The Stellar Abundances for Galactic Archaeology (SAGA) data base - II. Implications for mixing and nucleosynthesis in extremely metal-poor stars and chemical enrichment of the Galaxy

Abstract

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.

Refractory metal nuggets within presolar graphite: First condensates from a circumstellar environment

“…Some of these metal‐poor stars show significant carbon enrichments (e.g., CEMP giant stars; Suda et al. ) and thus would be more conducive to the formation of carbonaceous stardust. One might assume that a pRMN (a metal grain) would be less likely to condense in a metal‐poor star, but this is not necessarily true.…”

Lithium in CEMP-no stars: A new constraint on the lithium depletion mechanism in the early universe

“…Although these abundance anomalies may be a clue to the understanding of CEMP-no stars, it is beyond the scope of this paper. Figure 6 shows lithium abundances as a function of [Fe/H] for our targets together with the data taken from the SAGA database (Suda et al 2008;Suda et al 2011;Yamada et al 2013). The database compiles abundances of halo stars from various literature.…”

Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

“…Data for the observed stars to compare our simulation results with are taken from the SAGA (Stellar Abundances for Galactic Archaeology) database (e.g., Suda et al 2008, Suda et al 2011, Yamada et al 2013in particular [Eu/Fe] abundance observations are mainly from e.g., Francois et al 2007, Simmerer et al 2004, Barklem et al 2005, Ren et al 2012, Roederer et al 2010, Roederer et al 2014a, Roederer et al 2014b, Roederer et al 2014c, Shetrone, Côté, Stetson 2001, Shetrone et al 2003, Geisler et al 2005, Cohen & Huang 2009, Letarte et al 2010, Starkenburg et al 2013, McWilliam et al 2003. From the raw data, we excluded carbon enriched metal poor stars ("CEMPs") and stars with binary nature, since the surface abundances of such objects are expected to be affected by internal pollution from deeper layers or pollution from the binary companion.…”