投稿者: YamashitaKazuyoshi

Phenomenological turbulent effects of core-collapse supernovae

Speaker: Shunsuke Sasaki

Abstract:

It is not yet known how massive the star can explode as core collapse supernova (CCSN), how much explosive energy, neutrinos and other quantities will be observed when it explodes. Researches into simulations of CCSN mechanism have succeeded in showing that such explosions are possible even in three-dimensional (3D) simulations. It was also revealed that turbulence associated with neutrino heating plays an important role in the explosion. This has led to an active discussion on the relationship between the quantity of progenitor before the explosion and the physical quantity of CCSNe, which is called progenitor dependence. In recent years, the development of phenomenological one-dimensional simulations (1D+) introducing turbulence effects has become an urgent issue in order to investigate the progenitor dependence more realistically. We developed 1D+ and we got the result that our 1D+ can mimic the shock evolution in 3D. In this tolk, I will explain our 1D+ and preliminary results about progenitor dependence.

The assembly and dispersal of dense gas in star forming regions

Speaker: Lars Bonne

Abstract:

First, I will present work analyzing multiple spectral lines toward low- and high-mass star forming regions. The study employs archival HI data, CO observations from the NANTEN2, APEX, and IRAM 30m observatories, and [CII] observations with the SOFIA telescope. In these regions we demonstrated the presence of recurring organized velocity fields, also found by other authors, which suggests that star formation is initiated by the same mechanism. Namely, magnetic field bending in high-velocity (>7 km/s) colliding flows. This appears to be consistent with magnetic field observations in several nearby clouds, which suggests that the proposed scenario might be widespread and explain both low- and high-mass star formation.
In the second part, I will present observations of the [CII] spectral line by the FEEDBACK legacy program toward ~10 ionized (HII) regions surrounding massive O stars. [CII] is the main coolant of the neutral ISM in photodissociation regions (PDRs) and thus an excellent probe to study the effect of stellar feedback on the host molecular cloud. The [CII] emission reveals previously undetected high-velocity gas (10-20 km/s) in all regions. This high-velocity gas is the result of expanding bubbles and continuous mass ejection in flattened molecular clouds. The detection of this high-velocity gas has reignited the discussion whether radiation or stellar winds drive molecular dispersal. Quantifying the mass ejection rates also allows us to make a direct estimate of molecular cloud dispersal timescales which consistently points to a few (< 5) Myr. This provides direct observational evidence that molecular cloud are transient structures and not in quasi-static equilibrium.

Exploring the interior of supernovae and their progenitors using supernova remnants

Speaker: Toshiki Sato

Abstract:

It is difficult to observe the physical conditions inside the supernova or its progenitor star immediately before and after the supernova explosion, where the important physics of stellar evolution and supernova explosions are concentrated. Our research focuses on the X-ray study of “supernova remnants” in order to extract the internal information at the moment of a star’s death. The uniqueness of supernova remnants is that it is possible to observe different elements synthesized inside stars and supernovae, and to infer the internal physical states (electron fraction, density structure, etc.) from the amount of elements. In this colloquium, based on our recent X-ray research, we would like to discuss what kind of information can be obtained from supernova remnant observations, and what we can expect from future research on supernovae and their progenitors.

Early Fragmentation in Infrared Dark Clouds

Speaker: Kaho Morii

Abstract:

The study of infrared dark clouds (IRDCs) sheds light on the initial conditions governing the formation of high-mass stars and stellar clusters. We have conducted high-angular resolution and high-sensitivity observations toward thirty-nine massive IRDC clumps, mosaicked by the Atacama Large Millimeter/submillimeter Array. These clumps, characterized by their darkness at 70 μm, as well as their density and low temperature, are thought to be the ideal sites as the birthplace of high-mass stars. We succeeded in identifying an unprecedented number of 839 cores, with masses between 0.05 and 81 Msun. With this large sample, we investigated the fragmentation properties in the very early stage of high-mass star formation. By employing the minimum spanning tree method, we calculated core separations ranging from 0.1 pc to 0.4 pc. To discern the dominant mechanism behind early fragmentation, as well as the hierarchical nature of the process, we compared these observed core separations and masses with those expected from Jeans length and masses, respectively. Our analysis implies that thermal Jeans fragmentation of clumps is the dominant mechanism deriving the observed properties especially for the formation of gravitationally bound cores. Additionally, we find that some clumps exhibit a wide dynamic range of core masses, spanning from low to high masses while others show a narrower range. Clumps with a higher protostellar core fraction tend to display a wider range. Furthermore, our sample highlights the complex nature of fragmentation, characterized by various patterns such as aligned, spread, and concentrated distributions. These findings provide valuable insights into the mechanisms deriving high-mass star formation.

Light curves of electron capture and Fe core collapse supernovae: The diagnostic method of electron capture supernovae

Speaker: Masato Sato

Abstract:

While massive stars (M>~10Msun) explode as Fe core collapse supernovae (FeCCSNe) at their last moment, those have slightly lower mass (M~8-10Msun) are theoretically expected to form O+Ne+Mg degenerated core, become Super Asymptotic Giant Branches (SAGB) and finally explode as electron capture supernovae (ECSNe) if their envelope is remained (Miyaji et al. 1980; Nomoto et al. 1982; Nomoto 1984, 1987). However, such evolutionary path and the mass boundary between FeCCSN and ECSN are not confirmed and constrained by observation because of insufficient observations of ECSNe. The reasons why we could hardly diagnose ECSN clearly are that observational characteristics of ECSNe comparing to low-mass FeCCSNe are not understood sufficiently, and the diagnostic method of ECSNe is not established yet. Although Kozyreva et al. (2021) shows that ECSN has blue plateau, they don’t include circumstellar material (CSM) interaction. However, CSM interaction might change the light curve significantly (Moriya et al. 2018). Thus, we synthesized the multicolor light curves of ECSNe and low-mass FeCCSNe including CSM interaction using the multi-group radiation hydrodynamics code, STELLA (Blinnikov et al. 1993). As a result, ECSN is revealed to show bluer plateau than low-mass FeCCSN even if it has reasonably dense CSM. Using this characteristic, we propose the first diagnostic method of ECSN in which the transition time from plateau to tail phase (tPT) and the color index B-V at tPT/2 are used. In the talk, we will show the calculated light curves of ECSN and low-mass FeCCSN and discuss their characteristics. In addition, we will propose the diagnostic method of ECSN. Also, we will mention our future work in which we will try to find an ECSN and reveal its nature.

GRB Optical and X-ray Plateau Properties Classifier Using Unsupervised Machine Learning

Speaker: Shubham Bhardwaj

Abstract:

The division of Gamma-ray bursts (GRBs) into different classes, other than the “short” and “long”, has been an active field of research. We investigate whether GRBs can be classified based on a broader set of parameters, including prompt and plateau emission ones. Observational evidence suggests the existence of more GRB sub-classes, but results so far are either conflicting or not statistically significant. The novelty here is producing a machine-learning-based classification of GRBs using their observed X-rays and optical properties. We used two data samples: the first, composed of 203 GRBs, is from the Neil Gehrels Swift Observatory (Swift/XRT, (Gehrels et al. 2004; Burrows et al. 2005)), and the latter, composed of 134 GRBs, is from the ground-based Telescopes and Swift/UVOT (Roming et al. 2005). Both samples possess the plateau emission (a flat part of the light curve happening after the prompt emission, the main GRB event). We have applied Gaussian Mixture Model (GMM) to explore multiple parameter spaces and sub-class combinations to reveal if there is a match between the current observational sub-classes and the statistical classification. With these samples and algorithm, we spot a few micro-trends in certain cases, but we cannot conclude that any clear trend exists in classifying GRBs. These microtrends could point towards a deeper understanding of the physical meaning of these classes (e.g., a different environment of the same progenitor or different progenitors). However, a larger sample and different algorithms could achieve such goals. Thus, this methodology can lead to deeper insights in the future.

Constraints on the dust size distributions in the HD 163296 disk from the difference of the apparent dust ring widths between two ALMA Bands

Speaker: Kiyoaki Doi

Abstract :

The formation of planets begins with dust coagulation in protoplanetary disks. Therefore, constraints on the dust size distribution in the disks can be a clue for understanding planet formation. In previous studies, the dust size has been estimated by using the spectral index derived from multi-wavelength observations or dust polarization observations. However, these studies provide different results depending on their methods and models and do not reach a consensus.
In this work, we propose a new method to constrain the dust size distribution by using the wavelength dependency of the dust ring widths. Since larger dust grains are trapped more effectively in the gas pressure bump, they form narrower rings. As a result, the dust rings appear narrower at longer wavelength observations since observations are sensitive to the dust grains whose size is comparable to the observed wavelength.
We constrain the dust size distribution in the HD 163296 disk using ALMA high-resolution observations in Band 6 (1.25 mm) and Band 4 (2.14 mm). We focus on the two clear dust rings in the disk and find that the outer ring at 100 au appears narrower at the longer wavelength, while the inner ring at 67 au appears similar between the two bands. We model a dust ring assuming size-dependent dust trapping at a gas pressure maxima and investigate the relation between the wavelength dependency of the ring width and the spectral index, and the dust size distribution. By comparing the model with the observations, we constrain the maximum dust size a_max and the exponent of the power law dust size distribution p. We constrain that 0.9 mm < a_max < 5 mm and p < 3.3 in the inner ring, and 35 mm < a_max > 1000 mm and 3.4 < p < 3.7 in the outer ring. The larger maximum dust size in the outer ring suggests that the degree of dust growth is spatially dependent, which could affect the location of the planetesimal formation.

Modeling the thermal evolution of planet-forming disks

Speaker: Satoshi Okuzumi

The thermal structure of protoplanetary disks determines when and where planets of different compositions form. However, the thermal structure of these disks remains largely uncertain due to two main factors: (1) the existence of strong internal heating sources deep inside the disks is still unknown, and (2) the cooling rate of the disks is influenced by micron-sized dust grains and varies as the grains grow into larger solid bodies. Dust growth can even impact disk heating induced by magnetic fields, as the grains regulate the disks’ electric conductivity by capturing plasmas. All these factors indicate that the temperature structure of the disks evolves as planet formation (dust evolution) progresses. In this presentation, I will discuss our recent efforts to model the coupled evolution of dust and the thermal structure of protoplanetary disks. Specifically, I will highlight the roles played by magnetic fields, disk shadows, and planet-induced spiral shocks in shaping the disks’ temperature distribution.

Celebrating 15 years of Gamma-Ray Burst observations with Fermi

Speaker: Elisabetta Bissaldi

Abstract :

Gamma-Ray Bursts represent the most powerful explosions in the Universe.
Their emission, which covers the entire electromagnetic spectrum from
radio up to TeV energies, has been studied since the 1970s by many
ground and space-based observatories.
The Fermi Gamma-ray Space Telescope has been a major player in the field
of GRB studies during the last 15 years, providing unique insights into
their nature. With thousands of GRBs detected by the Gamma-ray Burst
Monitor (GBM) and hundreds by the Large Area Telescope (LAT), we have
learned the broad high-energy properties of the populations of these
events and obtained unique insights into their emission mechanisms and
physical characteristics.
In this talk, I will review the highlights of GRB science from low (keV)
to high (GeV) energies, as well as the most recent observations of very
high energy (TeV) emission, with particular emphasis on the GRB 221009A
event, revealed in October 2022, which holds the brightness record among
all GRBs observed by the Fermi telescope.

Simulation Study of Collisions Between Filamentary Molecular Clouds Threaded by Lateral Magnetic Field and Subsequent Evolution

Speaker: Raiga Kashiwagi

Abstract :

Most Stars are thought to be formed inside filamentary molecular clouds. Recent observations have proposed that star formation may be triggered through collisions between filamentary molecular clouds at nearby star-forming regions(Duarte-Cabral et al. 2010; Nakamura et al. 2014). Furthermore, according to Kumar et al. (2020), all luminous clumps with L > 10⁴L⊙ and L > 10⁵L⊙ at distances within 2 and 5 kpc respectively are located at the intersections (referred to as “hubs”) of filaments and this indicates the formation of massive stars preferentially form at the hubs. Based on the above, filament collisions are considered to be a universal and important phenomenon. Understanding the filament collision process will reveal the initial conditions for collision-induced star formation. Therefore, we have been working on 2D-MHD simulations of filament-filament collisions. In this colloquium, we will report on the condition of radial instability of the merged filament and its evolution. As the initial condition, we prepare two identical infinite long filaments, threaded by lateral magnetic fields. The filaments are in magnetohydrostatic equilibrium(Tomisaka 2014; Kashiwagi & Tomisaka 2021). The two filaments collide head-on along the magnetic field lines with relative velocities ranging from transonic to supersonic. If the total line mass of the initial filaments exceeds the critical line mass of the magnetized filament, the shocked region collapses radially. On the other hand, for collisions where the total line mass is below the critical value, the shocked region evolves into a structure that closely resembles the magnetohydrostatic equilibrium state. In addition, the results for the case where the collision direction is perpendicular to the magnetic field lines are also briefly introduced. Finally, we briefly introduce the preliminary results of the Orthogonal collision which is reproduced by the 3D-MHD simulation.