Neutron Capture Elements in s-Process-Rich, Very Metal-Poor Stars

By Wak Aoki, S. Ryan, J. Norris, Timothy C. Beers, H. Ando, N. Iwamoto, Toshitaka Kajino, G. Mathews, M. Fujimoto. 2001

133 citations recorded by the source corpus.

We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7 mbarn-1, significantly larger than that which best fits solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r ~ 0.1).

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/cc0b6c821da5330321f7c9a113bb93b9b5f2da61

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In Astronomy and Astrophysics

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

In Nuclear and High Energy Physics

1. C. Sneden
2. J. Cohen
3. Roberto Gallino
4. Jorge Meléndez
5. J. Cowan
6. Wak Aoki
7. Timothy C. Beers
8. J. E. Lawler

Citations in the corpus

• Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium (2004, unknown): “We estimate the carbon isotope ratios (12C/13C) from the CH lines around 4200 8 (Aoki et al. 2001).”
• Chemical abundance analysis of the open clusters Cr 110, NGC 2099 (M 37), NGC 2420, NGC 7789, and M 67 (NGC 2682) (2009, background): “For neodymium, we could only find three reliable lines, which apparently do not need any detailed HFS (hyper-fine splitting) analysis (Aoki et al., 2001), at 5092, 5249 and 5485 Å.”
• The Binary Frequency Among Carbon-enhanced, s-Process-rich, Metal-poor Stars (2004, background): “Despite extensive investigations of this class of objects by means of high-resolution spectroscopy (e.g., Norris, Ryan & Beers 1997a, b; Aoki et al. 2001, 2002a, b, c), the origin of carbon in these stars still remains unclear.”
• The s process: Nuclear physics, stellar models, and observations (2010, methodology): “Measurements of Pb abundances have been made by Aoki et al. (2000, 2001) for CEMP stars, by Van Eck et al. (2001) for CH stars, and by Allen and Barbuy (2006) for Ba stars using the absorption line of neutral Pb at 4058 Å.”
• Abundances In Very Metal-Poor Dwarf Stars (2004, result): “Pb, whose abundance ratios are shown in Figure 10, is detected in three of the four C-rich stars, which are similar to the two s-process rich EMP subgiants discussed by Aoki et al. (2001).”
• A Subaru/High Dispersion Spectrograph Study of Lead (Pb) Abundances in Eight s-Process Element-rich, Metal-poor Stars (2002, methodology): “The open circle shows the ratio of LP 706–7 determined by Norris, Ryan, & Beers (1997) and Aoki et al. (2001) based on the AAT/UCLES spectrum.”
• Improved Laboratory Transition Probabilities for Nd II and Application to the Neodymium Abundances of the Sun and Three Metal-poor Stars (2003, background): “Isotopic splittings vary from line to line, but are always in the range 0:002 GdD maxd0:004 Å (Aoki et al. 2001 and references therein).”
• Is HE 0107–5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars (2004, methodology): “It has been also pointed out (Aoki et al. 2001, 2002b; Ryan et al. 2001) that metal poor stars exhibit interesting variations in the relative abundances of s-process elements of different atomic weight.”
• Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields (2006, background): “…Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany; lutz@aip.de of heavy neutron-capture elements that are synthesized in the s-process (e.g., Aoki et al. 2001; Van Eck et al. 2001), or in the r-process (e.g., CS 22892−052, Sneden et al. 1996, or CS 31082-001 Cayrel et al. 2001).”