投稿者: Yuko Matsushita

Formation of clusters in granular streams – Toward collisional experiments of dust agglomerates

Speaker: Akiko Nakamura

Abstract: 

Cluster formation in free-falling granular streams has been studied experimentally and numerically, mainly for spherical particles. We experimentally investigate the growth process and the physical properties of the clusters, especially the packing fraction, in the hope that these clusters can be used to study the collisional process of dust agglomerates in the early solar system and planetary rings. In a comparison of granular streams of spherical and irregular particles, we showed that the stream of irregular particles evolved faster and formed clusters of larger sizes with lower packing fraction. 

Towards more detailed modeling of galaxy formation

Speaker: Takashi Okamoto

Abstract: 

Current cosmological simulations of galaxy formation have been successful in explaining many observed properties of galaxies. There are, however, some tensions between the simulations and observations. I briefly describe the current status of simulations of galaxy formation. I then introduce our recent attempts to develop new star formation recipes in galaxy scale simulations. The new method allows us to identify star-forming clouds and detect collisions between the clouds during galaxy-scale simulations. I show some preliminary results obtained by a collision-induced star formation model. 

Challenges in analyses of exo-atmospheres in the era of JWST and Ariel

Speaker: Quentin Changeat

Abstract: 

With the recent launch of the James Webb Space Telescope and with later space missions such as Ariel, the study of exoplanet atmospheres will be revolutionised. The complexity of interpreting exoplanet spectra, often tackled via atmospheric retrieval techniques, however, will increase and pose many new challenges. In this talk, a review of current analysis techniques as well as their potential evolution will be presented.

Dense core collision using MHD simulation 

speaker: Shinichi Kinoshita

Abstract:

The collision between clouds in the ISM is an important phenomenon to study, and there is increasing observational evidence that cloud–cloud collisions trigger stars and cluster formation in the Milky Way. Observational evidence of triggered star formation by cloud-to-cloud collisions comes mainly from relatively large scale star formation events that produce massive stars and clusters. Therefore, many numerical studies to date have been concerned with collisions between high mass clouds. However, in turbulent molecular clouds, a core collision may occur on a smaller scale. In turbulent molecular clouds, due to the randomness of gas velocities, small over-densities can be formed throughout the cloud and dense core collisions can occur due to the velocity dispersion between them.   We investigate, using three-dimensional magnetohydrodynamical (MHD) simulations, core collisions between two stable dense cores.  In our simulations, we considered the effect of magnetic fields on the collision process and focused on their observational features. In this presentation, I will discuss the evolution of dense core collisions and comparison with observational data. 

A new measurement method of isotopologue ratios in protoplanetary disks: a case study of the 12CO/13CO ratio in the TW Hya disk

speaker: Tomohiro Yoshida

Abstract:

Planetary systems are thought to be born in protoplanetary disks. Isotope ratios are a powerful tool for investigating the material origin and evolution from molecular clouds to planetary systems via protoplanetary disks. However, it is challenging to measure the isotope (isotopologue) ratios, especially in protoplanetary disks, because the emission lines of major species are saturated. We developed a new method to overcome these challenges by using optically thin line wings induced by thermal broadening. As a first application of the method, we measured the 12CO/13CO ratio in the TW Hya protoplanetary disk. 12CO/13CO was estimated to be ~23, which is significantly smaller than the typical ISM value (~69) in the inner disk. In contrast, it is suggested that 12CO/13CO is higher than >100 in the outer disk. Our results imply that the gas-phase 12CO/13CO can vary by a factor of >5 even inside a protoplanetary disk, and therefore, can be used to trace material evolution in disks.

The Physics of Fast Radio Bursts

Speaker: Bing Zhang

Abstract: 

Fast radio bursts (FRBs) are mysterious cosmological millisecond-duration bursts in the radio band. In this talk, I will review the latest observational status in the field including the identification of a Galactic magnetar as the source of FRBs. I will also review our current understanding of the physical mechanisms of FRBs in reference to two related astrophysical phenomena, namely, radio pulsars and gamma-ray bursts. I will discuss the observational evidence in favor of FRB emission involving neutron star magnetospheres. Some ideas and issues of various radiation mechanisms for FRBs will be critically discussed. Several open questions in the field regarding repeaters vs. non-repeaters and whether there are engines other than magnetars will be presented.

4 year status of research conducted via the M2O

speaker: Ross Burns

Abstract:

The maser monitoring organisation, commonly called the ‘M2O’ has been responding to maser flares since September 2017. In this colloquium a summary of the research conducted in this time, including published results, multi-wavelength and cross-facility collaboration, in addition to work done developing and supporting new and longstanding observational facilities will be presented. Based on past and present activities the future direction of the M2O will be considered in the context of upcoming missions and likely funding scenarios that observatories may encounter.

The Role of Filament-Filament Collision in Star Formation

speaker: Raiga Kashiwagi

Abstract:

Filamentary structures are recognized as a fundamental component of interstellar molecular clouds in observations made by the Herschel satellite.  In addition, most of the prestellar cores, which will become protostars in the future have been found along gravitationally unstable filaments. These observational results have led to a strong interest in the relationship between filaments and star formation. Recently, some observations imply that star formation is caused by filament-filament collisions. However, theoretical predictions are still insufficient: how they will evolve and which physical parameters change their evolution. Thus, we investigate filament-filament collisions by using numerical simulations. In this seminar, I will report the preliminary results of  2d-hydrodynamical simulations, and discuss the detailed evolution of filament-filament collisions.

The possibility of core growth with mass accretion and the core mass calculation method for continuum observations

speaker: Hideaki Takemura

Abstract:

Stars are formed in dense regions which are called dense cores in the molecular clouds. Thus, it is crucial to understand how dense cores form from parental molecular clouds to reveal star formation processes. Since the evolution process of a star strongly depends on the mass, the mass is one of the most important properties of a dense core as well. The mass functions of dense cores (CMFs), the mass spectrum of dense cores, are expected to have information of the core evolution and star formation processes. Many previous studies of CMFs toward nearby star-forming regions with single-dish telescopes have suggested that the CMFs resemble the stellar IMFs. However, top-heavy CMFs are reported from recent ALMA observations toward distant high-mass star-forming regions.  Therefore, the processes that link CMF and IMF have not been revealed. In this presentation, I will discuss the possibility of core growth suggested from the comparison of CMF and IMF in the Orion Nebula Cluster region and the core mass calculation method for dust continuum observation of ALMA.

Turbulent Lives of Stars

speaker: Fabian Schneider

Abstract:

Stars are the basic building blocks of the visible Universe. Understanding how they transformed the pristine Universe into the one we live in today is at the heart of astrophysical research and vital for many areas in astrophysics. For example, massive stars are cosmic powerhouses and their immense radiation, strong stellar winds and powerful supernova explosions helped to re-ionise the Universe after the Dark Ages, drive the evolution of galaxies and laid the foundation for life as we know it. At the end of their lives, stars produce compact objects whose mergers are now routinely observed thanks to gravitational-wave observatories. Yet, our understanding of the lives and final fates of stars is seriously incomplete. I will review some pressing challenges in stellar astrophysics with a particular emphasis on massive binary stars. In binary systems, stars can exchange mass, thereby completely changing their evolution and final fates. Stripping off the envelope of a massive star may allow it to explode in a supernova and produce a neutron star instead of collapsing into a black hole. In many cases, binary mass transfer results in stellar mergers and common-envelope episodes. I will present simulations that show the emergence of strong magnetic fields in such dynamic and turbulent phases with possibly far-reaching consequences. Merged stars may be highly magnetic and the progenitors of the strongest magnets in the Universe, so-called magnetars. In common-envelope episodes, the magnetic fields can drive bipolar, jet-like outflows and may help explain the shapes of asymmetric planetary nebulae.