[Paper Review] The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements

N. Christlieb, Timothy C. Beers+16 more|University of Hertfordshire Research Archive (University of Hertfordshire)|Aug 21, 2004

Stellar, planetary, and galactic studiesPhysics and Astronomy43 references83 citations

TL;DR

The HERES survey identifies metal-poor stars with strong r-process element enhancements using snapshot spectroscopy from VLT/UVES. It discovered two new r-II stars, CS 29497–004 and CS 29491–069, with [Eu/Fe] = 1.64 and 1.08, respectively, and found that CS 29497–004's abundance pattern matches a scaled solar r-process pattern with a 0.3 dex Th underabundance due to radioactive decay.

ABSTRACT

We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~2A) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H] < -2.5dex. For several hundred confirmed metal-poor giants brighter than B~16.5mag (most of them from the HES), ``snapshot'' spectra (R~20,000; S/N~30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, 8 from the HES, and 4 comparison stars. We discovered two new r-II stars, CS29497-004 ([Eu/Fe] = 1.64 +/- 0.22) and CS29491-069 ([Eu/Fe] = 1.08 +/- 0.23). A first abundance analysis of CS29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.

Motivation & Objective

• To identify and study metal-poor stars with strong r-process element enhancements ([r/Fe] > 1 dex), particularly r-II stars.
• To determine the frequency of r-II stars in the metallicity regime [Fe/H] < -2.5 dex using a large-scale survey.
• To investigate the nucleosynthetic origin of neutron-capture elements in metal-poor stars through detailed abundance analysis.
• To assess the potential of Th/Eu and other isotope ratios as chronometers for age determination in r-process-enhanced stars.
• To explore the metallicity range where r-II stars are most likely to be found, particularly in the [Fe/H] ≈ -2.0 to -2.5 regime.

Proposed method

• Utilized moderate-resolution (R ~ 20,000) snapshot spectroscopy with VLT/UVES on 35 metal-poor giant candidates from the Hamburg/ESO and HK surveys.
• Selected stars based on sharp lines and [Fe/H] < -2.5 dex from objective-prism surveys (HES and HK).
• Performed initial abundance analysis on snapshot spectra to identify r-I and r-II stars, focusing on [Eu/Fe] and [Ba/Eu] ratios.
• Used a scaled solar r-process pattern as a reference for comparing observed neutron-capture element abundances.
• Applied error analysis procedures to estimate uncertainties in key abundance ratios, including log ε(Th/Eu).
• Planned follow-up with higher-resolution and higher signal-to-noise ratio spectra for precise abundance and age determination.

Experimental results

Research questions

• RQ1 What is the true frequency of r-II stars among metal-poor giants with [Fe/H] < -2.5 dex?
• RQ2 Can snapshot spectroscopy effectively identify r-II stars in large-scale surveys without full high-resolution follow-up?
• RQ3 How do the abundance patterns of newly discovered r-II stars compare to theoretical r-process models and solar system r-process residuals?
• RQ4 Is the Th/Eu ratio a reliable chronometer in r-II stars, particularly when uncertainties are large?
• RQ5 What is the metallicity range in which r-II stars are most likely to be found, and how does this compare to previous assumptions?

Key findings

• Two new r-II stars were discovered: CS 29497–004 with [Eu/Fe] = 1.64 ± 0.22 and CS 29491–069 with [Eu/Fe] = 1.08 ± 0.23.
• The abundance pattern of CS 29497–004 shows average enhancement of Ba to Dy by 1.5 dex relative to Fe and the Sun, matching a scaled solar r-process pattern well.
• Thorium (Th) in CS 29497–004 is underabundant by 0.3 dex relative to the r-process pattern, consistent with radioactive decay over time.
• CS 29497–004 does not exhibit an 'actinide boost' like CS 31082–001 or CS 30306–132, indicating it is not a star enriched by a specific actinide-producing event.
• The star CS 29497–004 has [Fe/H] = -2.64 ± 0.12, making it significantly more metal-rich than previously known r-II stars (which are typically at [Fe/H] ≈ -3.0).
• The limited signal-to-noise ratio of the snapshot spectrum results in a 0.24 dex uncertainty in log ε(Th/Eu), precluding a meaningful age determination using the Th/Eu chronometer at this stage.