投稿者: Yuko Matsushita

Building blocks of the Milky Way revealed from the chemodynamics of halo stars

speaker: Tadafumi Matsuno

Abstract:
Since galaxy merger and accretion are fundamental processes in galaxy evolution, reconstructing the accretion history is key to understanding the evolution of a galaxy. In the Milky Way, we can study ancient accretion events from kinematics and chemical abundances of halo stars. In this talk, I will first summarize the discovery of candidates for past galaxy accretions to the Milky Way, which are made from the analysis of stellar kinematics available thanks to the Gaia mission. I will then talk about our follow-up observation programs to chemically characterize their members, allowing us to confirm the accretion events and characterize the accreted galaxies’ properties. I will finally summarize the new opportunities that these found accreted galaxies bring to the study of nucleosynthesis processes.

Filaments and their role in the star formation process

speaker: Seamus Clarke

Abstract:

It has long been known that filaments have been connected to the star formation process but Herschel observations have sparked a recent interest in them. However, how they play this role is not fully understood, nor is the effect of their geometry on the star formation process. Here I will present two recent works, one numerical and one observational, looking at helping to understand the place filaments occupy in star formation. First, I will present a numerical study looking at the fragmentation of an accreting, non-equilibrium filament. I show that filaments are prone to fragment into smaller sub-filaments due to internal accretion-driven turbulence. These sub-filaments play a key role in the formation of cores, affecting both their number and mass. Moreover, unlike in equilibrium models of filament fragmentation, no characteristic fragmentation length-scale is observed. Second, I will present a Herschel study of the outer Galaxy giant molecular filament (GMF) G214.5-1.8. I find that G214.5 has a mass of ~16,000 solar masses, similar to clouds such as Serpens and Mon R2, yet is mostly quiescent while those are highly star forming. I find that G214.5 has fragmented into numerous clumps and that there appears to be no characteristic fragmentation length-scale present. I show that the GMF is unusual in its high aspect ratio, narrowness, and paucity of dense gas, as well as having a highly asymmetric radial profile consistent with compression. Using HI survey data, I find that G214.5 lies on the edge of a HI superbubble and this may be the cause of G214.5’s properties, highlighting external effects and how they shape filaments within the bubble-dominated interstellar medium.

Phenomenological turbulent effects in core collapse supernovae

speaker: Sasaki Shunsuke

Abstract: 

We have developed phenomenological turbulent effects with one-dimensional (1D) simulation based on Reynolds decomposition. Using the method, we have conducted a systematic study of 320 models changing progenitor mass, mixing-length parameter, and diffusion coefficients for internal energy, turbulent energy, and electron fraction. With employed turbulent effects, supernova explosion can be achieved even in 1D geometry.  1D+ can reproduce results of 2D simulation at a later phase after the core bounce.     The progenitor dependence of explodability Explosion energy ,PNS mass and so on are also investigated in the mixing-length parameter plane. These simulations would help understand the role of turbulence in core-collapse supernovae.

Constraints on the dust size distribution in the protoplanetary disk around HD 163296 by ALMA multi-band observations

speaker: Doi Kiyoaki

Abstract:

Dust in protoplanetary disks is the material for planets, and how the dust grows is a critical problem in planet formation. Recently, high-resolution observations by ALMA made it possible to spatially resolve the dust distributions in protoplanetary disks. To characterize the disk and dust particles, we focused on the wavelength dependency of the dust distributions. We analyzed Band 6 (1.3 mm) and Band 4 images of the disk around HD 163296 and constrained the dust radial and vertical distributions. The disk has two clear rings at 67 au and 100 au. We found that the ring width is similar between the 2 Bands at the inner ring, whereas the ring width is narrower at Band 4 than at Band 6 at the outer ring. These results suggest that the maximum dust size is smaller than the observed wavelength at the inner ring, whereas the maximum dust size is larger than the wavelength in the outer ring, assuming the dust trapping scenario. We also found that the dust is puffed up in the inner ring and settled in the outer ring at both wavelengths. They are consistent with the dust size suggested by the radial profile. This local variation of the dust size and distribution suggests the possibility that the planetesimal formation is a localized process.

speaker: Alessio Traficante

From filaments to clumps: multi-scale dynamics in 70 micron quiet star-forming regions

Abstract:

The star formation mechanism occurs in well defined structures that can be identified and studied in great details in our own Galaxy: the process starts in giant molecular clouds, objects extended up to several tens of parsecs, within which elongated sub-structures, called filaments, may form. Inside filaments, round-like condensations extended up to ~1pc in radius, the so-called clumps, are the natural birth site of the pre- and proto- stellar cores, inside which will origin the future stars.
There are still many open questions in this hierarchical view of the star formation process: are these structures relatively confined from each other? Or is the large-scale environment affecting the dynamics of the formation down to clumps and cores? Is there a continuous interplay of the various forces involved in the process, namely turbulence, gravity and magnetic fields, at all scales? Or is there a relevant scale at which gravity will start to dominate the collapse, with critical implications on the star-formation mechanism?
After a general overview of the problem, I will present in details some recent results focused on the interplay between gravity and turbulence at the filament and clump scales. To investigate this interplay we have combined the dynamics of so-called 70 micron quiet clumps, i.e. very pristine regions not yet strongly affected by feedbacks, with the dynamics of the parent filaments in which they are embedded. Both clumps and filaments physical properties have been extracted from the Herschel survey of the Galactic Plane, Hi-GAL. The kinematics of filaments have been taken from ancillary CO data, while the kinematics of clumps has been studied with dedicated surveys carried out with the IRAM 30m telescope and aimed to observe high-density tracers such as N₂H⁺ (1-0) an HCO⁺ (1-0). We observe a continuous interplay between turbulence and gravity, where the former creates structures at all scales and the latter takes the lead above a critical value of the surface density is reached, ~ 0.1 g cm^-2. In the densest filaments this transition can occur at the parsec, or even larger scales, leading to a global, gravitationally driven collapse of the whole region and to the formation of the most massive objects in the Galaxy.

Novel modeling of magneto-rotational stellar evolution

speaker: Koh Takahashi

Abstract
While magnetic fields have been considered to influence the evolution of non-degenerate and compact stars, it has become clear in recent years that actually all stars are deeply affected. We propose a new framework of stellar evolution simulation, in which intertwined evolution between the magnetic fields, the stellar rotation, and the stellar structure is treated self-consistently. In this talk, I will report the results of the magneto-rotational evolution of 1.5 M stars, which have radiatively stratified envelopes during their main-sequence. We have found that the Lorentz force aided by the Omega-effect imposes torsional Alfven waves propagating through the magnetized medium, leading to near-rigid rotation within ~Alfven time. Our results of the hydrogen-burning stage can reproduce the main observed properties of Ap/Bp stars, and moreover, calculations continued to the red-giant regime show a pronounced core-envelope coupling, which reproduces the core and surface rotation periods determined by asteroseismic observations. In addition, I will report on the recent progress of developing a 1D scheme for representing the convective dynamo, which accounts for the magnetic activity of convective stars including the sun. Our new approach reproduces the most fundamental properties of this phenomenon, namely the magnetic field amplification as well as the cyclic behavior.

Star formation efficiency and destruction of giant molecular clouds with stellar feedback

speaker: Jeong-Gyu Kim

Abstract:

Galaxy evolution crucially depends on how gas is converted into stars in giant molecular clouds (GMCs) and on how ensuing stellar feedback from young massive stars (in the form of UV radiation, stellar winds, and supernovae) interacts with the surrounding interstellar medium. Observations indicate that GMCs in normal disk galaxies turn only a small fraction of gas mass into stars per free-fall time and over their lifetimes, while clouds in denser environments may form stars more efficiently. Although stellar feedback is believed to play an important role in controlling the efficiency of star formation and cloud lifecycle, details remain elusive. In this talk, I will present results from radiation (M)HD simulations of star-forming GMCs with stellar feedback. I will show how the star formation efficiency and destruction timescale depend on various integrated cloud properties such as surface density, turbulence level, and magnetization, and discuss the relative importance of different feedback mechanisms. We explain the observed star formation rate of the Milky Way by applying our finding that the efficiency per free-fall time decreases with the virial parameter of a molecular cloud. Time permitting, I will also briefly introduce our ongoing effort in modeling multiphase galactic disks with self-consistent star formation and feedback using the TIGRESS numerical framework.

AGN Variability: reverberation phenomena and variability-based AGN selection

speaker: Mitsuru Kokubo

Abstract:
AGN accretion disk continua show temporal luminosity variations over the entire wavelength range from X-rays to optical on time scales of days to years.
The continuum variability induces the reverberation phenomena in the outer accretion disk, Broad Line Region (BLR), and dust torus IR emission.
The reverberation mapping technique has been used to put constraints on the radial extent and physical conditions of the otherwise unresolved accretion disk, BLR, and dust torus around the central supermassive black holes.
In this talk I will review the recent observational progress and emerging issues in the AGN reverberation mapping.
In the latter half of the talk, I will also discuss our ongoing work on the variability-based AGN survey with the Subaru/HSC time-domain data.

UV & Lyα halos of Lyα emitters across environments at z = 2.84

speaker: Satoshi Kikuta

Abstract :

We present UV & Lyα radial surface brightness (SB) profiles of Lyα emitters (LAEs) at z = 2.84 detected with the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The depth of our data, together with the wide field coverage including a protocluster, enable us to study the dependence of Lyα halos
(LAHs) on various galaxy properties, including Mpc-scale environments. UV and Lyα images of 3490 LAEs are extracted, and stacking the images yields SB sensitivity of ~1e-20 erg/s/cm^2/arcsec^2 in Lyα. Fitting of the two-component exponential function gives the scalelengths of 1.56 and 10.4 pkpc.
Dividing the sample according to their photometric properties, we find that while the dependence of halo scalelength on environment outside of the protocluster core is not clear, LAEs in the central regions of protoclusters have very large LAHs and may be related to diffuse Lyα emission from abundant cool
gas permeating the forming protocluster core irradiated by active members including a hyperluminous QSO. For the first time, we identify “UV halos” around bright LAEs which are probably due to a few lower-mass satellite galaxies. Through comparison with recent numerical simulations, we conclude
that, while scattered Lyα photons from the host galaxies are the dominant contributor to LAHs, star formation in satellites evidently contributes significantly to LAHs, and that fluorescent Lyα emission may be boosted within protocluster cores at cosmic noon and/or near bright QSOs.

The effects of surface fossil magnetic fields on massive star evolution

speaker: Zsolt Keszthelyi

Abstract :

Magnetism plays a vital role in several astrophysical phenomena, from the scale of sub-atomic particles up to galaxy clusters. Studying stellar magnetic fields can help us better understand planetary habitability, stellar variability, and the overall magnetic flux evolution from star formation to compact objects.
There has been renewed interest in this field, partly due to the surprising discovery that some massive stars host strong, globally organized, large-scale magnetic fields. The long-term (years to decades-long) spectropolarimetric monitoring shows a lack of correlation with stellar parameters. This suggests that the observed fields of early-type stars are not produced by a dynamo mechanism. Instead, they are thought to be remnants from the earlier history of the star (from the star formation phase or, in part, produced by stellar mergers).
In a series of publications, we have been studying the long-term, evolutionary impact of such fossil fields on massive star evolution. Two main effects concern i) trapping wind material and thus reducing the mass loss and ii) magnetic braking leading to slowly spinning stars. Both have far-reaching consequences and could affect completely evolutionary pathways, stellar populations, predictions on stellar end products, and gravitational wave progenitors.
We recently computed and scrutinized a grid of models including these effects in three metallicity environments. The library of new stellar models is open source and available for the community via: https://zenodo.org/record/7069766