The Hamburg/ESO R-process enhanced star survey (HERES)

Abstract

We have derived abundances of 33 elements and upper limits for 6 additional elements for the metal-poor ([Fe/H] = −2.42) turn-off star HE 0338−3945 from high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ...). It is also heavily enriched in Eu, which is generally considered an r-element, and in other similar elements. It is less enriched, by about a factor of 10, in the lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesser degree, N and O. These abundance estimates are subject to severe uncertainties due to NLTE and thermal inhomogeneities which are not taken into detailed consideration. However, an interesting result, which is most probably robust in spite of these uncertainties, emerges: the abundances derived for this star are very similar to those of other stars with an overall enhancement of all elements beyond the iron peak. We have defined criteria for this class of stars, r + s stars, and discuss nine different scenarios to explain their origin. None of these explanations is found to be entirely convincing. The most plausible hypotheses involve a binary system in which the primary component goes through its giant branch and asymptotic giant branch phases and produces CNO and s-elements which are dumped onto the observed star. Whether the r-element Eu is produced by supernovae before the star was formed (perhaps triggering the formation of a low-mass binary), by a companion as it explodes as a supernova (possibly triggered by mass transfer), or whether it is possibly produced in a high-neutron-density version of the s-process is still unclear. Several suggestions are made on how to clarify this situation.Key words. stars: population ii -stars: fundamental parameters -stars: abundances -Galaxy: halo -Galaxy: abundances -Galaxy: evolution IntroductionElements with atomic numbers Z > 30 are believed to be almost exclusively synthesized in neutron-capture (n-capture) reactions. In the most metal-poor stars the overall abundance of these elements varies from star to star, by more than a factor of 100 at a given metallicity (McWilliam et al. 1995;Ryan et al. 1996). Also, the different abundance ratios vary, e.g. the Ba/Eu ratio tends to decline with decreasing [Fe/H]

Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 μm star J004441.04-732136.4

“…It is interesting to compare the abundance pattern of J004441.04 with the patterns observed in peculiar metal poor stars like the CEMP-s/r stars, that is, a carbon-enhanced metalpoor star with enrichments in both s and r-process elements (see discussions in Beers & Christlieb 2005;Jonsell et al 2006;Sneden et al 2008;Lugaro et al 2012). According to the definition given in Jonsell et al (2006) Abundance wise, J004441.04 could be seen as a more metal rich analogue of the CEMP-s/r stars. However, unlike many of the stars within the CEMP-s/r class, the distribution of heavy elements in J004441.04 is similar to that of a pure s-process, except for the anomalously low Pb abundance.…”

Three carbon-enhanced metal-poor dwarf stars from the SDSS

“…In the note to their Table 4, Sivarani et al (2006) attempted to provide a more quantitative description of the classes, based essentially on the abundance of Ba and for one class (CEMP-s) also on the Ba/Eu ratio. Jonsell et al (2006) defined the class CEMP-r+s, which is different from the class CEMP-r/s of Beers & Christlieb (2005). In Table 8 we provide the definitions found in the literature of the different classes.…”

“…Although our preferred carbon, nitrogen and oxygen abundances are those derived from the 3D analysis, we adopt the abundances from the 1D analysis to compare our stars with a sample of hot dwarf CEMP stars from the literature. The full sample of stars selected is compiled from Table 10 of Aoki et al (2008), Table 8 of Jonsell et al (2006) and Table 4 of Sivarani et al (2006).…”

Europium, Samarium, and Neodymium Isotopic Fractions in Metal‐Poor Stars

“…Adopting the original authors' classifications for the enrichment patterns for these stars, this sample consists of 6 r-process-enriched stars Hill et al 2002;Aoki et al 2003a;Sneden et al 2003;Plez et al 2004;Ivans et al 2006;Frebel et al 2007;A. Frebel, 2007, private communication), 15 s-process-enriched stars (Aoki et al 2001(Aoki et al , 2002Johnson & Bolte 2002b;Cohen et al 2003Cohen et al , 2006Barbuy et al 2005;Preston et al 2006), and 5 (r þ s)-process-enriched stars (Ivans et al 2005;Aoki et al 2006;Jonsell et al 2006;Masseron et al 2006) from the Galactic halo population. The r-process-enhanced sample also includes 10 stars from the metal-poor globular clusters M13 and NGC 6752 (Yong et al 2006).…”

“…Thus, many studies have employed [ Ba/Eu] 7 ratios in metal-poor stars to estimate the relative contributions from the s-and r-processes (e.g., Spite & Spite 1978;Gratton & Sneden 1994;Burris et al 2000;Mashonkina et al 2003;Honda et al 2004a;Barklem et al 2005;François et al 2007), which in turn constrain models of chemical evolution in the early Galaxy ( Travaglio et al 1999( Travaglio et al , 2001Cescutti et al 2006;Kratz et al 2007). Recently, [ La/Eu] ratios have been employed as an alternative to [ Ba/Eu] to avoid difficulties in Ba abundance determinations (see, e.g., Simmerer et al 2004;Jonsell et al 2006;Winckler et al 2006).…”