![[PukiWiki]](image/pukiwiki.png "[PukiWiki]")
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.
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.
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.
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.
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.
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.
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.
