-introduce discs, then what effect will it have on the formation of the protostar itself, or on large scales in star forming clusters? In this talk, I will first introduce the three non-ideal MHD processes: Ohmic resistivity, ambipolar diffusion, and the Hall effect. I will then discuss their effects on disc formation, the formation and evolution of the first and second hydrostatic core, and (if time permits) on star cluster formation.

582018-01-23 Tue
14:20~15:20
R1203
Olivia Jones
[University of Edinburgh]
*Colloquium*
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.

592018-01-18 Thu
14:20~15:20
R1203
Haifeng Yang
[University of Virginia]
*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.

602018-01-17 Wed
14:20~15:20
R1203
Andreas Schulze
[NAOJ]
*Colloquium*
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.

612018-01-12 Fri
14:20~15:20
R1203
Somnath Dutta
[S. N. Bose National Centre for Basic Sciences]
*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.

622018-01-11 Thu
14:20~15:20
R1203
Upasana Das
[University of Colorado]
*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.

632018-01-10 Wed
14:20~15:20
R1203
Takaya Nozawa
[NAOJ]
*Colloquium*
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.

642018-01-09 Tue
14:20~15:20
R1203
Tom Broadhurst
[University of the Basque Country]
*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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