[Paper Review] The Hamburg/ESO R-process Enhanced Star survey (HERES) III. HE 0338-3945 and the formation of the r+s stars

K. Jonsell, P. S. Barklem+5 more|ArXiv.org|Jan 20, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy122 references91 citations

TL;DR

This paper presents high-resolution spectroscopic analysis of the metal-poor star HE 0338-3945 ([Fe/H] = -2.42), revealing extreme overabundances of both $r$- and $s$-process elements, including Eu, Ba, La, and C, defining a new class of stars known as r+s stars. Despite uncertainties from non-LTE and thermal inhomogeneities, the data suggest a common origin involving mass transfer from a binary companion, though the exact mechanism for $r$-process production remains unclear among competing scenarios including supernova-triggered formation or high-neutron-density $s$-process.

ABSTRACT

We have derived abundances of 33 elements and upper limits for 6 additional elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE 0338-3945 from high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ..). It is also heavily enriched in Eu, which is generally considered an r-element, and in other similar elements. It is less enriched, by about a factor of 10, in the lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesser degree, N and O. These abundance estimates are subject to severe uncertainties due to NLTE and thermal inhomogeneities which are not taken into detailed consideration. However, an interesting result, which is most probably robust in spite of these uncertainties, emerges: the abundances derived for this star are very similar to those of other stars with an overall enhancement of all elements beyond the iron peak. We have defined criteria for this class of stars, r+s stars, and discuss nine different scenarios to explain their origin. None of these explanations is found to be entirely convincing. The most plausible hypotheses involve a binary system in which the primary component goes through its giant branch and asymptotic giant branch phases and produces CNO and s-elements which are dumped onto the observed star. Whether the r-element Eu is produced by supernovae before the star was formed (perhaps triggering the formation of a low-mass binary), by a companion as it explodes as a supernova (possibly triggered by mass transfer), or whether it is possibly produced in a high-neutron-density version of the s-process is still unclear. Several suggestions are made on how to clarify this situation.

Motivation & Objective

• To determine the abundance pattern of the metal-poor star HE 0338-3945 using high-resolution VLT-UVES spectra.
• To identify and characterize a new class of stars, r+s stars, defined by simultaneous overabundances of $r$- and $s$-process elements.
• To evaluate multiple astrophysical scenarios for the origin of r+s stars, particularly focusing on binary evolution and nucleosynthetic processes.
• To assess the role of non-LTE and 3D effects in abundance analysis, especially for Eu and other key $n$-capture elements.
• To explore the implications of the observed chemical homogeneity and spatial distribution of r+s stars compared to r-II giants.

Proposed method

• High-resolution, high signal-to-noise ratio spectroscopy using the VLT-UVES instrument at Paranal Observatory, Chile.
• Abundance analysis of 33 elements and upper limits for 6 additional elements using model atmospheres and line synthesis.
• Application of non-LTE corrections and consideration of thermal inhomogeneities to improve accuracy of elemental abundance determinations.
• Classification of stars into r+s and r-II categories based on [Eu/Fe], [Ba/Eu], and [Ba/Fe] ratios.
• Comparison of observed abundance patterns with scaled solar $r$-process and $s$-process models.
• Evaluation of nine astrophysical scenarios for r+s star formation, including binary mass transfer, supernova triggering, and high-neutron-density $s$-process.

Experimental results

Research questions

• RQ1 What is the detailed abundance pattern of the metal-poor star HE 0338-3945, and how does it compare to other r-process and s-process enhanced stars?
• RQ2 What physical mechanisms can explain the simultaneous overabundance of both $r$- and $s$-process elements in r+s stars like HE 0338-3945?
• RQ3 Why are r+s stars predominantly turn-off stars while r-II stars are all giants, and what does this imply about mass transfer and dilution?
• RQ4 To what extent can non-LTE and 3D effects alter the inferred abundances of Eu and other key $n$-capture elements in these stars?
• RQ5 Which astrophysical scenario—binary evolution, supernova-triggered formation, or high-neutron-density $s$-process—best explains the observed chemical homogeneity and abundance patterns of r+s stars?

Key findings

• The star HE 0338-3945 exhibits extreme overabundances: ~100× solar in heavy $s$-elements (Ba, La), ~10× solar in lighter $s$-elements (Sr, Y, Zr), and enhanced C, N, and O.
• Eu is overabundant by a factor of ~100 relative to iron, indicating significant $r$-process contribution, despite being a $s$-process element in standard models.
• The abundance pattern of HE 0338-3945 closely matches that of other r+s stars, supporting the existence of a distinct class of metal-poor stars with coeval $r$- and $s$-process enrichment.
• The observed chemical homogeneity across r+s stars suggests a common origin, likely involving binary mass transfer from an evolved companion.
• Among nine proposed scenarios, none are fully convincing, but the most plausible involve a binary system where the primary produces $s$-elements and CNO, with $r$-process elements possibly from a supernova or high-neutron-density $s$-process.
• The lack of radial velocity variations in several r+s stars challenges some scenarios, but the data remain consistent with binary systems where mass transfer occurred prior to the main sequence.