Europium, Samarium, and Neodymium Isotopic Fractions in Metal‐Poor Stars

Abstract

We have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high-resolution (R $ 120;000), high signal-to-noise ratio (S/N $ 160Y1000) spectra obtained with the 2d-coudé spectrograph of McDonald Observatory's 2.7 m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r-or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r-process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD 196944.

The Hobby-Eberly Telescope "Chemical Abundances of Stars in the Halo" (CASH) Project. I. The Lithium-, s-, and r-Enhanced Metal-Poor Giant HKII 17435-00532

“…[Fe/H] is defined by the Fe abundance for the final set of T eff , log(g), and v t . We increase the overall metallicity Z of the atmosphere by +0.4 dex to account for the extra electrons from the α-elements that contribute to the H − continuous opacity; this artificial increase alters our derived abundances of the electrondonating elements by 0.01 dex and is completely negligible when elemental abundance ratios are considered (see, e.g., § 4.3 of Roederer et al 2008). We adopt the following atmospheric parameters for HKII 17435-00532: T eff / log(g)/v t /[Fe/H] = 5200 K/2.15/2.0 km s −1 /−2.23.…”

Uncertainties in s-process nucleosynthesis in massive stars determined by Monte Carlo variations

“…Observationally, the isotopic composition of neutron capture elements in galactic stars has been measured only for few elements (e.g., Ba, Sm, Nd and Eu) so far (Roederer et al 2008;Gallagher et al 2015), whereas the elemental abundances are available for much more elements (with the notable exception of In). The uncertainty ranges and primary key reactions for each es-process element are summarized in Table 8.…”

r-process enhanced metal-poor stars

“…quality and quantity of Sm isotopic fractions first investigated by [17,23]. The substantial growth of n-capture abundances in very metal-poor stars over the past couple of decades has illuminated the role of the r-process in early Galactic nucleosynthesis.…”