2019/9/26 (木)


Speaker: Soichiro Hashiba

2019/7/18 (木)


Speaker: Yuuki Takei

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.

2019/5/9 (木)


Yusuke Yamada

Title: Supergravity cosmology


Quantization of gravity is one of the issues in theoretical physics. Quantized Einstein gravity has a problem of non-renormalizability, which indicates the necessity of extending Einstein gravity. Supergravity is one of the possible extension of GR, which improve the UV property of gravity. I will explain what is supergravity and the status of quantized supergravity.
I will also discuss the cosmological application of supergravity. In particular, I will show that (maximal) supergravity may be related to the class of inflation models consistent with the latest CMB observations.

2019/4/25 (木)

Kotaro Fujisawa

Title: Rotating stars


Almost all stars have rotation. Rotation plays an important role in many
astrophysical situations. Generally, structures of non-rotating stars
are spherical, but those of rotating stars are deformed due to the
centrifugal force. Non-sphericality is the essence of rotating stars.
In this talk, I introduce observations of rotating stars and sub-stellar
objects, and show our recent calculations of rotating sub-stellar
objects and rotating stars. I also discuss our new numerical scheme for
obtaining structures of rotating stars and a formulation of GR stars.

2019/4/18 (木)

Eiichiro Komatsu (16:00-)

2019/4/11 (木)

Minxi He

2019/4/4 (木)

Toshiki Sato (RIKEN) (RESCEU colloquium, 13:30-)

Title: The Origin of the X-ray Clumpy Ejecta in Type Ia Supernova Remnants

X-ray-emitting clumpy structures are generally observed in young Type Ia supernova remnants although the origin is still obscure. There are two candidates for explaining the formation of clumps; initial clumpiness in ejecta at the explosion (i.e., clumpy ejecta model) or hydrodynamic instabilities made from smooth ejecta profile (i.e., smooth ejecta model). This information should reflect the initial ejecta structure of SNe Ia, so it is important for understanding the Type Ia explosion itself. Our preliminary investigations into constraining the structure of SN Ia remnants using Fourier and wavelet-transform analyses did not turn out to be sufficiently powerful at discriminating the two hydro models and the observed Tycho image from each other. This led us to investigate an approach that would be more sensitive to patterns in the distribution of clumps and holes in the images, such as the "genus statistic".

In this study, for the first time, the genus statistics have been applied to a famous type Ia remnant, Tycho (SN 1572) to understand the formation of the clumps by comparing with hydrodynamical models (Sato et al. 2019, arXiv: 1903.00764). We found the genus curve from Tycho's supernova remnant strongly indicates a skewed non-Gaussian distribution of the ejecta clumps, which is similar to that of a hydrodynamical model for the clumpy ejecta model. In contrast, a hydrodynamical model for the smooth ejecta model has a genus curve that is similar to that of a random Gaussian distribution. Thus, our results support the initial clumpiness in the Type Ia ejecta is more reasonable for the origin of the clumps and demonstrate usefulness of the genus statistics for this field. In addition, we will also discuss the origin of "Fe-rich" ejecta clumps in Type Ia SNRs in this seminar.

2019/3/13 (水)

Tilman Troester

2019/3/7 (木)

Anna Lisa Varri

2019/2/28 (木)

Farewell Seminar & Party

2019/2/21 (木)


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Last-modified: 2013-11-20 (水) 11:11:54 (2064d)