The Hamburg/ESO R-process enhanced star survey (HERES). II. Spectroscopic analysis of the survey sample

By Paul Barklem, N. Christlieb, Timothy C. Beers, V. Hill, M. Bessell, J. Holmberg, B. Marsteller, S. Rossi, F. Zickgraf, D. Reimers. 2005

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We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars −3.8 ≤ (Fe/H) ≤− 1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically S /N ∼ 54 per pixel (ranging from 17 to 308), R ∼ 20 000, λ = 3760-4980 A. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3σ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S /N ,o r when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of (Fe/H) = −2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with (Fe/H) < −3 and 181 stars with −3 < (Fe/H) < −2. We find one star with (Fe/H) < −3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at (Fe/H) < −2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.

Subfields: Astronomy and Astrophysics, Instrumentation

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Where, oh where has the r-process gone? (2007, methodology): “From the new results it is found that the Ba/Eu ratio is almost constant between [Fe/H] ∼ −3.5 and −2 [Hill et al. (2002); Johnson & Bolte (2002); Aoki et al. (2005); Barklem et al. (2005); Otsuki et al. (2006); Ivans et al. (2006); see Fig.”
HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES (2009, positive): “Four of our six stars have Mg abundances of [Mg/Fe] ∼ 0.4, in good agreement (see Figure 10) with the general trend in MW halo stars (e.g., Cayrel et al. 2004; Barklem et al. 2005; Lai et al. 2008).”
First results from the IllustrisTNG simulations: a tale of two elements - chemical evolution of magnesium and europium (2017, background): “While some sites correspond to large regions of stars, many are more uniformly distributed throughout the galaxies, agreeing qualitatively with observations of the MW showing highly enhanced europium stars in the halo (Cohen et al. 2004; Venn et al. 2004; Barklem et al. 2005).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “…& Bolte (2001); Mishenina et al. (2001); Mashonkina et al. (2003); Sneden et al. (2003a); Christlieb et al. (2004); Simmerer et al. (2004); Honda et al. (2004); Barklem et al. (2005); François et al. (2007); Frebel et al. (2007); Aoki & Honda (2008); Hayek et al. (2009); Roederer et al. (2010c).”
The s process: Nuclear physics, stellar models, and observations (2010, neutral): “On the other hand, 13 CEMP-s stars do not show any r-process-enhancement, i.e. CS 22880-074 (Aoki et al., 2007), HE 0024-2523 (Lucatello et al., 2003), HE 2158-0348 (Cohen et al., 2006), and HE 0202-2203 (Barklem et al., 2005).”
The Frequency of Carbon-enhanced Metal-poor Stars in the Galaxy from the HERES Sample (2006, methodology): “…question anew, making use of the data obtained by the Hamburg/ESOR-process enhanced star (HERES) survey (Christlieb et al. 2004, hereafter Paper I; Barklem et al. 2005, hereafter Paper II), which presently provides the largest sample of high-resolution spectra available for very metal-poor (VMP)…”
The stellar halo of the Galaxy (2008, background): “In contrast, the neutron-capture elements behave very differently, showing a large spread at low metallicity (Barklem et al. 2005; François et al. 2007, and references therein).”
CHEMICAL EVOLUTION IN HIERARCHICAL MODELS OF COSMIC STRUCTURE. II. THE FORMATION OF THE MILKY WAY STELLAR HALO AND THE DISTRIBUTION OF THE OLDEST STARS (2009, background): “Similar arguments can be made for the chemical clock provided by the abundances of r-process elements in metal-poor stars (Barklem et al. 2005).”
A new implementation of the infrared flux method using the 2MASS catalogue (2009, methodology): “We completed the sample with extremely metal-poor dwarfs from Bonifacio et al. (2007), Christlieb et al. (2004), and Barklem et al. (2005).”
Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars (2006, methodology): “Barklem et al. (2005) included HD 221170 as a comparison object (S/N ∼50) in their survey of r-process enhanced stars taken with R ∼ 20,000 and wavelength coverage of 3760Å< λ < 4980Å.”
Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 , (2009, methodology): “Following Norris et al. (2001) and Barklem et al. (2005), errors due to observational uncertainties were estimated with the expression: σWλ = λ √ (n)/(R × S/N), wheren denotes the number of pixels spanning the full-width at half maximum for the line,R the spectral resolution, andS/N the…”
First stars VIII. Enrichment of the neutron-capture elements in the early Galaxy (2007, neutral): “The highly r-enriched (r-II) stars have all been found in a very narrow range around [Fe/H] = −2.9 (Barklem et al. 2005).”
The Hamburg/ESO R-process enhanced star survey (HERES) - III. HE 0338-3945 and the formation of the r + s stars (2006, methodology): “These spectra were analysed with an automatic procedure based on MARCS model atmospheres and synthetic spectra (Barklem et al. 2005, Paper).”
HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS FROM SDSS/SEGUE. I. ATMOSPHERIC PARAMETERS AND CHEMICAL COMPOSITIONS (2012, background): “Previous studies of large samples of candidate metal-poor stars from these surveys have revealed the chemical compositions of stars with [Fe/H]∼ −3 (Cayrel et al. 2004; Cohen et al. 2004; Honda et al. 2004; Barklem et al. 2005; Aoki et al. 2005; Lai et al. 2008; Bonifacio et al. 2009).”