投稿者: y.nagai

GRB Closure Relationship in Multi-wavelength

Speaker: Shubham Bhardwaj (D2, SOKENDAI/NAOJ)

Abstract:

Gamma-ray bursts (GRBs) are intense pulses of high-energy emission associated with massive stars’ death or compact objects’ coalescence. Their multi-wavelength observations help verify the reliability of the standard fireball model.  We analyze 14 GRBs observed contemporaneously in gamma-rays by the Fermi Large Area Telescope (LAT), in X-rays by the Swift Telescope, and in the optical bands by Swift and many ground-based telescopes. We study the correlation between the spectral and temporal indices using closure relations according to the synchrotron forward-shock model in the stratified medium (n ∝r^{-k}) with k ranging from 0 to 2.5. We find that the model without energy injection is preferred over the one with energy injection in all the investigated wavelengths.

Gas Infall and Core Growth in High-mass Star Formation

Speaker: Kaho Morii (D3, The University of Tokyo)

Abstract:

Understanding gas dynamics is a key to understanding star formation. Recent observations of infrared dark clouds (IRDCs), the birthplace of high-mass stars, imply the necessity of core growth by feeding gas from the surroundings. To explore this, we conducted ALMA observations of a 70 μm dark region within a massive, dense IRDC (~1180 Msun, 12 K). Our analysis of 19 cores, including two intermediate-mass cores (10 and 4 Msun), shows signs of gas infall, with velocities between 0.3-1.4 km/s and infall rates of 10^-4 to 10^-3 Msun/yr. These are higher than in low-mass regions and can be considered a strong indication of core growth, enabling the formation of high-mass stars from intermediate-mass cores that would not originally be able to form high-mass stars at their current mass.

Infrared scattered light observations of protoplanetary disks

Speaker: Ryo Tazaki (University of Tokyo)

Abstract:

Dust growth and evolution in protoplanetary disks is the first step in planet formation. However, these processes are still largely unknown. Thanks to the recent advent of ground- and space-based telescopes, we can study these processes in more detail through disk observations. In particular, infrared disk observations allow us to study the properties (such as grain size, porosity, and composition) and spatial distribution of micron-sized dust grains, which are major building blocks of planets. In this talk, I will present our recent observational and modeling efforts to understand dust properties and their spatial distribution using the Subaru Telescope, the Very Large Telescope, and the James Webb Space Telescope.

The contribution of the faint Lyman alpha galaxies to cosmic reionization as seen by MUSE/VLT

Speaker: TRAN Thi-Thai (Division of Science, NAOJ)

Abstract:

At the end of the Dark Ages, when the first structures of the Universe were formed, radiation from these structures ionized the neutral hydrogen atom surrounding its environment. This crucial phase is known as cosmic re-ionization. It is the last phase transition undergone by the Universe, finishing around z~6. Today, various hypotheses exist concerning the main contributors to this process, such as Active Galactic Nuclei (AGN), star-forming galaxies, etc.  I assess the contribution of the star-forming galaxies to cosmic reionization by studying the evolution of the luminosity function with redshift,  estimating their star formation rate density as well as the escape fraction of Lyman alpha photon at different redshift ranges through the use of VLT/MUSE as well as gravitational lensing. For this purpose, the work assembles the largest sample of lensed Lyman-alpha emitters: faint galaxies identified by the Lyman-α emission line (LAEs) to date (at redshifts between 2.9 and 6.7).  The best-fit results of the Schechter function at different redshift ranges allow us to determine the luminosity density and convert it to the star formation rate density. These results, compared with those of the critical value for the star formation density, suggest that galaxies selected by their Lyα emission could be responsible for reionization assuming a Lyα photon escape fraction of 8%, with a typical clumping factor of ∼ 3. On the other hand, when assessing the escape fraction of Lyman alpha photons from star-forming galaxies we found that the LAE population could have provided all the photons necessary for reionisation at z=6 using well-motivated assumptions about the ionising photon efficiency and the escape of ionising photons from these galaxies.

Where 3-dimensional analysis matters: dust distribution in protoplanetary disks under planet-disk interactions

 Speaker: Jiaqing Bi (Academia Sinica Institute of Astronomy and Astrophysics (ASIAA)/University of Toronto)

Abstract:

Protoplanetary disks are the birthplaces of planets. However, these disks also embed young planets, making them extremely difficult to detect. Substructures within the disks, especially those observed in millimeter-wavelength dust thermal emissions, such as gaps and rings, have been used to suggest the presence of planets. Nevertheless, planet-disk interactions are not the only mechanisms that can explain these features. Therefore, distinguishing substructures caused by planets from those of other origins is crucial for current planet-hunting efforts and understanding the planet formation process. In this talk, I will summarize our recent studies on dust dynamics under planet-disk interactions, utilizing three-dimensional numerical simulations. Our findings show that incorporating the vertical dimension in planet-disk models reveals a wealth of dynamics, leading to possible characteristic features in observations.

Galaxy evolution with galaxy mergers

Speaker: Takanobu Kirihara (Kitami Institute of Technology)

Abstract:

In the framework of hierarchical structure formation based on cold dark matter, galaxies like the Milky Way grow up through numerous interactions and mergers of many less massive galaxies. These signatures are mainly imprinted in the halo region. In recent years, the Gaia project and the Subaru HSC survey have revealed many tidal remnants in the haloes of the Milky Way and Andromeda galaxy, respectively. In this talk, I will provide an overview of the halo formation process revealed by a combination of observations and numerical simulations, referring to recent observational progress. I also focus on the interaction between massive galaxies. I will introduce the relationship between AGN activity, galaxy density, and interaction of galaxies based on our recent observations and numerical simulations. 

Collisional evolution from dust to planets 

Speaker: Hiroshi Kobayashi (Nagoya University)

Abstract:

Planets were believed to form via the accretion of planetesimals
generated from dust grains in protoplanetary disks. However, the
growth of planets is much slower than their migration due to
disk-planet interaction. Comparably rapid growth via pebble
accretion was then proposed, which requires very massive
protoplanetary disks because most pebbles fall into the central
star.  Although planetesimal formation, planetary migration, and
planetary growth have been studied with much effort, the full
evolution path from dust to planets was uncertain.  We have
investigated full collisional evolution from dust to planets.
For collisional evolution, collisional outcomes are not simply
characterized as fragmentation, bouncing, etc. The impact
simulations for dust aggregates showed the detailed
outcomes. According to the outcome model, the growth of dust
grains are not prevent from collisional fragmentation.  We thus
perform the full simulations (DTPSs) for collisional evolution
from dust to planet in whole protoplanetary disks.  Dust growth
with high porosity allows the formation of icy planetesimals in
the inner disk (< 10 au), while pebbles formed in the outer
disk drift to the inner disk and there grow to planetesimals.
The growth of those pebbles to planetesimals suppresses their
radial drift and supplies small planetesimals sustainably in the
vicinity of cores.  This enables rapid formation of sufficiently
massive planetary cores within 0.2-0.4 million years, prior to
the planetary migration.  However, such porous pebbles are
unlikely to reproduce the polarized millimeter wavelength light
observed from protoplanetary disks. We thus investigate gas-giant
core formation with non-porous pebbles via DTPSs. Even non-porous
bodies can grow into planetesimals and massive cores to be gas
giants are also formed in several 100 thousand years. The rapid core
formation is mainly via the accretion of planetesimals produced
by collisional coagulation of pebbles drifting from the outer
disk. The formation mechanism is similar to the case with porous
pebbles, while core formation occurs in a wider
region (5-10 au) than that with porous pebbles.  Although
pebble growth and core formation depends on the disk temperature,
core formation is likely to occur with disk temperatures in
typical optical thick disks around protostars.