投稿者: YamashitaKazuyoshi

Stellar triples with chemically homogeneously evolving inner binaries

Speaker: Andris Dorozsmai

Abstract:

Observations suggest that massive stellar triples are common. However, their evolution is not yet fully understood. In this talk, I discuss the evolution of hierarchical triples in which the stars of the inner binary experience chemically homogeneous evolution (CHE), particularly with the aim to explore the role of the tertiary star in the formation of gravitational-wave (GW) sources. To investigate these systems, I use the the triple-star population synthesis code TRES. I found that about 40 per cent of the all triples harboring a CHE inner binary (CHE triples) experience tertiary mass transfer episodes, an event which is rare for classically evolving stars (i.e. non-CHE stars). In the majority of tertiary mass transfer episodes, the accreting inner binary consists of two main sequence stars (58-60 per cent) or two black holes (24-40 per cent). I will also show that von Zeipel-Lidov-Kozai (ZLK) oscillations play an important role in the evolution of these systems. In particular, I find that for triples with initial outers pericentre smaller than 2000 solar radii, ZLK oscillations can result in eccentric (stellar or double compact object) mergers. Approximately 24 per cent of CHE triples become GW sources. Moreover, in a significant fraction of these sources, the tertiary star plays an important role and leads to configurations that are not predicted for isolated binaries. To conclude, the evolution of CHE binaries can be affected by a close tertiary companion, resulting in astronomical transients such as tertiary-driven massive stellar mergers and equal-mass BH-BH binaries that merge via gravitational-wave emission within Myrs.

The origin of extremely metal-poor star with weak r-process signature

Speaker: Hiroko Okada

Abstract:

The origin of the rapid neutron-capture process is a major question in astrophysics. The clue to answer this question is the chemical abundance patterns of metal-poor stars, which are believed to reflect the nucleosynthesis yields of a single event. Recent observations propose two r-process classes: the “main r-process” for light and heavy elements and the “weak r-process” for lighter ones. Aoki et al. (2017) suggested to use abundance ratios of first-peak neutron-capture elements (Sr-Ag) in metal-poor stars to identify the origin of weak r-process nucleosynthesis. However, their sample is not necessarily extremely metal-poor, and some contamination of main r-process, and even s-process, is suspected. To clarify the weak r-process’s pure abundance pattern, we studied the extremely metal-poor star SMSS J022423.27-573705.1, with a high lower limit on [Sr/Ba] ratio (Jacobson et al., 2015). Analyzing near UV spectrum data from the VLT/UVES, we measured 26 elemental abundances including first-peak neutron-capture elements and determine Ba abundance. We also compare our results with the latest nucleosynthesis models.

Formation of streamers by dense core collisions

Speaker: Aoto Yoshino

Abstract:

Stars are formed by the gravitational contraction of dense cores in molecular clouds. In the classical model, a nearly axisymmetric core gravitationally collapses to form a disk around the protostar (e.g., Terebey et al. 1984). On the other hand, recent high-resolution observations of protostellar cores have often revealed non-axisymmetric, elongated flows of material falling into the disk, called streamers (Pineda et al. 2020; Valdivia-Mena et al. 2022). Per-emb-2 is a protostellar system located in the Perseus Molecular Cloud at 300 pc, and interferometric observations with ALMA and other instruments have revealed a large, carbon-chain molecule-rich streamer structure (Pineda et al. 2020). However, the origin of streamers is still unknown, and core collisions are one candidate. In this study, we investigate the collision process of cores using three-dimensional hydrodynamic simulations and explore the formation process of non-axisymmetric streamers that appear in the circumstellar structure.

The Chemical enrichment and origin of Nitrogen-Rich Galaxies at High Redshift

Speaker: Kuria Watanabe

Abstract:

Chemical properties of young galaxies are important to understand the chemical evolution in galaxy formation. We investigate the origin of abundance ratios in JWST star-forming galaxies (z ~ 4-10) by comparing chemical evolution models.
The high N/O ratio in GN-z11, CEERS_01019, and GLASS_150008 observed with JWST cannot be explained by typical chemical evolution models including AGB stars. We focus on the three candidates of rich nitrogen, the Wolf-Rayet stars (WR), supermassive stars (SMS), and tidal disruption events (TDE). We develop the chemical evolution models of three candidates. Although the Ar/O values of the high N/O galaxies are not obtained, we find that the Ar/O values are the key to distinguishing between the three models. The three models can show the high N/O values as much as the N/O values in GN-z11, CEERS_01019, and GLASS_150008. Because the time scale of high N/O is very short due to the CCSNe, the massive stars (>25 Msun) should directly collapse into black holes.

Planetesimal accretion in planetesimal rings

Speaker: Yuki Kambara

Abstract:

In the standard planet formation scenario, planetesimals have been assumed to be smoothly distributed in the radial direction except for the snowline. Recently, however, simulations of gas and dust evolution have shown that planetesimals form only in radially limited locations, such as gas pressure bumps and snowlines, and are concentrated in ring-like regions.
The evolution process of planetesimals distributed in a ring is crucial to understanding planet formation theory. However, the evolution of planetesimal rings has not been studied in detail. In this work, we investigate the evolution of planetesimal rings using N-body simulations. We systematically change the initial width and the total mass of planetesimal rings and investigate the dependence of protoplanet properties on the initial conditions. In all simulations, protoplanets undergo oligarchic growth while ring width expands due to scattering by protoplanets. In planetesimal rings, massive protoplanets tend to be formed around the ring center, while protoplanets far from the center of rings are less massive. Scaled orbital separation depends neither on the initial ring width nor the total mass and is consistent with the estimate in the oligarchic growth model. The width of the expanded planetesimal ring does not depend on the initial ring width but on the total ring mass. The maximum mass of protoplanets depends strongly on the total ring mass and weakly on the initial ring width.

HST-dark galaxies: Unveiling the Nature of Infrared Bright, Optically Dark Galaxies with Early JWST Imaging and Spectroscopy

Speaker: Laia Barrufet

Abstract:

Over the past decade, low-resolution infrared observations have revealed a population of bright IR galaxies undetected with HST, termed “HST-dark galaxies. Nevertheless, their photometric redshifts and physical properties were uncertain due to the limited photometry even including ALMA data. This can result in an incomplete galaxy census at earlier times, due to UV-faint galaxy populations such as quiescent or dust-obscured sources. The key question is: how common are such sources at z >3-4 and what is their contribution to the cosmic star formation rate density? With JWST, we can study for the first time the physical properties of this enigmatic population. In this talk, I will summarise the progress in our understanding of HST-dark galaxies facilitated by JWST data, employing both photometry and spectroscopy. In the first part of the talk, I will expose the progress made with HST-dark galaxies thanks to JWST/NIRcam photometry. I will focus on the results of 30 HST-dark sources selected based on their red colours across 1.6 to 4.4 microns with NIRCam imaging from the Early Release Science Program CEERS. These galaxies are generally highly dust-attenuated, massive, star-forming sources at z~2-8. Our analysis underscores the unique capability of JWST in uncovering this previously elusive galaxy population and in conducting a more complete census of galaxies at z>3 based on rest-frame optical imaging. Interestingly, HST-dark galaxies present a considerable contribution to the obscured star formation rate density at z~7, already in the Epoch of Reionization. In the second part of the talk, I will present the initial NIRSpec spectra of HST-dark galaxies from my GO Cycle-1 program ‘Quiescent or dusty? Unveiling the nature of extremely red galaxies at z>3’. I will showcase spectra for highly attenuated dusty galaxies and quiescent galaxies and their physical characteristics. Despite the prevalence of dust in most galaxies, they exhibit distinct spectral features. Furthermore, HST-dark galaxies harbour hidden gems: quiescent galaxies at redshifts > 3, for which I will present spectra and demonstrate NIRSpec’s capacity to analyze these sources. Finally, I will explore potential avenues for future research involving HST-dark galaxies, leveraging the synergy between JWST and ALMA.

Review on Pan et al. (2022) “The terrestrial planet formation around M dwarfs: in situ, inward migration, or reversed migration”

Speaker: Ryota Kitamura

Abstract of the paper:

Terrestrial planets are commonly observed to orbit M dwarfs with close-in trajectories. In this work, we extensively perform N-body simulations of planetesimal accretion with three models of in situ, inward migration, and reversed migration to explore terrestrial formation in tightly compact systems of M dwarfs. In the simulations, the solid discs are assumed to be 0.01 per cent of the masses of host stars and spread from 0.01 to 0.5 au with the surface density profile scaling with r−k according to the observations. Our results show that the in-situ scenario may produce 7.77+3.23 −3.77 terrestrial planets with an average mass of 1.23+4.01 −0.93 M_⊕ around M dwarfs. The number of planets tends to increase as the disc slope is steeper or with a larger stellar mass. Moreover, we show that 2.55+1.45 −1.55 planets with a mass of 3.76+8.77 −3.46 M_⊕ are formed in the systems via inward migration, while 2.85+1.15 −0.85 planets with 3.01+13.77 −2.71 M_⊕ are yielded under reversed migration. Migration scenarios can also deliver plentiful water from the exterior of the ice line to the interior due to more efficient accretion. The simulation outcomes of the reversed migration model produce the best match with observations, being suggestive of a likely mechanism for planetary formation around M dwarfs.

Effect of stellar wind on SS433’s jet propagation

Speaker: Miho Tan

Abstract:

SS433 is an X-ray binary system consisting of a star and a compact star, and the compact star is known to emit spiral binaries. The jets propagate more than 100 pc; in Ohmura et al. (2021) and other groups, uniform jets injected from 1 pc propagated 100 pc. But one of the important questions is how to travel to 1pc from the ejection region. Therefore, in this study, we evaluate the propagation of the jet within 1pc including the effect of stellar winds. Since the orbital period of the binary is shorter than the jet propagation timescale, we assume the propagation area becomes turbulence formed by the stellar wind. We put turbulent velocity fields of 0.1% and 0.01% of the jet velocity in the ambient around the jet to see the effect of the turbulence on the jet propagation.

Effects of internal heating sources on hydrogen-rich supernova light curves

Speaker: Tatsuya Matsumoto

Abstract:

Core-collapse supernovae (SNe) are caused by the death of massive stars, and their light curves provide a lot of information about the stellar evolution and physical processes of explosions. In particular, the light curves of hydrogen-rich SNe have a characteristic slowly-evolving phase, so-called the plateau phase, whose luminosity and duration are related to the SN parameters such as ejecta mass and energy. Recent observations revealed that some H-rich SNe exhibit evidence for a sustained energy source powering their light curves, resulting in a brighter and/or longer-lasting plateau phase.

We present a semi-analytic light curve model that accounts for the effects of an arbitrary internal heating source such as 56Ni/Co decay, a central engine (millisecond magnetar or accreting compact object), and shock interaction with a dense circumstellar disk.

While a sustained internal power source can boost the plateau luminosity commensurate with the magnitude of the power, the duration of the recombination plateau can typically be increased by at most a factor ∼2−3 compared to the zero-heating case. For a given ejecta mass and initial kinetic energy, the longest plateau duration is achieved for a constant heating rate at the highest magnitude that does not appreciably accelerate the ejecta. This finding has implications for the minimum ejecta mass required to explain particularly long-lasting supernovae, and for confidently identifying rare explosions of the most-massive hydrogen-rich (e.g. population III) stars.

Subaru/IRD high-resolution spectroscopy of a T-type brown dwarf and investigation of its atmospheric properties with high-resolution spectrum model ExoJAX

Speaker: Yui Kawashima

Abstract:

While brown dwarf atmospheres share composition and temperature with those of extrasolar gas giant planets, in general, brown dwarfs are observable with a higher signal-to-noise ratio when compared to exoplanets. Thus, the observation of brown dwarf atmospheres helps us establish our understanding of various processes in the atmospheres of such temperature and composition, including chemistry, thermal structure, dynamics, and cloud formation. Also, their high-resolution spectra serve as excellent templates for the observational validation of the molecular line lists at such high temperatures. The accuracy of molecular line lists holds the key to detecting chemical species in exoplanet atmospheres, which are often observed with a lower signal-to-noise ratio.

Recently, we observed the high-resolution spectrum of a T6.5-type brown dwarf Gl 229B with the InfraRed Doppler (IRD) spectrograph mounted on the Subaru Telescope. We have constrained its atmospheric properties, such as the molecular abundances and thermal structure, using an inverse-problem approach with our high-resolution spectrum model ExoJAX (Kawahara, Kawashima et al. 2022). We have also investigated the possibility of inferring the object mass using the embedded information on collision-induced absorption uniquely accessible by high-resolution spectroscopy. Furthermore, we have revealed that in some wavelength regions, specific molecular line lists do not match the observed absorption features.