天文学教室談話会(2024年度)

last-update:2024/06/20

  第1781回 天文学教室談話会

2024年4月9日(火) 15:00-16:00

Title: Search for phosphine in prestellar cores
Speaker: Kenji Furuya 古家健次 (Department of Astronomy 天文学教室)
Language: English
Understanding in which chemical forms phosphorous exists in star- and planet-forming regions and how phosphorus is delivered to planets are of great interest in terms of the origin of life on Earth. Phosphine (PH3) is considered an important species for understanding phosphorus chemistry, but has never been detected in star- and planet-forming regions.
In this talk, I will present our recent observational and theoretical studies on PH3 in prestellar cores. We performed sensitive observations of PH3 toward low-mass prestellar core L1544 with the ACA stand-alone mode of ALMA. While the line was not detected, the non-detection provides the upper limit to the gas-phase PH3 abundance in L1544. Based on our gas-ice astrochemical modeling, we find the scaling relationship between the gas-phase PH3 abundance and the volatile (gas and ice with larger volatility than water) P elemental abundance for given physical conditions. This characteristic and well-constrained physical properties of L1544 allow us to constrain the upper limit to the volatile P elemental abundance, which is a factor of 60 lower than the overall P abundance in the ISM. Then the majority of P should exist in refractory forms. The volatile P elemental abundance of L1544 is smaller than that in the coma of comet 67P/C-G, implying that the conversion of refractory phosphorous to volatile phosphorous could have occurred along the trail from the presolar core to the protosolar disk through e.g., sputtering by accretion/outflow shocks.

 

  第1782回 天文学教室談話会

2024年4月16日(火) 16:15-17:15

Title: Dust Dynamics and Growth during Star and Disk Formation Using Non-ideal MHD Simulation with Lagrangian Particles
Speaker: Naoto Harada 原田直人 (Department of Astronomy 天文学教室)
Language: Japanese

The first step in planet formation is the collisional growth of dust grains in circumstellar disks. Recent observations suggest the existence of grown dust in protostellar systems during the early stages of star formation. Therefore, it is necessary to understand the star, disk, and planet formation processes in a unified way. For this purpose, we have developed a new numerical simulation code to simultaneously calculate the gas dynamics, the dust dynamics, and the dust growth process. A two-dimensional axisymmetric non-ideal magnetohydrodynamic simulation is performed to compute the process of protostar and disk formation from the gravitational collapse of a molecular cloud core. We also compute the evolution of the dust grains using a Lagrangian formulation, which allows us to track the trajectory and size history of individual dust particles. In this study, we calculated the gas and dust evolution from the formation of the protostar to 1 × 10^4 yr later. While particles in the envelope grew less, particles in the disk grew rapidly to centimeter sizes within the limits of our calculations. Individual particles followed a similar size history after falling into the disk; the small dust grew exponentially, and the growth curve of the large grains became quadratic. The protostellar system is filled with grown dust in the early stage and may have been prepared for planet formation. In addition, in this calculation, large grains were not lifted by the outflow and transported into the envelope, because the launching point of the outflow is slightly away from the disk. This may lead to the diversity of the dust opacity spectral index observed in the envelopes of Class 0/I sources.

 

  第1783回 天文学教室談話会

2024年4月23日(火) 16:15-17:15

Title: Circumgalactic medium around Lyman alpha emitters probed with deep narrowband images
Speaker: Satoshi Kikuta 菊田智史 (Department of Astronomy 天文学教室)
Language: Japanese

The circumgalactic medium (CGM) is a source for a galaxy’s star-forming fuel, the venue for galactic feedback and recycling, and thought to play an important role in galaxy evolution. To characterize the CGM around forming galaxies, we conducted deep imaging observations of Lyα emitters (LAEs), low-mass star-forming galaxies, with the Hyper Suprime-Cam on the Subaru Telescope to detect diffuse Lyα emission from their CGM, or Lyα halos (LAHs) using a narrowband filter which can capture Lyα emission from z = 2.84. The depth of our data, together with the wide-field coverage including a protocluster, enable us to study the dependence of the shape of LAHs on various properties, including Mpc scale environments. UV and Lyα images of 3490 LAEs are extracted, and stacking the images yields surface brightness sensitivity of ~1e-20 erg/s/cm^2/arcsec^2 in Lyα. Dividing the sample according to their photometric properties, we find that LAEs in the central regions of protoclusters appear to have very large LAHs. This could be caused by combined effects of source overlapping and diffuse Lyα emission from cool intergalactic gas permeating the forming protocluster core irradiated by active members. For the first time, we identify “UV halos” around bright LAEs that are probably due to a few lower-mass satellite galaxies. Through comparison with recent numerical simulations, we conclude that, while scattered Lyα photons from the host galaxies are dominant origin for LAHs, star formation in satellites evidently contributes to LAHs, and that fluorescent Lyα emission may be boosted within protocluster cores at cosmic noon and/or near bright QSOs. We would also like to report on Lyα surface brightness profiles of LAEs during the pre- and post-reionization epochs (at z=5.7 and 6.6) using a catalog of LAEs obtained from narrowband data of the recently conducted HSC-SSP survey.

 

  第1784回 天文学教室談話会

2024年4月30日(火) 16:15-17:15

Title: Zooming in on two aspects of star cluster evolution: gas expulsion and dynamics of Cepheid variable stars
Speaker: Frantisek Dinnbier (Department of Astronomy 天文学教室)
Language: English

Star clusters evaporate and eject stars over their entire life-times, which eventually results in their dissolution. In addition, some stars are released soon after the cluster formation when the gravitational potential is lowered as feedback from massive stars pushes residual gas out of the cluster. We attempt to estimate the time-scale of gas expulsion by self-consistent 3D hydrodynamical simulations performed by the AMR code Flash. These models suggest that gas expulsion occurs on the time-scale of ~1 Myr, and that it is necessary to resolve the cluster to individual stars to obtain a more realistic picture of gas expulsion; models assuming spherical symmetry are too idealised. We find that the stars released due to gas expulsion form
a distinctive tidal tail, the properties of which are different from the S-shaped tidal tail forming due to gradual evaporation of stars. We also suggest a new method for estimating the age of star clusters from the orientation of its tidal tail; the main advantage of this method is that it is independent on models for stellar evolution.
In the second part of my talk, I will present results of dynamical evolution of stars which evolve to Cepheids in open star clusters. We investigate the fraction of Cepheids seen in clusters as the function of the initial cluster mass, age, metallicity, and position within the cluster hosting galaxy. Most of the clusters contain only one Cepheid at the given time; only the most massive clusters can contain several Cepheids at the same time. The models predict a higher fraction of Cepheids in clusters (~30%) than what is observed (~10%), which suggests an additional disruption mechanism for clusters, which is most likely interaction with giant molecular clouds. The dynamical environment of clusters has an important influence on evolution of binary Cepheids, 10% of which have exchanged their companion with another binary during their life-time. While orbital period is mostly unaltered, eccentricity of binary Cepheids increases due to the cluster dynamical environment. To account for the observed number of companions to Cepheids despite the significant destruction of binary mid-B stars, it is natural to assume that mid-B stars typically form with a higher companion frequency than expected (at least two companions per a mid-B star). The models also provide an estimate of the number of Cepheids which have coalesced with their companions, as well as the evolutionary types of companions to Cepheids, which are in ~4% of cases compact objects.

 

  第1785回 天文学教室談話会

2024年5月14日(火) 16:15-17:15

Title: The Origin of Short-Lived Radioactive Nuclides in the Early Solar System
Speaker: Ryo Sawada 澤田涼 (Institute for Cosmic Ray Research, UTokyo 東京大学宇宙線研究所)
Language: Japanese

The early solar system contained a short-lived radionuclide, 26Al (its half-life time t1/2 = 0.7 Myr). The decay energy of 26Al is thought to have controlled the thermal evolution of planetesimals and, possibly, the water contents of planets. Many hypotheses have been proposed for the origin of 26Al in the solar system. One of the possible hypotheses is the “disk injection scenario”: when the protoplanetary disk of the solar system had already formed, a nearby (<1 pc) supernova injected radioactive material directly into the disk. Such a 26Al injection hypothesis has been tested so far with limited setups for disk structure and supernova distance, which have treated disk disruption and 26Al injection separately. We have revisited this problem, to investigate whether there are self-consistent conditions under which the surviving disk radius can receive enough 26Al to account for the abundance in the early solar system. Our results place a strong constraint on the disk injection scenario. Rather, we suggest that the fresh 26Al of the early solar system must have been synthesized/injected in other ways. Furthermore, the origin of short-lived radionuclides in the early solar system other than 26Al will also be discussed.

 

  第1786回 天文学教室談話会

2024年5月21日(火) 16:15-17:15

Title: The molecular ISM and astrochemistry in the starburst galaxy NGC 253: The ALCHEMI survey
Speaker: Nanase Harada 原田ななせ (National Astronomical Observatory of Japan 国立天文台)
Language: English

Starburst galaxies are characterized by a very high rate of star formation. The interstellar medium (ISM) in starburst galaxies is expected to have different physical properties from that in the Milky Way due to this star formation activity. This difference should also appear in chemical composition. We conducted the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) to analyze this ISM property in one of the nearest starburst galaxies. It is a spectra scan towards the nearby starburst galaxy NGC 253. It covers a wide frequency range of ALMA Bands 3-7, and was conducted as an ALMA Large Program in Cycle 5. The survey detected about 1500 transitions and more than 100 species. They include complex organic molecules that originated from the hot and dense starburst region. Comparing chemical models and observed chemical abundances, our results show 1-2 orders of magnitude higher cosmic-ray ionization rates than the Galactic Center. They also demonstrate multiple signs of shocks. We also conduct a statistical study called principal component analysis (PCA) to extract physical features from the survey images.
A recent press release of this survey: https://alma-telescope.jp/news/starfactory-202403 (Japanese)
https://alma-telescope.jp/en/news/starfactory-202403 (English)

 

  第1787回 天文学教室談話会

2024年5月28日(火) 16:15-17:15

Title: Roles of Cosmic Rays in Astrophysics: The History of Milky Way
Speaker: Jiro Shimoda 霜田治朗 (Institute for Cosmic Ray Research, UTokyo 東京大学宇宙線研究所)
Language: Japanese

We study the long-term evolution of our galaxy over cosmic time by modeling the star formation, metallicity, cosmic-rays (CRs), outflows, and inflows of the galactic system to obtain various insights into the galactic evolution. Considering the consistencies between the observed Galactic diffuse X-ray emissions (GDXEs) and possible conditions to drive the Galactic wind by CRs, we find that the star formation rate becomes half of the mass accretion rate of the disk. The mass accretion of the disk is modeled by using the results of the dark matter N-body simulations. We find that if the gas accretion is characterized by the dark matter core radius, the rotation curve of the low-mass stars in our galaxy can be explained. These results simultaneously provide consistent amounts of metals and CRs with the current conditions of our galaxy. The most significant model predictions are that there is unidentified accretion flow with a possible number density of ∼ 0.01 /cc and the (part of) GDXEs originated from the diffuse plasma formed by consuming a roughly tenth of supernova explosion energy. The latter is the science case of future X-ray missions; XRISM, Athena, and JEDI. We also discuss further prospects of our results in terms of multi-messenger astrophysics.

 

  第1788回 天文学教室談話会

2024年6月4日(火) 16:15-17:15

Title: Direct Imaging of Exoplanets; from investigation of Jovian planet formation/evolution to future explorations for the Second Earth
Speaker: Taichi Uyama 鵜山太智 (California State University Northridge / Astrobiology Center)
Language: English

Since the first detection of an extrasolar planets (exoplanets), thousands of exoplanets have been reported through a variety of detection techniques. Direct imaging is a unique way proving the presence of exoplanets, providing useful information for their characterizations. I will discuss how direct imaging tackles challenges of controversial planet formation/evolution theories, by presenting current direct imaging observations of young Jovian planets and protoplanetary disks with adaptive optics observation with ground-based telescopes and cycle-1 JWST programs, including my programs with Subaru, Keck, and JWST. I will also introduce prospects of future direct imaging explorations for terrestrial planets and extraterrestrial lives for the next decades.

 

  第1789回 天文学教室談話会

2024年6月11日(火) 16:15-17:15

Title: Recent JWST insights into Little Red Dots and unresolved mysteries
Speaker: Kohei Ichikawa 市川幸平 (Waseda University 早稲田大学)
Language: English

The James Webb Space Telescope (JWST) has unveiled numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGN), even though their survey areas are only ~10^2 arcmin^2, which were previously considered too small for significant AGN discoveries before the JWST launch. This discovery highlights the presence of a dust-reddened AGN population, referred to as little red dots (LRDs), which are more abundant than X-ray selected AGNs that are less influenced by obscuration. In this talk, we summarize the recent observational properties of LRDs and discuss what we can learn from them about the growth of supermassive black holes (SMBHs) in the early Universe at z>=5. The large number density of LRDs indicates that the cosmic growth rate of BHs within this population does not decrease but rather increases at higher redshifts beyond z~6. The BH accretion rate density deduced from their luminosity function is significantly higher than that from other AGN surveys in X-ray and infrared bands. To align the cumulative mass density accreted by BHs with the observed BH mass density at z~4-5, as derived from the integration of the BH mass function, the radiative efficiency must be doubled from the canonical 10% value, achieving significance beyond the >2sigma confidence level. This suggests the presence of rapid spins, with 96% of the maximum limit among these BHs, maintained by prolonged mass accretion instead of chaotic accretion with randomly oriented inflows. Accordingly, we propose a hypothesis that the dense, dust-rch environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts a potential association between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe. If time allows, we will explore the future prospects of wide-area surveys such as UNIONS and Euclid as well as VLA 3 GHz radio surveys, which could further uncover high-luminosity and/or low-z analogs of LRDs.

 

  第1790回 天文学教室談話会

2024年6月18日(火) 16:15-17:15

Title: Substructures in Disks around Very Low Mass Stars
Speaker: Jun Hashimoto 橋本淳 (Astrobiology Center アストロバイオロジーセンター)
Language: English

Very low-mass stars (VLMS) with a mass range from approximately 0.1 to 0.2 Msun are remarkably common in our galaxy, comprising approximately 10-20% of the total number of stars. They serve as intriguing laboratories for investigating planet formation in extreme conditions characterized by low temperatures and densities. This interest arises from the observed higher frequency of Earth-sized planets in habitable zones around VLMS, as exemplified by notable cases like the TRAPPIST-1 system and Proxima Centauri b. Meanwhile, the population of giant planets around VLMS is less than 2% of the detected exoplanets so far. The trends of exoplanet demographics may be reflected in the properties of the birth places of planets, a.k.a., protoplanetary disks, around VLMS. In the talk, I will review recent high-angular resolution observations of protoplanetary disks around VLMS with ALMA.