High resolution study of the abundance pattern of the heavy elements in very metal‐poor field stars

Abstract

The abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic “rapid” process. In a careful and homogeneous analysis of the “First Stars” sample (eighty per cent of the stars have a metallicity [Fe/H] ≃ –3.1 ± 0.4), it has been shown that at this metallicity [Eu/Ba] is constant, and therefore the europium‐rich stars (generally called “r‐rich”) are also Ba‐rich. The very large variation of [Ba/Fe] (existence of “r‐poor” and “r‐rich” stars) induces that the early matter was not perfectly mixed. On the other hand, the distribution of the values of [Sr/Ba] vs. [Ba/Fe] appears with well defined upper and lower envelopes. No star was found with [Sr/Ba] < –0.5 and the scatter of [Sr/Ba] increases regularly when [Ba/Fe] decreases. To explain this behavior, we suggest that an early “additional” process forming mainly first peak elements would affect the initial composition of the matter. For a same quantity of accreted matter, this additional Sr production would barely affect the r‐rich matter (which already contains an important quantity of Sr) but would change significantly the composition of the r‐poor matter. The abundances found in the CEMP‐r+s stars reflect the transfer of heavy elements from a defunct AGB companion. But the abundances of the heavy elements in CEMP‐no stars present the same characteristics as the the abundances in the EMP stars. Direct stellar ages may be found from radioactive elements, the precision is limited by the precision in the measurements of abundances from faint lines in faint stars, and the uncertainty in the initial abundances of the radioactive elements. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates From the Skymapper Survey

“…Except for the star SMSS J022423.27-573705.1 which has [Sr/Ba] > 2 (see Section 5.2), our sample follows the same behavior as those of, e.g., Spite & Spite (2014) and Cohen et al (2013). The Ba-poor objects show the largest range of [Sr/Ba] ratios, while the most Ba-rich objects show less scatter.…”

“…The first two are predominantly formed via the s-process in low-mass AGB stars, while Eu is almost entirely formed via the r-process (Sneden et al 2008;Jacobson & Frebel 2014). The large variation of [X/Fe] versus [Fe/H] (>1 dex) for neutron-capture elements, in strong constrast to the relative constancy of the α-elements, has also been wellestablished in the literature Barklem et al 2005;Lai et al 2008;Roederer et al 2010aRoederer et al , 2014aCohen et al 2013;Roederer 2013;Yong et al 2013a;Spite & Spite 2014). Our sample shows similar behavior (Figure 12) Figure 12).…”

The low Sr/Ba ratio on some extremely metal-poor stars

“…They seem to have the same abundance pattern of neutron-capture elements as the EMP stars (Spite & Spite 2014), and are generally not binaries (Starkenburg et al 2014). They are classified CEMP-no (see also Carollo et al 2014).…”

“…Generally, [Eu/Fe] is used to measure the r-process enrichment. In the EMP stars it is not always easy to measure the abundance of Eu, but at these low metallicities there is a very good correlation between [Eu/Fe] and [Ba/Fe] (see, e.g., Mashonkina et al 2010;Spite & Spite 2014), and thus [Ba/Fe] can be used to estimate the r-process enrichment.…”

Homogeneity in the early chemical evolution of the Sextans dwarf spheroidal galaxy

“…We were not able to detect clean Eu features in our spectra. Nonetheless, [Eu/Fe] seems to correlate well with [Ba/Fe] for [Fe/H] for metallicities [Fe/H] ≤-2.5 (e.g., Mashonkina et al 2010;Spite & Spite 2014). At very low metallicity, even Ba has therefore been formed by the r-process.…”