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  第1752回 天文学教室談話会

2023年4月11日(火) 16:15-17:15

Title: X-ray study on the synchrotron emission in Kepler’s SNR
Speaker: Vincenzo SAPIENZA (University of Palermo, staying at Department of Physics, UTokyo)
Language: English
Abstract:
Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high energy electrons accelerated at the shock front. We performed a spatially resolved spectral analysis of the young Kepler’s SNR, where we identified two different regimes of particle acceleration. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with dense CSM. We also studied the temporal evolution of the synchrotron flux, from 2006 to 2014. A number of regions show a steady synchrotron flux and equal cooling and acceleration times. However, we found some regions where we measured a significant decrease in flux from 2006 to 2014. Our results display a coherent picture of the different regimes of electron acceleration observed in Kepler’s SNR.

  第1753回 天文学教室談話会

2023年4月18日(火) 16:15-17:15

Title: Atomistic insights on the chemistry of the interstellar medium
Speaker: German Molpeceres (天文学教室)
Language: English
Abstract:
Despite the harsh physical conditions in the dense interstellar medium, there is plenty of evidence that these regions act as the onset of organic chemistry. This chemistry arises from the interplay between gas-phase reactions and reactions on ice-coated dust particles. In most cases, the chemistry that takes place in these regions is unconventional for Earth-based standards and is hardly predictable. Hence, astrophysical models require of abundant input from experiments and theoretical simulations. In this talk, I will review my contributions to the theoretical investigation of gas phase and surface chemistry operating in dense molecular clouds, emphasising the latter. Finally, I will connect my past investigations with our recent efforts to understand the chemistry occurring on ices different from water ice.

  第1754回 天文学教室談話会

2023年4月25日(火) 16:15-17:15

Title: Initial mass function and star cluster dynamics: the impact of massive stars
Speaker: Long Wang (Sun Yat-sen University 中山大学、中国)
Link(s): http://orcid.org/0000-0001-8713-0366
Language: English
Abstract:
Massive stars have a significant impact on the dynamical evolution of star clusters. They play a crucial role during star formation, as their radiation can push surrounding gas away and inhibit further star formation. Additionally, strong mass loss from massive stars via strong winds can rapidly reduce the gravitational potential of star clusters and trigger their fast expansion. Once these massive stars evolve into black holes, they continue to drive the expansion of the cluster by forming binary black holes at the center. Recent observations from Gaia have provided abundant kinetic data on stars, which can be used to identify the long stellar streams of star clusters. In this talk, we will explore how these observations can help us to constrain the initial conditions of star clusters, particularly the properties of initial mass functions, by analyzing the dynamical impact of massive stars.

  第1755回 天文学教室談話会

2023年5月9日(火) 16:15-17:15

Title: Chemical abundances of red supergiants over a large area of the Galactic disk
Speaker: Daisuke Taniguchi 谷口大輔 (National Astronomical Observatory of Japan 国立天文台)
Link(s): https://d-taniguchi-astro.github.io/homepage/index_en.html
Language: Japanese 日本語
Abstract:
Our Galaxy, also referred to as the Milky Way, is the “nearest” galaxy in the Universe, making it an ideal laboratory for investigating the (chemical) evolution of galaxies. This advantage has enabled us to measure elemental abundances, such as [Fe/H] and [Mg/Fe], of individual stars within several kpc from our Solar system. A natural step forward is to expand observations to more distant objects, such as those located at the tip of the Galactic bar (bar end), at the Galactic center, or even in nearby galaxies. To observe stars located in such far regions (up to ~1 Mpc) with high-resolution spectrographs, it has recently been proposed to use red supergiants (RSGs) as a luminous (>10^4 Lsun) tracer of chemical abundances. In this talk, I will present our recent study to measure chemical abundances of RSGs, utilizing the near-infrared high-resolution, high-throughput spectrograph WINERED (YJ bands; 0.91-1.35 micron). This wavelength range has an advantage of the weakest strengths of molecular lines contaminating to atomic lines. We have developed and tested a method to determine effective temperatures and chemical abundances of RSGs including amongst others Fe, Mg, and Y. Moreover, we have measured the chemical abundances of four RSGs located at around the Galactic bar-end region, and confirmed that their metallicities are lower than expected from the radial metallicity gradient of the Galactic disk.

  第1756回 天文学教室談話会

2023年5月16日(火) 16:15-17:15

Title: The molecular composition of shadowed proto-solar disk midplanes beyond the water snowline
Speaker: Shota Notsu 野津翔太 (Department of Astronomy 天文学教室)
Link(s): Notsu et al. 2022, ApJ, 936, 188, Website of Shota Notsu:
Language: English
Abstract:
Planets form and obtain their compositions in protoplanetary disks around young stars. The chemical compositions of gas and solid dust grains in these planet-forming disks will decide planetary compositions. The thermal structure in the protoplanetary disk plays an important role in controlling the disk chemical structure. The disk midplane temperature is potentially affected by the disk substructures such as dust traps/rings, which have been detected with recent ALMA observations. The dust depletion beyond the water snowline will cast a shadow (Ohno & Ueda 2021).
In our study (Notsu et al. 2022, ApJ, 936, 188), we adopted a detailed gas-grain chemical reaction network, and investigated the radial gas and ice abundance distributions of dominant carbon-, oxygen-, and nitrogen-bearing molecules in disks with shadow structures beyond the water snowline around a protosolar-like star. We also investigated the dependance of the disk chemical structures on ionisation rates and initial abundances.
In shadowed disks, the dust grains at r∼3−8 au are predicted to have more than ∼5−10 times amounts of ices of organic molecules such as H2CO, CH3OH, and NH2CHO, saturated hydrocarbon ices (such as CH4 and C2H6), in addition to H2O, CO, CO2, NH3, N2, and HCN ices, compared with those in non-shadowed disks which are composed mostly H2O, CO2, and NH3 ices. In the shadowed regions, we find that hydrogenation (especially of CO ice) is the dominant formation mechanism of complex organic molecules, rather than radical-radical reactions and gas-phase reactions. The gas-phase N/O ratios show much larger spatial variations than the gas-phase C/O ratios, and thus the N/O ratio is predicted to be a useful tracer of the shadowed region. N2H+ line emission is a potential tracer of the shadowed regions beyond the water snowline. We conclude that a shadowed region allows the recondensation of key volatiles onto dust grains, provides a region of chemical enrichment of ices that is much closer to the star than within a non-shadowed disk, and may explain to some degree the trapping of O2 ice in dust grains that formed comet 67P/Churyumov-Gerasimenko. We discuss that in the shadowed disks, Jupiter does not need to have migrated vast distances, and complex organic molecules can be formed in situ rather than being fully inherited from molecular clouds.
In this talk, first I will briefly review recent topics about astrochemistry in star and planet forming regions, and then present our recent modeling studies about molecular composition of shadowed disk midplanes (Notsu et al. 2022). We also shortly discuss future prospects for molecular line observations of protoplanetary disks with much higher spatial resolutions (ALMA and ngVLA).

  第1757回 天文学教室談話会

2023年5月23日(火) 16:15-17:15

Title: Star Formation Quenching in Galaxy Clusters and Their Progenitors
Speaker: Makoto Ando 安藤誠 (Department of Astronomy 天文学教室)
Language: Japanese 日本語
Abstract:
It is well known that galaxies that dwell in denser environments are more likely to be quenched (i.e. star formation is stopped), suggesting that surrounding environments influence galaxy evolution, referred to environmental effect. Galaxy clusters, the densest environments in the Universe, are dominated by the quenched population, which is thought to be built up through many environmental effects. Therefore, they are good laboratories to test our understanding of how dense environments quench galaxy star formation. In this talk, I will present our research on galaxy evolution in clusters and their progenitors, referred to protoclusters.
In the first part, I will present the results of a systematic search for protoclusters at z∼2 to reveal when the quenched population emerges in (proto)clusters. We find the excess of the quenched galaxy fraction in protoclusters compared to the general field at z∼2. Still, the excess is insignificant at z>1, suggesting that significant quenching may occur at the later epoch when whole protocluster structures violently collapse to the core regions. In the second part, I will discuss detecting the anisotropic distribution of quenched galaxies in matured clusters up to z∼1. The detected anisotropy infers that some fast quenching mechanisms, like ram-pressure stripping, work anisotropically within clusters.