投稿者: Yuko Matsushita

Numerical study on star formation process induced by collisions between filamentary molecular clouds

speaker: Raiga Kashiwagi

Abstract:

Recent observations have revealed that filamentary molecular clouds are closely related to star formation. In particular, where the filaments appear to overlap, active star formation has been observed. One possible interpretation is that the filaments collide with each other and then star formation is triggered. Therefore, we focus on collisions between filamentary molecular clouds and investigate these evolutionary processes by using 2D hydrodynamical simulations. We assumed a head-on collision of isothermal filaments in hydrostatic equilibrium. One of the main results is that the critical line mass, which governs the radial gravitational instability of the filament, also governs the radial gravitational instability of the shocked region, which is formed by the collision. In other words, if the total line mass of the initial filaments exceeds the critical line mass, we can expect that filament-filament collisions trigger a radial gravitational collapse of the shocked region. On the other hand, when the total line mass is less than the critical line mass, the equilibrium filament will be formed in the shocked region. In this presentation, I will report not only the results of the hydrodynamic calculation but also the magnetohydrodynamic calculation and discuss the properties of the magnetic field in filament-filament collisions.

The origin of the proton-rich and neutron-rich isotopes

speaker: Yuta Yamazaki

Abstract:

The origin of heavy elements is still unclear. The process and astrophysical site of the nucleosynthesis of neutron-rich and proton-rich nuclei have long been under debate. This study tries to reveal the origin of these nuclei through galactic chemical evolution calculations. Neutron-rich nuclei are synthesized by a fast neutron capture process, r-process. The observational evidence suggests that galactic stars have very similar compositions of the elements synthesized in this process. We have discovered the possibility that there is diversity in the mass A-distribution, while maintaining the universality of the elemental abundance. This also allows us to quantitatively discuss the contributions from multiple candidates of r-process sites. For the source of proton-rich nuclei, the gamma process, which has been studied for a long time, is difficult to produce a sufficient amount of. The vp-process involving neutrinos has recently been widely studied as a candidate of proton-rich production process. In general, it is difficult to find observational evidence of proton-rich nuclei nucleosynthesis because of their low fraction in the elemental abundance. However, we have discovered the possibility of observing a contribution from the vp process to the Molybdenum elemental abundances of metal-poor stars in galaxies.

Aspherical Shock Breakout

speaker: Chris Irwin

Abstract:
The first light that emerges from a supernova is the shock breakout emission, which in the spherical case is characterized by a brief UV flash. In the case of an axisymmetric, prolate explosion, the shock first breaches the surface along the symmetry axis, then marches towards the equator; the resulting breakout light curve may differ substantially from the spherically symmetric case. We study the properties of the bolometric light curve associated with the emergence of a non-relativistic, bipolar shock from a spherical star, and identify four possible classes of breakout light curves, depending on the degree of asphericity. Compared to spherical breakouts, we find that significantly aspherical breakouts are distinguished by 1) a longer and fainter initial breakout flash and 2) an extended phase of slowly-declining emission which is produced as ejecta flung sideways during the breakout expand and cool. We show that, even when allowing for asphericity, the duration of the breakout flash cannot exceed roughly ∼R_∗/v_{bo}, where R_∗ is the stellar radius and v_{bo} is the velocity of the fastest-moving ejecta. Applying this result, we find that the long duration of the X-ray flash seen in SN 2008D cannot be explained as an aspherical breakout from a standard Wolf-Rayet star, and the same is true for the prompt X-ray emission associated with low-luminosity GRBs such as GRB 060218. We therefore suggest that these events originate from non-standard progenitors with larger radii.

Hydrogen line emission from accreting gas-giant planets

speaker: Yuhiko Aoyama

Abstract:
Gas giant planets such as Jupiter are mainly composed of hydrogen gas. They form in the circum-stellar gaseous disk (so-called protoplanetary disk) by gathering its surrounding gas. Recent instruments have successfully detected forming giant planets embedded in the protoplanetary disk by direct imaging. Particularly, the detection of hydrogen lines including H-alpha (Balmer-alpha) is a key to understanding their formation. Hydrogen lines require hot gas of ~10^4 K. However, at the hydrogen line emitting planets, the photospheric temperature is estimated as a few thousand K by IR observation. Therefore, hydrogen lines are thought to come from the gas accretion towards the forming planets.
In this talk, I will show our numerical model of hydrogen line emission at the accretion shock on the planetary surface. We confirmed the shock emission can reproduce the observation and estimated their accretion properties such as mass accretion rate by the flux and spectral profile of the observed hydrogen lines. Then, I will introduce the competitive line emission model of “magnetospheric accretion column” and how we can distinguish the emission mechanism with some future observations.

Comparison of N-body simulations of Earth-like planets with observations

speaker: Hoshino Haruka

Abstract:
Many exoplanets have been discovered so far. Among them, Earth-like planets are the majority of the population. It is also known that short-period Earth-like planets around low-mass stars are the main targets of the study to understand planet formation. We investigated the effect of stellar mass on the structure of planetary systems. We explored whether the observed results can be reproduced by N-body simulations using the MMEN model, which is a disk model based on observations. Although the number of observed planets around low-mass stars is still small, we discuss whether it can be used as a prediction for future observations.

Observability of magnetic field in lensing galaxies using broadband radio polarization data

speaker: Omae Rikuto

Abstract:
External galaxies often intervene in front of background radio sources such as quasars and radio galaxies. Linear polarization of the background emission is depolarized by Faraday rotation of inhomogeneous magnetized plasma of the intervening galaxies. Exploring the depolarizing intervening galaxies can be a powerful tool to investigate the cosmological evolution of the galactic magnetic field. Recently, Mao et al. (2016) detected coherent μG magnetic fields in the lensing disk galaxy by exploiting the scenario where the polarized radio emission from a background source is gravitationally lensed by an intervening galaxy using broadband radio polarization data. The method is based on the difference in Faraday depths where the background source emission passes through different positions of the intervening galaxies due to the gravitational lensing effect. Using a galactic magnetic field model, we investigate how background polarized sources are observed due to gravitational lensing effects of intervening galaxies. We will discuss whether spectral changes and galactic magnetic fields can be estimated when RM structures are present in the region through which the background source emission passes.

Construction of dense core mass function with 3D MHD simulation data

speaker: Hideaki Takemura

Abstract:

Stars are formed in 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 (Core Mass Functions, 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 mass functions (Initial Mass Functions, IMFs). However, top-heavy CMFs are reported from recent ALMA continuum observations toward distant high-mass star-forming regions. Therefore, more detailed study is necessary to reveal the link CMF and IMF. As a first step, we compare CMFs constructed with 3D density data and 2D column density data of molecular cloud made from a result of 3D MHD simulation. In this presentation, I will show the result of dense core survey and discuss the properties of resultant CMFs.

From Arecibo to CRAFTS: Bridging the Centuries

speaker: Di Li

Abstract:

Inspired by the Arecibo telescope, a technical and scientific wonder of the 20th century, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) was originally proposed as part of a SKA concept. Since its operation commenced in 2020, FAST data have resulted in more than 120 peer-reviewed papers, including five on the Nature magazine and one on Nature Astronomy. Such productivity evinces the unparalleled sensitivity of FAST as well as the originality of a series of experimental techniques such as the high-cadence CAL, which enables the Commensal Radio Astronomy FasT Survey (CRAFTS). CRAFTS realizes the world’s first commensal survey of pulsars, HI imaging, HI galaxies, and fast radio bursts (FRBs). Through CRAFTS, we have so far discovered more than 160 pulsars, including one double-neutron-star system (DNS), and 6 new high DM (>1000 pc cm-3) fast radio bursts (FRBs), including one new repeater. The HI images and galaxies from CRAFTS are close to publication quality. I will introduce a few highlights from CRAFTS as well as PI programs, including the first successful HI narrow self-absorption (HINSA) Zeeman detection, which was featured on the cover of Nature, the first detection of the characteristic energy for a FRB source, a unified model describing the frequency evolution of active repeating FRBs, etc. A plan to build an array of FAST-like antennae is being formulated to achieve the point-source sensitivity rivaling that of the SKA. Along with a slew of major survey programs, CRAFTS and FASTA poise to extend the legacy of Arecibo.

Fast Transient Studies with Deep/High-cadence Time-domain Surveys

speaker: Jian Jiang

Abstract:

Transients with fast brightness variance in UV/optical wavelengths (”fast transients,” such as early-phase supernovae, fast blue optical transients, kilonovae) are of great interest in time-domain astronomy. Although great progresses in time-domain astronomy have been made via wide, shallow, and day(s)-cadence surveys, the limited observing depth and low time resolution make traditional time-domain surveys hard for fast-transient studies. In this talk, I will focus on our previous/ongoing studies of early-phase supernovae and fast blue optical transients via deep (Subaru/HSC) and high-cadence (Kiso/Tomo-e) wide-field surveys. As one of the most powerful time-domain survey facilities in the 2020s, I will also briefly talk about the upcoming Chinese 2.5m Wide-field Survey Telescope (WFST) project.

Unveiling non-gray surface of cloudy exoplanets: the influence of wavelength-dependent surface albedo and cloud scattering properties on retrieval solutions

speaker: Fei Wang

Abstract:

Direct-imaging spectra hold rich information about a planet’s atmosphere and surface, and several space-based missions aiming at such observations will become a reality in the near future. Previous spectral retrieval works have resulted in key atmospheric constraints under the assumption of a gray surface, but the effect of wavelength-dependent surface albedo on retrieval has not been shown. We explore the influence of the coupling effect of cloud and wavelength-dependent surface albedo on retrieval performance via modeling suites of Earth-like atmospheres with varying cloud and surface albedo parameterizations. Under the assumption of known cloud scattering properties, the surface spectral albedos can be reasonably recovered when the surface cover represents that of Earth-like vegetation or ocean, which may aid in characterizing the planet’s habitability. When the cloud scattering properties cannot be assumed, we show that the degeneracy between the cloud properties and wavelength-dependent surface albedo leads to biased results of atmospheric and cloud properties. The multi-epoch visible band observations offer limited improvement in disentangling this degeneracy. However, the constraints on atmospheric properties from the combination of UV band (R ~ 6) + visible band (R = 140) are consistent with input values to within 1σ. If short bandpass data is not available, an alternative solution to reduce the retrieval uncertainties would be to have the prior constraints on planetary cloud fraction with less than 20% uncertainty.