Galactic Chemical Evolution of Heavy Elements: From Barium to Europium

Abstract

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable to reproduce a large set of Galactic (local and non local) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources:the dominant reaction 13 C(α,n) 16 O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22 Ne(α,n) 25 Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, it shows a sharp production of the Ba-peak elements around Z ≃ Z ⊙ /4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution given by the Galactic chemical evolution model at the epoch of the solar system formation.We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars) showing that the observed trends can be understood in the light of the present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

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*

“…It is worth noting that Ba II λλ4554 and 4934 are rather weak in each of these stars, and a change in the Ba abundance derived from these lines does not exceed a few hundredths when moving from the solar mixture 134 Ba: 135 Ba: 136 Ba: 137 Ba: 138 Ba = 2.4:6.6:7.9:11.2:71.7 (Lodders et al 2009, pp. 44-54) to the r-process one 135 Ba: 137 Ba: 138 Ba = 24:22:54 (Travaglio et al 1999). For example, Ba II λ4554 in HD140283 has an equivalent width (EW) of 20 mÅ, and the abundance shift between using the solar and the r-process Ba isotope mixture amounts to 0.02 dex.…”

“…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). Our data on Eu/Ba (top right panel in Figure 14) provide evidence for a dominant contribution of the r-process to the production of Ba and Eu in the early Galaxy, when the halo and thick-disk stellar population formed, and rapidly growing enrichment of the Galactic matter by s-nuclei, when metallicity increased from [Fe/H] ; −0.8 to the solar value.…”

Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

“…Based on nucleosynthesis predictions for stellar deaths, a number of detailed analyses have been performed, from light elements up to the Fe-group (e.g., Timmes et al 1995, Goswami & Prantzos 2000, Gibson et al 2003, Pagel 2009, Kobayashi 2012, Matteucci 2012. Such approaches have recently also been applied to understand the enrichment of heavy elements in the galaxy (including r-process contributions) as a function of time or metallicity [Fe/H] (see e.g., Ishimaru & Wanajo 1999, Travaglio et al 1999, De Donder & Vanbeveren 2003, Matteucci 2012, Vangioni et al 2015.…”

The Yields of r-Process Elements and Chemical Evolution of the Galaxy

“…The site or sites of the r-process are not known, although suggestions include the ν-driven wind of Type II SNe (e.g., Woosly and Hoffman, 1992; Woosly et al, 1994) and the mergers of neutron stars (e.g., Lattimer and Schramm, 1974; Rosswog et al, 2000). Particular attention to the Galactic evolution of elements produced by neutron-capture nucleosynthesis was given by Mathews et al, (1992), Pagel and Tautvaisiene (1997), and more recently by Travaglio et al (1999). These authors adopted the standard approach to Galactic chemical evolution, assuming that stars form from a chemically homogeneous medium at a continuous rate.…”