We describe the construction of a database of extremely metal-poor (EMP) stars in the Galaxy. Our database contains detailed elemental abundances, reported equivalent widths, atmospheric parameters, photometry, and binarity status, compiled from papers in the literature that report on studies of EMP halo stars with [Fe=H] Ä 2.5. The compilation procedures for this database were designed to assemble data effectively from electronic tables available from online journals. We have also developed a data retrieval system that enables data searches by various criteria and illustrations to explore relationships between stored variables. Currently, our sample includes 1212 unique stars (many of which are studied by more than one group) with more than 15000 individual reported elemental abundances, covering relevant papers published by 2007 December. We discuss the global characteristics of the present database, as revealed by the EMP stars observed to date. For stars with [Fe=H] Ä 2.5, the number of giants with reported abundances is larger than that of dwarfs by a factor of two. The fraction of carbon-rich stars (among the sample for which the carbon abundance is reported) amounts to 30% for [Fe=H] Ä 2.5. We find that known binaries exhibit different distributions of the orbital period, according to whether they are giants or dwarfs, and also as a function of the metallicity, although the total sample of such stars is still quite small.

Recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ∼1.5-3 M ⊙ stars. However, no planets have been found inside 0.6 AU around clump giants, in contrast to solar-type main-sequence stars, many of which harbor short-period planets such as hot Jupiters. In this study we examine the possibility that planets were engulfed by host stars evolving on the red-giant branch (RGB). We integrate the orbital evolution of planets in the RGB and helium burning (HeB) phases of host stars, including the effects of stellar tide and stellar mass loss. Then we derive the critical semimajor axis (or the survival limit) inside which planets are eventually engulfed by their host stars after tidal decay of their orbits. Especially, we investigate the impact of stellar mass and other stellar parameters on the survival limit in more detail than previous studies. In addition, we make detailed comparison with measured semimajor axes of planets detected so far, which no previous study did. We find that the critical semimajor axis is quite sensitive to stellar mass in the range between 1.7 and 2.1 M ⊙ , which suggests a need for careful comparison between theoretical and observational limits of existence of planets. Our comparison demonstrates that all those planets are beyond the survival limit, which is consistent with the planet-engulfment hypothesis. However, on the high-mass side (> 2.1M ⊙ ), the detected planets are orbiting significantly far from the survival limit, which suggests that engulfment by host stars may not be the main reason for the observed lack of short-period giant planets. To confirm our conclusion, the detection of more planets around clump giants, especially with masses 2.5M ⊙ , is required.

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We have constructed the database of stars in local group galaxies using the extended version of the SAGA (Stellar Abundances for Galactic Archaeology) database that contains stars in 24 dwarf spheroidal galaxies and ultra faint dwarfs. The new version of the database includes more than 4500 stars in the Milky Way, by removing the previous metallicity criterion of [Fe/H] ≤ 1 −2.5, and more than 6000 stars in the local group galaxies. We examined a validity of using a combined data set for elemental abundances. We also checked a consistency between the derived distances to individual stars and those to galaxies in the literature values. Using the updated database, the characteristics of stars in dwarf galaxies are discussed. Our statistical analyses of α-element abundances show that the change of the slope of the [α/Fe] relative to [Fe/H] (so-called "knee") occurs at [Fe/H] = −1.0 ± 0.1 for the Milky Way. The knee positions for selected galaxies are derived by applying the same method. Star formation history of individual galaxies are explored using the slope of the cumulative metallicity distribution function. Radial gradients along the four directions are inspected in six galaxies where we find no direction dependence of metallicity gradients along the major and minor axes. The compilation of all the available data shows a lack of CEMP-s population in dwarf galaxies, while there may be some CEMP-no stars at [Fe/H] < ∼ −3 even in the very small sample. 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. We do not find any evidence of anti-correlations in O-Na and Mg-Al abundances, which characterises the abundance trends in the Galactic globular clusters.

We discuss the characteristics of known extremely metal‐poor (EMP) stars in the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA) data base. We find the transition of the initial mass function to be at [Fe/H]∼−2 from the viewpoint of the distribution of carbon abundance and the frequency of carbon‐enhanced stars. Analyses of carbon‐enhanced stars in our sample suggest that nucleosynthesis in asymptotic giant branch (AGB) stars can contribute to carbon enrichment in a different way depending on whether the metallicity is above or below [Fe/H]∼−2.5, which is consistent with the current models of stellar evolution at low metallicity. For observed EMP stars, we confirm that some, though not all, observed stars might have undergone at least two types of extra mixing to change their surface abundances. One is the depletion of lithium abundance during the early phase of the red giant branch; the other is a decrease of the C/N ratio by one order of magnitude during the red giant branch phase. Observed small scatters of abundances for α‐elements and iron‐group elements suggest that the chemical enrichment of our Galaxy takes place in a well‐mixed interstellar medium. The abundance trends of α‐elements are highly correlated with each other including α‐enhanced and depleted stars, while the abundances of iron‐group elements are subject to different slopes relative to the iron abundance. This implies that the supernova yields of α‐elements are almost independent of mass and metallicity, while those of iron‐group elements have a metallicity dependence or mass dependence on the variable initial mass function. The occurrence of the hot‐bottom burning for M≳ 5 M⊙ is consistent with an initial mass function of the Galaxy peaked at ∼10–12 M⊙, compatible with the statistics of carbon‐enhanced stars with and without s‐process element enhancement and nitrogen‐enhanced stars. For s‐process elements, we find not only a positive correlation between carbon and s‐process element abundances, but also an increasing slope of the abundance ratio between them with increasing mass number of s‐process elements. The dominant site of the s‐process is still an open question because none of the known mechanisms for the s‐process is able to account for this observed correlation. In spite of the evidence of AGB evolution in observed abundances of EMP stars, any evidence of binary mass transfer is elusive by pursuing the effect of dilution in the convective envelope. We find the dependence of sulphur and vanadium abundances on the effective temperatures, in addition to the previously reported trends for silicon, scandium, titanium, chromium and cobalt.