[Paper Review] The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample

P. S. Barklem, N. Christlieb+8 more|ArXiv.org|May 3, 2005

Stellar, planetary, and galactic studiesPhysics and Astronomy88 references260 citations

TL;DR

This study presents a spectroscopic analysis of 253 metal-poor halo stars from the HERES survey using high-resolution VLT/UVES snapshot spectra (S/N ~54, R~20,000). It derives moderate-precision abundances for 22 elements and identifies 8 r-II stars centered at [Fe/H] = -2.81 with minimal scatter, revealing cosmic scatter in neutron-capture elements at low metallicity, particularly for C, Sr, Y, Ba, Eu, and possibly Zr.

ABSTRACT

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 spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy codes of Valenti & Piskunov. Elemental abundances of moderate precision 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. Among the sample of 253 stars, we find 8 r-II stars and 35 r-I stars. 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. (*** abridged ***)

Motivation & Objective

• To conduct a homogeneous spectroscopic analysis of a large sample of metal-poor halo stars from the HERES survey to study r-process and s-process enrichment.
• To determine elemental abundances with moderate precision using automated analysis of low-to-moderate signal-to-noise snapshot spectra.
• To investigate the distribution and scatter of abundance ratios in metal-poor stars, particularly for neutron-capture elements, to probe early Galactic nucleosynthesis and stellar evolution.
• To identify and characterize r-process-enhanced stars, including r-II and r-I types, and s-process-rich stars with enhanced Eu.
• To assess the significance of cosmic scatter in abundance ratios by comparing observed scatter to analysis uncertainties.

Proposed method

• Employed automated line profile analysis via the Spectroscopy Made Easy (SME) code based on Valenti & Piskunov (1996), using synthetic spectra to derive stellar parameters and elemental abundances.
• Analyzed high-resolution (R ~ 20,000) VLT/UVES snapshot spectra covering 3760–4980 Å with typical signal-to-noise ratio of ~54 (range 17–308).
• Used a grid of model atmospheres and synthetic spectra to fit observed line profiles, determining effective temperature, surface gravity, microturbulent velocity, and elemental abundances.
• Applied atomic data from multiple sources (e.g., VALD, SSK04, BGHL81) with careful selection of lines, excluding those affected by blending or strong CH features in carbon-enhanced stars.
• Calculated abundance uncertainties using error propagation and compared observed scatter to estimated errors to distinguish cosmic scatter from measurement noise.
• Classified stars as r-II or r-I based on Eu and other r-process element abundances relative to Fe and other elements.

Experimental results

Research questions

• RQ1 What is the distribution of r-process-enhanced stars in the metallicity range [Fe/H] ≤ -1.5, and how do their abundance patterns compare to known r-process standards?
• RQ2 To what extent is the observed scatter in abundance ratios of α-elements (Mg, Ca, Ti) and iron-peak elements (Sc, V, Cr, Mn, Fe, Co, Ni) due to measurement error versus intrinsic cosmic scatter?
• RQ3 How does the scatter in neutron-capture element abundance ratios (e.g., Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu) vary with metallicity and r-process enrichment level?
• RQ4 What fraction of the sample consists of very metal-poor stars ([Fe/H] < -3) and stars with extreme r-process enhancement (r-II stars), and what are their key abundance characteristics?
• RQ5 Are there stars with strong Eu enhancements that are not r-process enhanced, suggesting s-process dominance or binary contamination?

Key findings

• Among 253 stars, 8 are classified as r-II stars, with a metallicity centered at [Fe/H] = -2.81 and a very small scatter of only 0.16 dex, indicating a tight r-process pattern.
• The r-I stars are distributed across the full metallicity range of the sample, from [Fe/H] = -3.8 to -1.5, showing no strong clustering.
• The study confirms 49 stars with [Fe/H] < -3 and 181 stars with -3 < [Fe/H] < -2, including one star with [Fe/H] < -3.5, significantly expanding the sample of ultra-metal-poor stars.
• Scatter in abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni relative to Fe and Mg is consistent with analysis errors, suggesting minimal or no cosmic scatter for these elements.
• Significant scatter in C, Sr, Y, Ba, Eu, and possibly Zr at [Fe/H] ≤ -2.5 exceeds measurement uncertainties, indicating intrinsic cosmic scatter of astrophysical origin.
• A significant number of stars show strong enhancements in Eu and s-process elements, indicating s-process-rich or binary-contaminated systems, with three such stars identified.