Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium

By Satoshi Honda, Wak Aoki, Toshitaka Kajino, H. Ando, Timothy C. Beers, H. Izumiura, K. Sadakane, M. Takada-Hidai. 2004

286 citations recorded by the source corpus.

We have obtained high-resolution, high signal-to-noise near-UV-blue spectra of 22 very metal-poor stars ([Fe/H] < -2.5) with the Subaru High Dispersion Spectrograph and measured the abundances of elements from C to Th. The metallicity range of the observed stars is -3.2 < [Fe/H] < -2.4. As found by previous studies, the star-to-star scatter in the measured abundances of neutron-capture elements in these stars is very large, much greater than could be assigned to observational errors, in comparison with the relatively small scatter in the α- and iron-peak elements. In spite of the large scatter in the ratios of the neutron-capture elements relative to iron, the abundance patterns of heavy neutron-capture elements (56 ≤ Z ≲ 72) are quite similar within our sample stars. The Ba/Eu ratios in the 11 very metal-poor stars in our sample in which both elements have been detected are nearly equal to that of the solar system r-process component. Moreover, the abundance patterns of the heavy neutron-capture elements (56 ≤ Z ≤ 70) in seven objects with clear enhancements of the neutron-capture elements are similar to that of the solar system r-process component. These results prove that heavy neutron-capture elements in these objects are primarily synthesized by the r-process. In contrast, the abundance ratios of the light neutron-capture elements (38 ≤ Z ≤ 46) relative to the heavier ones (56 ≤ Z ≤ 70) exhibit a large dispersion. Our inspection of the correlation between Sr and Ba abundances in very metal-poor stars reveals that the dispersion of the Sr abundances clearly decreases with increasing Ba abundance. This trend is naturally explained by hypothesizing the existence of two processes, one that produces Sr without Ba and another that produces Sr and Ba in similar proportions. This result should provide a strong constraint on the origin of the light neutron-capture elements at low metallicity. We have identified a new highly r-process element enhanced, metal-poor star, CS 22183-031, a giant with [Fe/H] = -2.93 and [Eu/Fe] = +1.2. We also identified a new, moderately r-process-enhanced, metal-poor star, CS 30306-132, a giant with [Fe/H] = -2.42 and [Eu/Fe] = +0.85. The abundance ratio of the radioactive element Th (Z = 90) relative to the stable rare-earth elements (e.g., Eu) in very metal-poor stars has been used as a cosmochronometer by a number of previous authors. Thorium is detected in seven stars in our sample, including four objects for which the detection of Th has already been reported. New detections of thorium have been made for the stars HD 6268, HD 110184, and CS 30306-132. The Th/Eu abundance ratios [log(Th/Eu)], are distributed over the range -0.10 to -0.59, with typical errors of 0.10 to 0.15 dex. In particular, the ratios in two stars, CS 31082-001 and CS 30306-132, are significantly higher than the ratio in the well-studied object CS 22892-052 and those of other moderately r-process-enhanced metal-poor stars previously reported. Since these very metal-poor stars are believed to be formed in the early Galaxy, this result suggests that the abundance ratios between Th and stable rare-earth elements such as Eu, both of which are presumably produced by r-process nucleosynthesis, may exhibit real star-to-star scatter, with implications for (1) the astrophysical sites of the r-process, and (2) the use of Th/Eu as a cosmochronometer.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/2cab204dbb8fff2a6988db6c0339b6ce24838625

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In Astronomy and Astrophysics

1. Timothy C. Beers
2. C. Sneden
3. A. Frebel
4. Roberto Gallino
5. A. Karakas
6. J. Cowan
7. J. Norris
8. N. Christlieb

Citations in the corpus

• THE r-PROCESS NUCLEOSYNTHESIS IN THE VARIOUS JET-LIKE EXPLOSIONS OF MAGNETOROTATIONAL CORE-COLLAPSE SUPERNOVAE (2015, methodology): “The rprocess in low entropy environment is much depend on the degree of neutron-richness, so that further lower Ye cases, which are between the prompt and delayed jet models, are expected to be reproduce entire pattern of HD122563.”
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, unknown): “…004 by Christlieb et al. (2004) ([Eu/Fe] = 1.64), CS 31078– 018 by Lai et al. (2008) ([Eu/Fe] = 1.23), CS 29491–069 ([Eu/Fe] = 1.0) and HE 1219-0312 ([Eu/Fe] = 1.4) by Hayek et al. (2009), CS 22183–031 by Honda et al. (2004) ([Eu/Fe] = 1.2), CS 22953–003 by François et al. (2007) ([Eu/Fe] = 1.05).”
• Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars (2006, background): “…additional observational and theoretical studies of this star, and others with enhanced actinide abundances, such as the recently discovered r-process-rich Th-rich star CS 30306-132 (Honda et al. 2004), will be needed to better understand and further refine the chronometric age determinations.”
• NEUTRINO-DRIVEN WINDS IN THE AFTERMATH OF A NEUTRON STAR MERGER: NUCLEOSYNTHESIS AND ELECTROMAGNETIC TRANSIENTS (2015, background): “This kind of pattern (“Honda-like pattern”, see Honda et al. (2004, 2007)) is observed in a few very old stars and its origin is still unknown.”
• Is HE 0107–5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars (2004, background): “The abundances of many neutron-rich elements vary considerably from star to star even among those stars which have the abundance ratio [Ba/Eu] very close to the solar r-process value (McWilliam 1998; Aoki et al. 2002a; Honda et al. 2004).”
• Quantifying the uncertainties of chemical evolution studies II. Stellar yields (2010, background): “Several authors (e.g. Honda et al. 2004; Lai et al. 2008; Bonifacio et al. 2009) have noticed that an offset exists between the abundances derived from Cr I and Cr II lines.”
• Neutron-Capture Elements in the Very Metal Poor Star HD 122563 (2006, methodology): “The estimated uncertainties are TeA ¼ 100 K, log g ¼ 0:3 dex, v turb ¼ 0:5 km s 1, and ½Fe/H ¼ 0:19 dex (Honda et al. 2004).”
• 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).”
• The Intermediate r-process in Core-collapse Supernovae Driven by the Magneto-rotational Instability (2016, methodology): “…range from the extreme cases of m-models, which produce the full r-process abundances up to the heaviest nuclei, down to the i- or h-models, responsible for weak r-process patterns as seen in “Hondatype” r-process patterns (Honda et al. 2004) or only nuclei up to the second peak (A = 130).”
• NEW RARE EARTH ELEMENT ABUNDANCE DISTRIBUTIONS FOR THE SUN AND FIVE r-PROCESS-RICH VERY METAL-POOR STARS (2009, result): “We also compared our CS 31082-001 abundances with those of Honda et al. (2004), with similar results: < log ǫHonda − log ǫus > = +0.07 ± 0.03 (σ = 0.09, 12 elements in common).”
• A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. VI. DETAILED ABUNDANCES OF 313 METAL-POOR STARS (2014, methodology): “…abundance versus E.P. as the primary means of determining Teff b Studies that use Fe i abundance versus E.P. as the primary means of determining Teff (2004), Honda et al. (2004b), Preston et al. (2006a), Lai et al. (2008), Bonifacio et al. (2009), Ishigaki et al. (2010), and Roederer et al. (2010).”