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Register a new userThe Stellar Abundances for Galactic Archaeology (SAGA) Database II - Implications for Mixing and Nucleosynthesis in Extremely Metal-Poor Stars and Chemical Enrichment of the Galaxy
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We discuss the characteristics of known 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 analyses of carbon-enhanced stars in our sample suggest that the nucleosynthesis in AGB stars can contribute to the 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. We find the transition of the initial mass function at [Fe/H] ~ -2 in the viewpoint of the distribution of carbon abundance and the frequency of carbon-enhanced stars. For observed EMP stars, we confirmed that some, not all, of observed stars might have undergone at least two types of extra mixing to change their surface abundances. One is to deplete the lithium abundance during the early phase of red giant branch. Another is to decrease the C/N ratio by one order of magnitude during the red giant branch phase. Observed small scatters of abundances for alpha-elements and iron-group elements suggest that the chemical enrichment of our Galaxy takes place in a well-mixed interstellar medium. We find that the abundance trends of alpha-elements are highly correlated with each other, while the abundances of iron-group elements are subject to different slopes relative to the iron abundance. This implies that the supernova yields of alpha-elements are almost independent of mass and metallicity, while those of iron-group elements have a metallicity dependence or mass dependence with the variable initial mass function.The occurrence of the hot bottom burning in the mass range of 5 <~ M / Msun <~ 6 is consistent with the initial mass function of the Galaxy peaked at ~ 10 - 12 Msun to be compatible with the statistics of carbon enhanced stars with and without s-process element (truncated)
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We describe the construction of a database of extremely metal-poor (EMP) stars in the Galactic halo whose elemental abundances have been determined. Our database contains detailed elemental abundances, reported equivalent widths, atmospheric parameters, photometry, and binarity status, compiled from papers in the recent literature that report studies of EMP halo stars with [Fe/H] < -2.5. The compilation procedures for this database have been designed to assemble the 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 permits the user to explore relationships between the stored variables graphically. 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 all of the relevant papers published by December 2007. We discuss the global characteristics of the present database, as revealed by the EMP stars observed to date. For stars with [Fe/H] < -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) amount to ~30 % for [Fe/H] < -2.5. We find that known binaries exhibit different distributions of orbital period, according to whether they are giants or dwarfs, and also as a function of metallicity, although the total sample of such stars is still quite small.
A tutorial for the Stellar Abundances for Galactic Archaeology (SAGA) database is presented. This paper describes the outline of the database, reports the current status of the data compilation and known problems, and presents plans for future updates and extensions.
The first supernovae enrich the previously pristine gas with metals, out of which the next generation of stars form. Based on hydrodynamical simulations, we develop a new stochastic model to predict the metallicity of star-forming gas in the first galaxies. On average, in internally enriched galaxies, the metals are well mixed with the pristine gas. However, in externally enriched galaxies, the metals can not easily penetrate into the dense gas, which yields a significant metallicity difference between the star-forming and average gas inside a halo. To study the consequences of this effect, we apply a semi-analytical model to Milky Way-like dark matter merger trees and follow stellar fossils from high redshift until the present day with a novel realistic metal mixing recipe. We calibrate the model to reproduce the metallicity distribution function (MDF) at low metallicities and find that a primordial IMF with slope of $mathrm{d}N/mathrm{d}M propto M^{-0.5}$ from $2 Msun$ to $180 Msun$ best reproduces the MDF. Our improved model for inhomogeneous mixing can have a large impact for individual minihalos, but does not significantly influence the modelled MDF at [Fe/H]$gtrsim -4$ or the best-fitting Pop~III IMF.
We have constructed the database of stars in the local group 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] <= -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 alpha-element abundances show that the change of the slope of the [alpha/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 study the enrichment histories for nine elements, C, four alpha-elements of Mg, Si, Ca, and Ti, Sc, and three iron-peak elements of Co, Ni, and Zn, by using a large number of stellar data, collected by the Stellar Abundances for Galactic Archaeology (SAGA) database. We find statistically significant changes, or breaks, of the mean abundance ratios to iron at three metallicities of [Fe/H]-1.8, -2.2, and -3.3. Across the first one, the mean abundance ratios decrease with the metallicity by similar extents for all the elements with the sufficient data. Across the latter two, downward trends with the metallicity are also detected but for limited elements, C, Co, Zn, and possibly Sc, and for two of Co and Zn, respectively. The breaks define four stellar populations with the different abundance patters which are dominant in each metallicity range divided by the breaks, Pop IIa, IIb, IIc, and IId in order of increasing metallicity. We also explore their spatial distributions with the spectroscopic distances to demonstrate that Pops IIa and IIb spread over the Galactic halo while Pops IIc and IId are observed near the Galactic plane. In particular, Pop IIc stars emerge around [Fe/H] -2.6 and coexist with Pop IIb stars, segregated by the spatial distributions. Our results reveal two distinct modes of star formation during the early stages of Galaxy formation, which are associated with the variations of IMF and the spatial distribution of remnant low-mass stars. For the two lower-metallicity populations, the enhancements of Zn and Co indicate a high-mass and top-heavy IMF together with the statistics on the carbon-enhanced stars. We discuss the relevance to the kinematically resolved structures of the Galactic halo and the possible sites of these populations within the framework of hierarchical structure formation scenario.
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