2019/11/28 (木)


Speaker: Taizo Okabe

2019/11/21 (木)


Speaker: Masataka Aizawa

2019/11/14 (木)


Speaker: Hiroaki Tahara

2019/11/7 (木)


Speaker: Natsuki Hayatsu & Soichiro Morisaki

2019/10/31 (木)


Speaker: Yuta Tarumi & Hiroto Mitani

2019/10/24 (木)


Speaker: Saku Iwata & Takuya Tsutsui

2019/10/17 (木)


no seminar

2019/10/10 (木)


Speaker: Junya Kume & Takumi Hayashi

2019/10/3 (木)


Speaker: Soichiro Hashiba

Title: The hierarchy problem, and the brane world

Many physicists (especially particle physicists) believe that Nature loves the order of one.
In this sense, huge hierarchies between some physical constants are thought to be quite “unnatural”.
Why does Nature seem to be “unnatural”? This is the hierarchy problem (the naturalness problem).

L. Randall and R. Sundrum (1999) proposed the brane world scenario as a solution for one of hierarchy problems.
In this scenario, our 4-dimensional universe is a brane floating in a 5-dimensional bulk.
When we construct 4D effective field theory, the structure of 5D spacetime exponentially enhances deferences
between parameters in 5D bulk and realizes large hierarchies between effective parameters on 4D brane.

In the first half of this talk, I will briefly introduce various kinds of hierarchy problems and possible solutions.
In the latter half, I will explain how the brane world scenario works.

2019/9/26 (木)


Speaker: Kenneth Wong (IPMU)

2019/7/18 (木)


Speaker: Yuki Takei

Title: A light curve model for type IIn supernovae

Type IIn supernovae (SNe IIn) exhibit narrow hydrogen emission lines in their spectra, which indicates the existence of dense circumstellar media (CSM). Ejecta collides with dense CSM and interaction between them occurs, making SNe IIn much more luminous than other normal SNe.
We construct a light curve model by resolving interaction region. In this talk I will present how to resolve the region.

2019/7/11 (木)


Speaker: Daichi Tsuna

2019/7/4 (木)


Speaker: Kana Moriwaki

Cross-correlation between the 21-cm signal and [OIII] emitters during early cosmic reionization

The 21-m line provides a direct method to probe the distribution of neutral hydrogen in the Universe. Telescopes such as SKA try to measure the power spectrum of the 21-cm at the epoch of reionization (EoR). By taking the cross-correlation between the 21-cm with some other tracers at the same redshift, we can mitigate the foreground contamination. We propose to use [OIII} line emitting galaxies to cross-correlate with the 21-cm signals at the early phase of the EoR. In this talk, I will introduce ongoing and planned observations of the EoR. Then I will discuss the detectability of the cross-power spectrum between the 21-cm and [OIII] line emitters at the beginning of the EoR.

2019/6/27 (木)


Haruki Nishino

Status of the upcoming project for CMB observations from Chile: the
Simons Array experiment

The observations of Cosmic Microwave Background (CMB) have been
playing a key role for us to understand the beginning of our universe.
In recent years, at least for experimental cosmologists, the
polarization component of CMB is considered to be the next frontier
because the signature by the primordial gravitational waves from the
cosmic inflation is predicted to be imprinted as a curl-like pattern
called B-mode in the polarization pattern of CMB. The POLARBEAR
experiment is one of the experiments that have made significant
contributions to the understanding of the B-mode polarization of the
CMB. Some experiments including POLARBEAR has been confirming the
existence of the B-mode signal from the weak lensing effects of
large-scale structures. On the other hand side, the B-mode signal from
the primordial gravitational waves has not yet been detected. Several
experimental projects have been making efforts to achieve higher
sensitivities, and the upgrade of POLARBEAR, the Simons Array
experiment, is one of the world-leading upcoming CMB experiments in
the Atacama desert in Chile. Its first receiver system was developed
in KEK in Japan and deployed to Chile last year. The first telescope
achieved the "first-light" in the last austral summer. In this talk,
the results from the POLARBEAR experiment and the recent developments
from the Simons Array experiment are presented.

2019/6/20 (木)


Toshinori Hayashi

A strategy to search for an inner binary black hole from the motion of tertiary star

LIGO's detections of gravitational wave from binary black hole (BBH) merger events
give us an important open question for the origin and evolution of them.
Current proposed scenarios require long-term orbital evolutions before coalescence.
This fact implies the presence of many wide-separation BBHs as progenitors of merger events.
Currently, there are many proposals to search for star-black hole binaries with Gaia and TESS. It is expected that many such binaries are discovered in near future and some of them may be triples consisting of inner BBHs and tertiary stars.
Thus, we propose a methodology to search for such BBHs through radial velocity (RV) modulations of stars.We first consider a coplanar and near-circular triple and apply perturbation theory to obtain approximate RV formula and understand its nature.
Incidentally, we consider a degeneracy between a S-type circumbinary planet and inner unseen binary and investigate the correspondence of their parameters.
We also consider effects of eccentricities and inclinations on RV signals and discuss their importance to estimate orbital parameters.
Finally we present a preliminary progress of an on-going research on observational feasibility under RV errors considering Gaia and TESS fiducial binaries.

2019/6/13 (木)


Yu Niino

Seeking the origin of short and intense explosions in the universe

Short and intense transients like gamma-ray bursts (GRBs), fast radio bursts (FRBs), and gravitational wave (GW) events provide us with irreplaceable laboratories of physics and unique probes of the evolution of the universe. However, observationally obtaining evidence that clarifies the nature of those transients is often challenging due to their short timescale and large distance toward them. In this talk, I will discuss multi-wavelength and/or statistical approaches taken to understand the origin of GRBs and FRBs. I will also briefly introduce the ongoing optical followup program of GW events using the 1.08 m Kiso Schmidt telescope.

2019/6/6 (木)


Speaker: Kojiro Kawana

Emission from thermonuclear explosions in white dwarf TDEs

In tidal disruption events (TDEs) of a white dwarf (WD) by a black hole (BH), the WD is not only tidally disrupted but possibly ignites nuclear burning with the help of tidal compression. MacLeod et al. (2016) show that emission from thermonuclear explosion in a TDE of a CO WD with 0.6 Msun is reminiscent of Type I supernovae. However, nucleosynthesis and dynamics of tidal debris in WD TDEs have a large variety depending on BH/WD mass and orbital parameters (Kawana et al. 2018). Here, we study a variety of emission emerging from thermonuclear explosion in WD TDEs by performing hydrodynamic simulations coupled with nuclear reactions and post-process radiative transfer calculations. We find that emission from WD TDEs of a He WD is significantly different from that of CO WDs. Because of lower mass of unbound debris, it shows short timescale and blue color. We will also discuss comparisons of He WD TDEs with so-called calcium rich transients and rapid transients.

2019/5/30 (木)


Renyue Cen (Princeton University)
RESCEU colloquium (16:45-)

Computing the Universe: from Intergalactic to Interstellar Medium

This talk will highlight some progress made based on our efforts of computing the universe, in order to understand the dynamics of gas in the universe, from intergalactic to interstellar medium. Select observables presented include the cosmic web from redshift zero to z=2-4, and the escape fraction of Lyman continuum photons from galaxies at the epoch of reionization, an essential parameter underlying the feasibility of stellar reionization of the universe.

2019/5/23 (木)


Tilman Hartwig

Title: Machine Learning for Classification of Astronomical Data


I will give an overview of various machine learning methods
and their scientific applications. As one specific example, I will
present decision trees in more detail since they are a very efficient
method to classify astronomical data: a labelled training sample is
split according to available features by requiring that each split
minimises the information entropy of the assigned classes. This elegant
mathematical formulation allows us to construct decision trees with
supervised learning, which can then be applied to classify new observations.
Eventually, I will present recent results of my own research: by
classifying the chemical abundance patterns of metal-poor stars in the
Milky Way, we can derive the multiplicity of the first generation of
stars in the Universe. Furthermore, this approach provides the feature
importance to identify crucial chemical elements to classify metal-poor
stars, which can be used to optimise future spectroscopic surveys.

2019/5/16 (木)


Conor Omand

Radio and Submillimetre Constraints on the Pulsar-Driven Supernova Model

Several classes of energetic transients, including superluminous supernovae (SLSNe) and gamma-ray bursts (GRBs), require more energy than conventionally available in a supernova. Several energy sources have been suggested, including a rapidly-rotating highly-magnetized pulsar, but can not be distinguished by the thermal emission of the transient. The smoking gun for this model should be late-time non-thermal emission, detectable after the ejecta becomes optically thin. We predicted the emission from several sources, and conducted follow-up observations in both radio (using VLA) and submillimetre (using ALMA and NOEMA). We found a weak signal from PTF10hgi, which was also detected at higher frequency by Eftekhari+ (2019), but no other detections, even though several observations had sensitivities well below our lower limits. I talk about the implications for the pulsar driven model, and some revisions which may explain the previous non-detections.

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Last-modified: 2013-11-20 (水) 11:11:54 (2213d)