Explorations of ther‐Processes: Comparisons between Calculations and Observations of Low‐Metallicity Stars

Abstract

Abundances of heavier elements (barium and beyond) in many neutroncapture-element-rich halo stars accurately replicate the solar system r-process pattern. However, abundances of lighter neutron-capture elements in these stars are not consistent with the solar system pattern. These comparisons suggest contributions from two distinct types of r-process synthesis events -a so called main r-process for the elements above the 2 nd r-process peak and a weak r-process for the lighter neutron-capture elements. We have performed r-process theoretical predictions to further explore the implications of the solar and stellar observations. We find that the isotopic composition of barium and the elemental Ba/Eu abundance ratios in r-process-rich low metallicity stars can only be matched by computations in which the neutron densities are in the range 23 log n n 28, values typical of the main r-process. For r-process conditions that successfully generate the heavy element pattern extending down to A = 135, the relative abundance of 129 I produced in this mass region appears to be at least ∼ 90% of the observed solar value. Finally, in the neutron number density ranges required for production of the observed solar/stellar 3 rd r-process-peak (A ≈ 200), the predicted abundances of inter-peak element hafnium (Z=72, A ≈ 177-180) follow closely those of 3 rd -peak elements and lead. Hf, observable from the ground and close in mass number to the 3 rd r-process-peak elements, might also be utilized as part of a new nuclear chronometer pair, Th/Hf, for stellar age determinations.

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…Europium is referred to as an r-process element, because only 6% of solar Eu originates from the s-process (Travaglio et al 1999). Theoretical predictions of a pure r-process production of Eu and Ba give [Eu/Ba] r ;0.67 in the classical waiting-point (WP) approximation (Kratz et al 2007) and [Eu/Ba] r ;=0.87 in the large-scale parameterized dynamical network calculations of Farouqi et al (2010) in the context of an adiabatically expanding high-entropy wind, as is expected to occur in core-collapse SNe. The solar r-residual, i.e., the difference between solar total and s-abundance, where the s-abundance is deduced from the Galactic chemical evolution models, ranges between [Eu/Ba] r = 0.71 (Travaglio et al 1999) and 0.80 (Bisterzo et al 2014).…”

Abundance Signatures in Halo Stars: Clues to Nucleosynthesis in the First Stars

“…Examination of Figure 3 indicates some deviations between the solar curve and the stellar data for the hghter n-capture elements. This has led to suggestions of a second (or even multiple) r-process sites (see [ 15] for further discussion).…”

Cu i resonance lines in turn-off stars of NGC 6752 and NGC 6397

“…occurring in the inter-shell region of low-mass asymptotic giant branch stars) and the weak sr-process. The latter occurs in massive stars in the C-burning shell when neutron densities reach high values, intermediate between typical s-process neutron densities (10 9 −10 11 cm −3 , Despain 1980) and r-process neutron densities (10 20 −10 30 cm −3 , Kratz et al 2007). The contribution of the s-process, both weak and main, to the solar system Cu abundance is estimated by Travaglio et al (2004a) to be 27%.…”