First stars. I. The extremer-element rich, iron-poor halo giant CS 31082-001

Abstract

AstronomyAbstract. We present a high-resolution (R = 75 000, S/N ∼ 500) spectroscopic analysis of the bright (V = 11.7), extreme halo giant CS 31082-001 ([Fe/H] = −2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We find CS 31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS 22892-052. As a result of the extreme overabundance of the heaviest r-process elements, and negligible blending from CH and CN molecular lines, a reliable measurement is obtained .7 dex (a factor of 50), very similar to that of CS 22892-052. Pb, in contrast, seems to be below the shifted Solar r-process distribution, possibly indicating an error in the latter, while thorium is more enhanced than the lighter nuclides. In CS 31082-001, log(Th/Eu) is −0.22 ± 0.07, higher than in the Solar System (−0.46) or in CS 22892-052 (−0.66). If CS 31082-001 and CS 22892-052 have similar ages, as expected for two extreme halo stars, this implies that the production ratios were different by about 0.4 dex for the two objects. Conversely, if the Th/Eu production ratio were universal, an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 ≤ Z ≤ 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating, for two reasons: (i) because of its faster decay rate and smaller sensitivity to observational errors, and (ii) because the inital production ratio of the neighboring nuclides 238 U and 232 Th is more robustly predicted than the 151 Eu/ 232 Th ratio. Our current best estimate for the age of CS 31082-001 is 14.0 ± 2.4 Gyr. However, the computed actinide production ratios should be verified by observations of daughter elements such as Pb and Bi in the same star, which are independent of the subsequent history of star formation and nucelosynthesis in the Galaxy.

On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

“…Solar abundances were taken from Asplund, Grevesse & Sauval (2005). Atomic lines used by Turbospectrum are sourced from VALD, Kupka & Ryabchikova (1999), Hill et al (2002), and Masseron (2006). Line-lists for the molecular species are provided for CH (T. Masseron et al 2014, in press), and CN, NH, OH, MgH and C2 (T. Masseron, in prep); the lines of SiH molecules are adopted from the Kurucz linelists and those from TiO, ZrO, FeH, CaH from B. Plez (private communication).…”

Chemical evolution of the Galactic halo and the Origin of Precious Metals

“…While Ba and heavier elements seem to fit the solar r-process pattern, lighter elements show wide varieties (e.g., Hill et al 2002, Sneden et al 2003. In particular, a large dispersion has been found in [Sr/Ba] at low metallicity (e.g., Ryan, Norris, & Beers 490 Ishimaru et al 1996, Honda et al 2004 fig.…”

The r-process Pattern of a Bright, Highly r-process-enhanced Metal-poor Halo Star at [Fe/H] ∼ −2

“…Table 3 also provides the abundance offsets that occur when LTE parameters are adopted. Figure 2 shows the r-process pattern in J1538-1804, along with reference patterns in the Sun, a very metal-poor, actinide boost r-II star (Hill et al 2002), and an extremely metal-poor r-I star (Roederer et al 2014a). The pattern in J1538-1804 is well fit by the solar r-process pattern and does not agree with the solar s-process pattern.…”