Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-poor Giants

By D. Burris, C. Pilachowski, T. Armandroff, C. Sneden, J. Cowan, H. Roe. 2000

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New abundances for neutron-capture (n-capture) elements in a large sample of metal-poor giants from the Bond survey are presented. The spectra were acquired with the KPNO 4 m echelle and coudé feed spectrographs, and have been analyzed using LTE fine-analysis techniques with both line analysis and spectral synthesis. Abundances of eight n-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, and Dy) in 43 stars have been derived from blue (λλ4070-4710, R ~ 20,000, S/N ratio ~ 100-200) echelle spectra and red (λλ6100-6180, R ~ 22,000, S/N ratio ~ 100-200) coudé spectra, and the abundance of Ba only has been derived from the red spectra for an additional 27 stars. Overall, the abundances show clear evidence for a large star-to-star dispersion in the heavy element-to-iron ratios. This condition must have arisen from individual nucleosynthetic events in rapidly evolving halo progenitors that injected newly manufactured n-capture elements into an inhomogeneous early Galactic halo interstellar medium. The new data also confirm that at metallicities [Fe/H] ≲ -2.4, the abundance pattern of the heavy (Z ≥ 56) n-capture elements in most giants is well-matched to a scaled solar system r-process nucleosynthesis pattern. The onset of the main r-process can be seen at [Fe/H] ≈ -2.9; this onset is consistent with the suggestion that low mass Type II supernovae are responsible for the r-process. Contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H] ~ -2.75 and are present in most stars with metallicities [Fe/H] > -2.3. The appearance of s-process contributions as metallicity increases presumably reflects the longer stellar evolutionary timescale of the (low-mass) s-process nucleosynthesis sites. The lighter n-capture elements (Sr-Y-Zr) are enhanced relative to the heavier r-process element abundances. Their production cannot be attributed solely to any combination of the solar system r- and main s-processes, but requires a mixture of material from the r-process and from an additional n-capture process that can operate at early Galactic time. This additional process could be the weak s-process in massive (~25 M☉) stars, or perhaps a second r-process site, i.e., different from the site that produces the heavier (Z ≥ 56) n-capture elements.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/d79eaac451dd11a72b72d0d5e250f4c3231648a4

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First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001 - Implications for the r-process site(s) and radioactive cosmochronology (2002, methodology): “Middle: residual abundance of CS 31082-001 after the Solar System r-process (Burris et al. 2000) has been subtracted.”
THE DISCOVERY AND ANALYSIS OF VERY METAL-POOR STARS IN THE GALAXY (2005, background): “Adopting the values of Burris et al. (2000) for the solar Ba and Eu abundances and r- and s-fractions of these elements, it follows that a star purely enriched by neutron-capture elements produced in the s-process would have [Ba/Eu]s = +1.5, whereas [Ba/Eu]r = −0.8 applies for a pure…”
Stellar Chemical Signatures and Hierarchical Galaxy Formation (2004, background): “For La ii, we compare with Burris et al. (2000) and Johnson (2002) only.”
IMPROVED LABORATORY TRANSITION PROBABILITIES FOR Ce ii, APPLICATION TO THE CERIUM ABUNDANCES OF THE SUN AND FIVE r-PROCESS-RICH, METAL-POOR STARS, AND RARE EARTH LAB DATA SUMMARY (2009, background): “…of a class of metal-poor Galactic halo stars with variable n(eutron)-capture elemental abundances (e.g. Sneden et al. 1995, Smith et al. 1995, Cowan et al. 1996, Woolf et al. 1995, Sneden et al. 1996, Burris et al. 2000, Hill et al. 2002, Sneden et al. 2003a) is particularly significant.”
Neutron-Capture Elements in the Early Galaxy (2008, background): “F o r p er so n al u se o n ly . abundances in Solar-system material have been estimated by, e.g., Käppeler, Beer & Wisshak (1989), Arlandini et al. (1999), Burris et al. (2000), Simmerer et al. (2004), and Cowan & Sneden (2006).”
The Extremely Metal-poor, Neutron Capture-rich Star CS 22892-052: A Comprehensive Abundance Analysis (2003, background): “It is clear from the comparisons summarized in both panels of Figure 8 that the agreement with r-process in the mass region 130dAd208 is extremely good, with a mean offset of 1.3, similar to that found for Burris et al. (2000) and Arlandini et al. (1999; see Table 6).”
Neutron-Capture Elements in the Very Metal Poor Star HD 122563 (2006, methodology): “Burris et al. (2000) also determined the abundances of eight neutron-capture elements, although their resolving power (R ¼ 20;000) is not as high as those in the above studies.”
Neutron star mergers versus core-collapse supernovae as dominant r-process sites in the early Galaxy (2003, methodology): “…(1993), François et al. (1993), Beveridge & Sneden (1994), McWilliam et al. (1995a), Ryan et al. (1996), Jehin et al. (1999), Aoki et al. (2000), Burris et al. (2000), Mashonkina & Gehren (2000), Sneden et al. (2000a), Mashonkina & Gehren (2001), Mishenina & Kovtyukh (2001), Koch & Edvardsson…”
Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium (2004, methodology): “The [Sr/Fe] ratios in our program stars are distributed over a very wide range, from 1.7 to +0.5, confirming the large dispersions in this ratio found by previous studies (e.g., McWilliam et al. 1995; Burris et al. 2000).”
A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies (2005, background): “To examine the s-process and r-process contributions separately, we examine Y (its s-process in the Sun is 74%; Travaglio et al. 2004), La (its s-process in the Sun is 75%; Burris et al. 2000), and Ba (its s-process in the Sun is 81%; Arlandini et al. 1999) relative to Eu.”
The Rise of the s-Process in the Galaxy (2004, methodology): “There is only a small offset between [Fe II/H] values derived in this study and those found in Burris et al. (2000) and Johnson (2002), as shown in Fig.”
The Origin of r-process Elements in the Milky Way (2017, methodology): “Eu in the solar system is almost entirely made by the r-process (Burris et al. 2000) and is therefore used as a tracer.”
Spectroscopic Studies of Very Metal-poor Stars with the Subaru High Dispersion Spectrograph. III. Light Neutron-Capture Elements (2005, background): “The 16 Indeed, the decomposition of solar system abundances by Burris et al. (2000) indicates a larger fraction of the r-process component for Y, resulting in ½Y/Zr r-ss ¼ 0:17, much higher than that found in r-process-element-enhanced stars (see Hill et al. 2002).”
Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies (2000, background): “An exception is Burris et al. (2000), who report a solar system r-process fraction of 0.66 for Zn.”
Stellar Abundances for the Galactic Archeology (SAGA) Database — Compilation of the Characteristics of Known Extremely Metal-Poor Stars (2008, methodology): “…that have been conducted (Gilroy et al. 1988; Ryan et al. 1991; McWilliam et al. 1995; Ryan et al. 1996; Fulbright 2000; Preston & Sneden 2000; Burris et al. 2000; Mishenina & Kovtyukh 2001; Aoki et al. 2002; Cohen et al. 2002; Carretta et al. 2002; Johnson 2002; Nissen et al. 2002; Cayrel et…”
VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. I. Nucleosynthesis and Abundance Ratios (2002, background): “8.6. s-Process and r-Process Elements In the Sun Eu is largely an r-process element (95%, Burris et al. 2000).”