IoA_Seminar

 

講演リスト

No. 393: 22 November 2022 (Tue.), 16:15-17:15

Speaker: Zsolt Keszthelyi (国立天文台)

Title: The effects of surface fossil magnetic fields on massive star evolution

Language: English

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 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

No. 392: 15 November 2022 (Tue.), 16:15-17:15

Speaker: Anish Amarsi (Department of Physics and Astronomy, Uppsala University)

Title: Solar and stellar spectroscopy using 3D non-LTE radiative transfer

Language: English

Abstract: Stars leave their signatures on the light that they emit, in the form of spectral lines. These signatures can be decoded to reveal the fundamental parameters and chemical compositions of stars, which can then shed light on the structure and evolution of stars, and of the Galaxy. However, such spectroscopic analyses are heavily model dependent. Shortcomings in the models often restrict the accuracy of the final results. For late-type stars like our Sun, two of the main problems in present-day methods are that they assume that stellar atmospheres are a) one-dimensional (1D) and hydrostatic, and b) satisfying local thermodynamic equilibrium (LTE). These assumptions may be relaxed simultaneously, by performing detailed 3D non-LTE radiative transfer post-processing of 3D radiative-hydrodynamic model stellar atmospheres. I shall describe the method, before discussing their application to understand the solar chemical composition, in the context of the solar modelling problem. I shall then present results on carbon, oxygen, and iron abundances in a sample of around 200 stars in the Mlky Way disk and halo, for which we find the 1D LTE and 3D non-LTE analyses to imply different conclusions about the chemical evolution of our Galaxy, and the compositions of planet-hosting stars. Finally, I shall discuss our efforts to use 1D non-LTE and 3D non-LTE models to surveys of millions of stars, including GALAH and 4MOST.

No. 391: 11 November 2022 (Fri.), 13:15-14:15

Speaker: Victor Debattista (University of Central Lancashire)

Title: The formation of the box/peanut bulge of the Milky Way

Language: English

Abstract: The bulge of the Milky Way constitutes some 25-30% of its stellar mass. Understanding how it formed therefore is vital to understanding galaxy formation in general. Historically it has been thought that the bulge is comprised of an accreted component and an in-situ formed population, varying in the shape, rotation rate, and metallicities. I will demonstrate how these properties arise naturally in a bulge that formed wholly in situ, with only the minor contamination of the stellar halo. I further show how the chemical bimodality of the bulge can be understood if its chemical evolution includes an episode of high star formation rate density clumps, as observed in high redshift galaxies. Taken together these facts point to a bulge that is substantially just the thickened part of the bar.

No. 390: 25 October 2022 (Tue.), 16:15-17:15

Speaker: Takami Kuroda (Max Planck Institute)

Title: The death of massive stars and the birth of various compact stars.

Language: English

Abstract: The final fate of massive stars is the moment of birth of diverse compact stars: neutron stars (NSs), black holes (BHs), magnetars, and exotic stars. Such diversity depends strongly on the explosion (i.e. core-collapse supernova, CCSN) mechanism as well as on the progenitor mass. Although recent CCSN simulations have gradually revealed the explosion mechanism of less massive stars (<~20 Msun), which are considered to leave NSs behind, our understanding of the fate of more massive stars (>~40 Msun) is remaining patchy and incomplete. A major reason for this incompleteness is due to the difficulty of CCSN simulations in full general relativity (GR), which is particularly important for more massive stars. Utilizing our original state-of-the-art GR neutrino-radiation code and employing an up-to-date EOS considering the hadron-quark phase transition, we have recently reported for the first time how various compact stars can be formed. Our results suggest that less massive stars may successfully explode by the standard neutrino heating mechanism and leave NSs behind, while very massive stars (>~70 Msun) tend to fail and their remnants are BHs. This picture is basically the same as our previous understanding. The most noteworthy is that some modest massive stars (~40-50 Msun) could explode when their nascent NS experiences the phase transition and forms a quark core surrounded by normal hadronic mattes, i.e., the so-called hybrid star. The explosion is so strong that such type of explosion could be a potential candidate for super-luminous SNe. In this talk, I will present our latest CCSN simulations and explain a possible scenario of various compact star formations and their multi-messenger signals.

No. 389: 18 October 2022 (Thu) 15:30 - 16:30

Speaker: 小林 将人 (国立天文台)

Title: Lifecycle of molecular clouds driven by HI gas accumulation

Language: English

Abstract: Molecular clouds host star formation so that they are the key structure to determine the galactic star formation. Recent ALMA observations reveal the statistics of molecular cloud populations in nearby galaxies (e.g., molecular cloud mass functions), as well as their inner structures on sub-pc scales. The connection between such large-scale statistics and inner structures is still poorly understood. To investigate the formation and evolution of molecular clouds, we calculate a semi-analytic evolution of molecular cloud mass functions and also perform a series of MHD simulations of supersonic converging HI gas flows. We find that many of the observed properties of molecular clouds are inherited from the cold component of HI gas, so called the cold neutral medium. These results suggest the importance of HI gas accumulation to drive molecular cloud formation and subsequent star / star cluster formation. Our results also indicate that most of molecular clouds die in 5 Myr once they start to host massive stars, but only a limited population without significant massive star formation survive to grow to 10^6 Msun clouds, which takes ~ 100 Myr. If time allows, we may briefly mention some indications to massive molecular clouds at high-redshift star-forming galaxies, based on the metallicity-dependence measured in our simulations.

No. 388: 28 July 2022 (Thu) 15:30 - 16:30

Speaker: 細川 隆史(京都大学)

Title: High-mass star formation in the early universe and their analogous observational signatures in the present-day universe

Language: English

Abstract: I present our recent efforts toward understanding the high-mass star formation across cosmic history. The extreme case is the Pop III star formation, for which we have continuously conducted multi-dimensional radiation-hydrodynamic simulations for a decade. Our latest updates allow us to study the evolution in the protostellar accretion stage, where the fragmentation of the star-forming gas and radiative feedback from accreting protostars operate in concert. Whereas simulations reveal the Pop III star formation in great detail, directly detecting it is beyond the abilities of upcoming observational facilities. An alternative way is searching for observational signatures of the nearby analogues in their formation stage. One of such candidates is young massive clusters containing many metal-poor high-mass stars. I also present our ongoing study to extract their possible observational signatures based on numerical simulation results.

No. 387: 21 July 2022 (Thu) 15:30 - 16:30

Speaker: Mason Leist (University of Texas San Antonio)

Title: Resolving Polar Dust in AGN with JWST: Going Beyond the PSF

Language: English

Abstract: The launch of the James Webb Space Telescope (JWST) promises to revolutionize infrared astronomy and our understanding of inflows and outflows in active galactic nuclei (AGN). David Rosario (Newcastle University, U.K.) has been awarded JWST science time to explore and characterize diffuse polar dust emission found in AGN using JWST Mid-infrared Instrument (MIRI) imaging. The Galactic Activity, Torus and Outflow Survey (GATOS2) collaboration will have access to these data, of which I am a member. Thanks to JWST’s exquisite low surface brightness sensitivity in the mid-infrared (MIR; 5-25 μm) observations of the diffuse polar dust emission found in AGN will be enabled unprecedented sensitivity. Relying on JWST’s stable PSF, we plan to use deconvolution to establish the structure of this diffuse emission below the resolution of the telescope. We will explore five different deconvolution techniques and select the best method based on comparisons of the flux conservation, FWHM, and Strehl ratios of the deconvolved images with an input model. To explore each technique, we have used the MIRI simulation software (MIRISim) to simulate JWST’s complex PSF convolved with a toy model of an AGN consisting of a resolved bicone and an unresolved AGN point source. Here I discuss the preliminary results on our assessment of the optimum deconvolution strategy.

No. 386: 12 July 2022 (Tue) 16:15 - 17:15

Speaker: Yutaro Kofuji(Department of Astronomy, The University of Tokyo)

Title: Imaging of the Supermassive Black Hole in Our Galaxy, Sgr A* with the Event Horizon Telescope

Language: English

Abstract: The first event horizon scale image of the supermassive black hole in our Galaxy, Sgr A* was captured by the Event Horizon Telescope(EHT) Collaboration. EHT is the Very Long Baseline Interferometry (VLBI) that links radio dishes around the world to create an Earth-sized telescope virtually. High-resolution observations of EHT enable us to see the vicinity of the black hole. We analyzed the reconstructed images and concluded that it is highly likely that Sgr A* has ~50 uas ring structure and this is consistent with the shadow of the Kerr black hole which weighs ~4 million solar masses. In this seminar, I would like to summarize the major results, then explain the details of our imaging process and briefly introduce the theoretical interpretations of our images.

No. 385: 7 July 2022 (Thu) 15:30 - 16:30

Speaker: Dr. Paul Hertz (NASA Astrophysics Division Director)

Title: NASA Astrophysics: From the past to the future

Language: English

No. 384: 5 July 2022 (Tue) 16:15 - 17:15

Speaker: 伊藤 慧(東京大学天文学教室)

Title: Environment and AGN activity of distant quiescent galaxies revealed by multi-wavelength surveys in the COSMOS field

Language: Japanese

Abstract: Recent multi-band observations have found that galaxies with suppressed star formation activity exist even in the high redshift universe. The state-of-art spectrograph has now confirmed them up to z~4. On the other hand, it is not well understood why they get quenched at such a high redshift, even though the cold streams are expected to supply gas. One of the preferable quenching mechanisms is the feedback from active galactic nuclei (AGNs). In addition, it has been challenging to see how quenching is connected to their living environment inside the large-scale structure of the distant universe. The cosmic evolution survey (COSMOS) is one of the preferable datasets to reveal these points since it has both a large survey area and deep multi-band photometry. In this talk, I will introduce our recent two papers on quiescent galaxies at high redshift. The first explores the spatial distribution differences among massive quiescent galaxies, mass star-forming galaxies, and Lyα emitters at 2<z<4.5. The second conducts a stacking analysis of X-ray and radio images for massive quiescent galaxies up to z~5 to explore the contribution of AGNs to quenching.

No. 383: 21 June 2022 (Tue) 16:15 - 17:15

Speaker: Paul K. H. Yeung(東京大学物理学教室)

Title: Cosmic-ray Acceleration & Escape from Intermediate-Aged Supernova Remnant Kes 79

Language: English

Abstract: Hadronic γ-ray sources of supernova remnant — molecular cloud (SNR–MC) interactions can serve as stopwatches for the escape of cosmic rays (CRs) from SNRs, which gradually develops from highest-energy particles to lowest-energy particles with time. Recent multiwavelength studies confirmed the interaction of the intermediate-aged SNR Kes 79 with molecular clouds. In this work, we analyze the 11.5 yr Fermi-LAT data to investigate the γ-ray feature in/around the Kes 79 region. With ≥5 GeV data, we detect two extended sources: Src-N (the brighter one; radius ≈0.31°) concentrated at the north of the SNR while enclosing a powerful pulsar — PSR J1853+0056, and Src-S (radius ≈0.58°) concentrated at the south of the SNR. Their spectra have distinct peak energies (≈1.0 GeV for Src-N and ≲0.5 GeV for Src-S), suggesting different origins for them. In our hadronic model that includes the leaked cosmic rays (CRs) from the shock-cloud collision, even with extreme values of parameters, SNR Kes 79 can by no means provide enough CRs reaching clouds at Src-N to explain the local GeV spectrum. We propose that the Src-N emission could be predominantly reproduced by a putative pulsar wind nebula powered by PSR J1853+0056. On the other hand, our same hadronic model can reproduce a majority of the GeV emission at Src-S with typical values of parameters. Also, SNR Kes 79's relatively softer γ-ray spectrum peaked at a relatively lower energy is in agreement with its intermediate age, based on the modelled evolution of cosmic-ray escape from an SNR.

No. 382: 6 June 2022 (Thu) 15:30 - 16:30

Speaker: 堀内 貴史(東京大学天文学教育研究センター)

Title: Evaluation of magnitudes of the Starlink satellites by simultaneous multicolor observations

Language: English

Abstract: The U.S. company, SpaceX plans to launch 42,000 Starlink satellites by the mid-2020s. However, these satellites orbit at relatively low altitudes (e.g. 550 km), and there are concerns that the light pollution from sunlight reflection likely affects observations. In January 2020, SpaceX launched a test satellite, Darksat with a black coating on its surface to reduce the reflection flux. In order to verify the effect of its black coating, we observed Darksat and unpainted Starlink satellites by simultaneous multicolor observations (g’: green, Rc: red, and Ic: near-infrared) with the 105 cm Murikabushi telescope/MITSuME. While the magnitude of Darksat is about 7, being difficult to see with the naked eyes, this brightness is sufficient to affect any observations. In June 2020, SpaceX launched Visorsat, which is a satellite with a sun visor to reduce the reflected sunlight. At the present stage, there is no sufficient verification on whether the sun visor is effective at various wavelengths. In this study, we are therefore conducting the simultaneous multicolor observations of Visorsat and the ordinary Starlink satellites through the OISTER campaign. The preliminary observations of Visorsat with the Murikabushi telescope have already shown that the apparent magnitude of Visorsat is not fainter than that of Darksat, ranging from 6.5 mag (g' band) to 5.7 mag (Ic band). Since most of the satellites to be launched by SpaceX in the future will be visor satellites, it is very important to investigate the effects of the Starlink satellites as soon as possible to reduce the light pollution in astronomical observations.

No. 381: 10 May 2022 (Tue) 16:15 - 17:15

Speaker: 西村優里(東京大学天文学教室) / Yuri Nishimura (University of Tokyo, Department of Astronomy)

Title: Molecular spectroscopy in local and high-redshift galaxies: What do "dense gas tracers" mean?

Language: Japanese

Abstract: Thanks to the technical advancement of (sub)millimeter observing facilities, a great number of molecular lines are now routinely observed with high sensitivity and high angular resolution. These molecular lines and line ratios are useful tools for studying physical, kinematic, and chemical properties of extragalactic systems. To fully exploit line diagnostics, it is important to relate different size scale observations: detailed understanding of nearby systems by spatially-resolved observations is essential to better interpretation of more distant objects which cannot be observed at the same physical resolution. In this talk, I will present (1) 10 pc-scale multiline mapping toward a Galactic star-forming region W3(OH); (2) spatially- and spectrally-resolved HCN and HCO+ observations of local (ultra)luminous infrared galaxies from the CONquest sample (for more detail on CONquest, see Falstad+2021); and (3) ALMA Band 3 (rest-frame ~350 GHz) line survey toward the Cloverleaf, a gravitationally-lensed quasar at z=2.56. I will first discuss which molecular emission arises from which part of the molecular cloud, and then focus on how galactic-scale dynamics, such as outflows, can alter line ratios.


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Last-modified: 2022-11-29 (火) 00:52:45