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In Kawahara et al. (2018) and Masuda et al. (2019), we reported the discovery of four self-lensing binaries consisting of F/G-type stars and (most likely) white dwarfs whose masses range from 0.2 to 0.6 solar masses. Here we present their updated system parameters based on new radial velocity data from the Tillinghast Reflector Echelle Spectrograph at the Fred Lawrence Whipple Observatory, and the Gaia parallaxes and spectroscopic parameters of the primary stars. We also briefly discuss the astrophysical implications of these findings.
We analyse the chemical abundances of stars in the local group dwarf galaxies using the SAGA database. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements.
Very metal-poor (VMP) stars preserve chemical signatures of early generations of stars, and are crutial to understand the early nucleosynthesis and first stars. Millions of stellar spectra obtained by LAMOST provide an unprecedented chance to enlarge the currently limited VMP star sample. Since 2014, a joint project on searching for VMP stars has been conducted based on the LAMOST survey and Subaru follow-up observations. So far, the project has obtained chemical abundances for about 250 VMP stars and a number of chemically interesting objects, e.g., three ultra metal-poor stars with [Fe/H] ~ − 4.0, a dozen Li-rich VMP stars distributed in a wide range of evolutionary stages. Statistics of the large homogeneous sample of VMP stars will be of great interest and importance to probe the chemical enrichment in the early Galaxy and low-mass star evolution.
We report progresses of a joint project on searching for extremely metal-poor (EMP) stars based on LAMOST survey and Subaru follow-up observation. Follow-up high-resolution snapshot spectra have been obtained for 70 objects, resulting in 42 EMP stars. A number of chemically interesting objects have already been identified, including (1) Two UMP (ultra metal-poor) stars with [Fe/H] ~ −4.0. One of them is the second UMP turnoff star with Li detection. (2) A super Li-rich (A(Li) ~ 3.1) EMP giant. This is the most metal-poor and extreme example of Li enhancement in giants known to date, and will shed light on Li production during the evolution of red giants. (3) A few EMP stars showing extreme overabundance in heavy elements. Detailed abundances of these extreme objects and statistics obtained by the large sample of EMP stars will provide important constraints on the Galactic halo formation.
IAU Commission 29 - Stellar Spectra has been one of the IAU commissions from the onset, until its dissolution at the most recent IAU General Assembly in Honolulu in 2015. This commission belonged to IAU Division G (“Stars and Stellar Physics”), the latter committed with fostering research in stellar astrophysics. Within the general field of stellar astrophysics, stellar spectroscopy plays a key role, as stellar spectra are a powerful tool providing a view into the detailed physical properties of stars and the physical processes occuring within them.
Large surveys and follow-up spectroscopic studies in the past few decades have been providing chemical abundance data for a growing number of very metal-poor ([Fe/H] <−2) stars. Most of them are red giants or main-sequence turn-off stars having masses near 0.8 solar masses. Lower mass stars with extremely low metallicity ([Fe/H] <−3) are yet to be explored. Our high-resolution spectroscopic study for very metal-poor stars found with SDSS has identified four cool main-sequence stars with [Fe/H] <−2.5 among 137 objects (Aoki et al. 2013). The effective temperatures of these stars are 4500–5000 K, corresponding to a mass of around 0.5 solar masses. Our standard analysis of the high-resolution spectra based on 1D-LTE model atmospheres has obtained self-consistent chemical abundances for these objects, assuming small values of micro-turbulent velocities compared with giants and turn-off stars. The low temperature of the atmospheres of these objects enables us to measure their detailed chemical abundances. Interestingly, two of the four stars have extreme chemical-abundance patterns: one has the largest excesses of heavy neutron-capture elements associated with the r-process abundance pattern known to date (Aoki et al. 2010), and the other exhibits low abundances of the α-elements and odd-Z elements, suggested to be signatures of the yields of very massive stars (> 100 solar masses; Aoki et al. 2014). Although the sample size is still small, these results indicate the potential of very low-mass stars as probes to study the early stages of the Milky Way's halo formation.
Chemical diversity among metal-poor stars in the old stellar components in the Milky Way (MW), namely the thick disk and stellar halo, provides clues to understanding the early chemodynamical evolution of our Galaxy. We present our results on a homogeneous chemical abundance analysis for nearby metal-poor stars likely belonging to the MW thick disk, inner and outer stellar halos. Abundances of alpha, sodium, iron-peak and neutron-capture elements in the sample stars have been estimated using high-resolution (R 50000) spectra obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. The derived abundances are used to examine differences and similarities in elemental abundance ratios among the kinematically defined thick disk, inner and outer halo subsamples in the metallicity range of −3.3 < [Fe/H] < −0.5. We show that, in the metallicity range of [Fe/H] < −2, the three subsamples are similar in most of the elemental abundances. On the other hand, in the higher metallicities, particularly in [Fe/H] > −1.5, the thick disk and the inner/outer halo subsamples show systematically different abundance ratios for some elements including alpha, sodium, zinc and europium. A modest difference in the sodium and zinc abundances between the inner- and outer halo subsamples is also identified. The observed distinct abundances of some elements among the three subsamples implies that their constituent stars originally formed in progenitor systems that have experienced different star formation and chemical enrichment histories.
We are carrying out near-infrared spectroscopy of Cepheids in the Galactic nuclear disk. The H-band spectra taken with SUBARU/IRCS indicate that their kinematics are consistent with the rotation of the nuclear disk.
Asymptotic Giant Branch (AGB) stars play a fundamental role in s-process nucleosynthesis during their thermal pulsing phase. The theoretical predictions obtained by AGB models at different masses, s-process efficiencies, dilution factors and initial r-enrichment, are compared with spectroscopic observations of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s), collected from the literature. We discuss here five stars as example, CS 22880-074, CS 22942-019, CS 29526-110, HE 0202-2204 and LP 625-44. All these objects lie on the main sequence or on the giant phase, clearly before the thermally pulsing AGB stage. The hypothesis of mass transfer from an AGB companion, would explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are CEMP(s + r) objects, and are interpreted assuming that the molecular cloud, from which the binary system formed, was already enriched in r-process elements by SNII pollution. In several cases, the observed s-process distribution may be accounted for by AGB models of different initial masses with proper 13C-pocket efficiencies and dilution factors. Na (and Mg), produced via the neutron capture chain starting from 22Ne, may provide an indicator of the initial AGB mass.
Commission 29 consists of members of the International Astronomical Union carrying out theoretical and observational studies of stars using spectroscopy, developing instrumentation for spectroscopy and producing and collecting data for interpretation of spectra.
The business meeting was attended by 23 members of the Commission. The meeting started at 16:00 a short report of the activities during the triennium 2006-2009. The focus of the activities was the sharing of expertise between spectroscopic techniques in various areas of astronomical research. In particular, the progress in instrumentation, detectors, data reduction, data analysis and archiving. The second activity was the analysis of to IAU meeting proposals followed by recommendations for improvements and eventually support. The sponsored symposia included Sponsoring symposia The Ages of Stars and The Disk Galaxy Evolution in the Cosmological Context. The Commission was also disseminating information about the Commission activities and relevant meetings to the Commission members. In this respect the Commission web page is playing a crucial role.
We performed a 1D LTE chemical abundance analysis of an extremely metal-poor star BD+44°493 ([Fe/H]= −3.7), and set a very low upper limit for its Be abundance: A(Be) < −2.0. It may indicate that the decreasing trend of Be abundances with lower [Fe/H] still holds at [Fe/H] < −3.5, and demonstrate that high C and O abundances do not necessarily imply high Be abundances. However, since the star is a subgiant with Teff ~ 5500K, Be may be depleted.
A significant fraction of metal-poor stars have large over-abundances of carbon, and are called Carbon-Enhanced Metal-Poor (CEMP) stars. Most of CEMP stars also show excesses of heavy neutron-capture elements like Ba, indicating that their origin is the nucleosynthesis in AGB stars. Remaining CEMP stars that have Ba abundances as low as non-carbon-rich stars appear in the lowest metallicity range ([Fe/H]≲−2.5), and connections with the two most iron-deficient stars (so-called Hyper Metal-Poor stars) are suggested. Although the origins of the carbon-excesses in these objects have not been well identified, some objects suggest contributions of faint supernovae. Remaining problems on CEMP stars, such as the binary fraction, excess of r-process elements, are discussed.
We explore the general characteristics of extremely metal-poor (EMP) stars in the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA) database (Suda et al. 2008, PASJ, 60, 1159). The overall trend of EMP stars suggests that there are at least two types of extra mixing to change the surface abundances of EMP stars. One is to deplete lithium abundance during the early phase of giant branch and another is to decrease C/N ratio by one order of magnitude during the red giant branch or AGB phase. On the other hand, these mixing processes are different from those suggested in the Galactic globular clusters because of the different relations between O, Na, Mg, and Al abundances.
We performed a chemical abundance analysis of the very bright (V = 9.1) carbon-enhanced metal-poor (CEMP) star BD+44°493, which is the first star found with metallicity [Fe/H] < −3.5 and an apparent magnitude V < 12. The star is classified as a CEMP-no” subgiant, and its abundance pattern implies that a first-generation faint supernova is the most likely origin of its carbon excess. We set an very low upper limit on this star's beryllium abundance, which demonstrates that high C and O abundances do not necessarily imply high Be abundances.
We have been determining abundances of Th, Pb and other neutron-capture elements in metal-deficient cool giant stars to constrain the enrichment of heavy elements by the r- and s-processes. Our current sample covers the metallicity range between [Fe/H] = −2.5 and −1.0. (1) The abundance ratios of Pb/Fe and Pb/Eu of most of our stars are approximately constant, and no increase of these ratios with increasing metallicity is found. This result suggests that the Pb abundances of our sample are determined by the r-process with no or little contribution of the s-process. (2) The Th/Eu abundance ratios of our sample show no significant scatter, and the average is lower by 0.2 dex in the logarithmic scale than the solar-system value. This result indicates that the actinides production by the r-process does not show large dispersion, even though r-process models suggest high sensitivity of the actinides production to the nucleosynthesis environment.
Observed large scatters in abundances of neutron-capture elements in metal-poor stars may suggest incomplete mixing of the interstellar medium at the beginning of the Galaxy. Comparing predictions by an inhomogeneous chemical evolution model and new observational results with Subaru HDS, we attempt to constrain the origins of r-process elements.
We have observed seven giants in the metal-poor globular cluster M15 using Subaru/HDS. We confirmed that there are significant star-to-star variations in the neutron-capture elemental abundances. This abundance variation means there were primordial chemical inhomogeneities in the proto-globular cluster cloud of M15. This result implies that there was insufficient time for complete mixing after r-process nucleosynthesis. It suggests that the main r-process occurs probably in supernovae which explode in later stages of globular cluster formation.