The Hamburg/ESOR-process enhanced star survey (HERES)

Abstract

AstrophysicsThe Hamburg/ESO R-process enhanced star survey (HERES) V ABSTRACTAims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327−5642, for which [Fe/H] = −2.78, [r/Fe] = +0.99. Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between Fe i and Fe ii, and Ca i and Ca ii.Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327−5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327−5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutroncapture peak elements, Sr to Pd, in HE 2327−5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = −0.92 ± 0.13, while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327−5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327−5642 is a radial-velocity variable based on our high-resolution spectra covering ∼4.3 years.

The Hamburg/ESO R-process Enhanced Star survey (HERES)

“…There are larger scatters among the results of Th correlated with Ba and Sr, and usually the scatter comes from the unclear type stars, which indicates the existence of multiple nucleosynthesis prcesses. Scatter of [Ba/Fe] and [Sr/Fe] were also reported by other studies (McWilliam 1998;Norris et al 2001). All these elements follow similar abundance pattern as thorium as expected, and stars of the same enhancement type have quite consistent distribution, supporting that they might form from the materials experienced similar chemical enrichments.…”

“…It appears that among the r-process enhanced stars with measured Th or other actinide elements (e.g., U), except for an actinide normal group, which means no obvious enhancement of actinide element abundances with respect to the scaled solar r-process pattern, an actinide boost group (e.g., CS31082-001, HE1219-0312, CS30306-132) also came to be known, although the former group seems to be more common among r-process enhanced metal-poor stars (e.g., CS22892-052, CS29497-004). This imples that for elements in the range of Z ≥ 90, significantly different chemical yields might be produced due to the various conditions of the star formation regions (Hill et al 2002;Roederer et al 2009;Mashonkina et al 2010).…”

“…The HERES survey offers a good opportunity to perform such a study. More than 22 elemental abundances (not including Th) for the sample stars have been reported in previously published papers in this series (e.g., Barklem et al 2005;Zhang et al 2011), and thorium abundances have been studied furtherly with better quality spectra in a few stars (Hayek et al 2009;Mashonkina et al 2010). …”

The Hamburg/ESO R-process Enhanced Star survey (HERES)

“…With this correction, we derived the relative abundance [N/Fe] = −0.32. It is worth noting that several papers applied "solar oscillator strengths" of the NH molecular lines, where Kurucz (1993) log g f -values were reduced by a factor of 0.807 dex (Hill et al 2002), 0.4 dex (Aoki et al 2006;Hayek et al 2009;Mashonkina et al 2010), and 0.3 dex (Shavrina et al 1996;Johnson et al 2007). Cayrel et al (2004) and this paper.…”

“…Observations of MP halo stars appear to infer a distinct production mechanism for Sr-Zr and heavy elements beyond Ba in the early Galaxy (Aoki et al 2005;François et al 2007;Mashonkina et al 2007b). Mashonkina et al (2010) found a clear distinction in Sr/Eu abundance ratios between the r-II and r-I stars, namely, the r-II stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = −0.92 ± 0.13, while the r-I stars have 0.36 dex higher Sr/Eu values.…”