Neutron-capture elements

Elements up to iron-peak are produced by stellar nucleosynthesis and heavier elements are formed by SNe nucleosynthesis. Nuclei beyond zinc are formed through neutron capture process free from any Coulomb barrier. The mechanisms is the following : nuclei capture neutrons and if the resulting nuclei are unstable they can transform neutrons into protons

viaβ decay (emitting an electron and an electron antineutrino), producing progressively all

heavier elements. Two types of neutron-captures processes can be defined depending on the competition between neutron-capture andβ decay time scales.

• rapid (r)-process : when the time scale for neutron capture is faster than radioactiveβ decay time scale. It occurs in high neutron density regions where the nuclei have no time to decay between two neutron captures. A large number of neutron are captured per second and the typical neutron density is of 1024− 1028cm−3(Kratz et al. 2007). Following successive neutron captures the atomic number remain unchanged but the nuclei mass increase creating radioactive and neutron-rich elements far from the Valley

of stability. The highly unstable nuclei obtained decay afterwards throughβ decay increasing their atomic number.

• slow (s)-process : when the time scale for neutron captures is slower than radioactive

β decay time scale. It occurs in small neutron densities (≤ 108_cm−3_{) where a single}

neutron is captured by a nucleus. The time between two neutron captures is of the order of hundreds to thousands of years. If the nuclei just obtained is unstable it will decay instead of capturing an other neutron. The nuclei created by s-process are close to the Valley of stability.

As the nuclei involved in the s-process are relatively long-lived their properties (mass, half-live) can be obtained experimentally. On the contrary we are lacking experimental properties of nuclei involved in r-process because their typical radioactive half-lives are of the order of 0.01- 0.1 seconds. This is why most of r-process nuclei data rely only on theoretical developments (e.g., Lunney et al. (2003)). Some elements are called pure r- or s- process because they can be formed by only one channel. For example, some stable isotopes are pure r-process because they are preceded by an unstable isotope of short half-life. In between stable isotopes could be produced by both s- and r-processes through different paths .

The observed peaks (after iron) in Solar System abundance distribution presented in figure 2.3 can be explained by the s-and r-processes. Particular nuclei presenting magic number of nucleons are known to be extremely stable. For magic nucleon number, the neutron capture cross-section decreases a lot, stopping the s-process and leading to an accumulation of magic isotopes. Sr, Y and Zr form the first s-process peak (light-s element), Ba, La, Ce, Pr and Nd the second peak (heavy-s elements). A third peak is located at the end of the s-process at Pb and Bi. The heavier element produced by s-process is Bi. The r-process also induced peaks that are horizontally shifted (lower Z than in s-process). An enhancement of neutron-rich unstable elements with magic neutron number is produced by r-process, leading afterβ decays to an enhancements of elements at lower Z.

Sources of r- and s-process need very different astrophysical environments depending on the contrast in neutron densities. Thanks to theoretical studies and observations, sources of s-process elements are identified as : i) low and intermediate mass thermally pulsating AGBs (Sneden et al. 2008) and ii) massive stars during He and C burning phases. In the case of AGBs as sources of s-process, the contribution to chemical enrichment is delayed by 100-300 Myr allowing to probe the star formation in similar time-scale to alpha-elements. According to Burbidge et al. (1957) and Cameron (1957a,b) the sources of r-process required an explosive environment, like during SNe events. The nature of the r-process elements is still a puzzling question. The first and usual considered channel production is the core-collapse supernovae (Woosley et al. 1994; Qian & Wasserburg 2007). If production of r-process elements are associated with massive stars nucleosynthesis their contribution to enrichment should not suffer from time delay. However Wanajo (2013) show that SNeII seem to fail to synthesize

-2 0 2 4 6 8 10 12 0 50 100 150 200 250

log

(X/H) + 12

Atomic Mass

Elements up to the Fe peak Neutron-capture elements

Figure 2.3: From Karakas & Lattanzio (2014) : Solar abundance distribution using data from Asplund et al. (2009).

the heavy r-process elements. Alternatively other models have been recently proposed for the production of r-process elements : neutrino-driven proto-neutron-star PNS wind of core- collapse supernovae (Wanajo 2013), and binary neutron star (NS-NS) mergers (Wanajo et al. 2014; Just et al. 2014). Rare events but able to reproduce light and heavy elements r-process elements according to their models.

The two r- and s-process can be separated into weak and main contributions.

The weak s-process occurred in massive stars at the end of the He and C burning phase and create light neutron capture elements (A ≤ 90, up to first peak elements). The main s-process occurred in AGBs as explained above and form the heavy neutron capture elements (A ≥ 90, beyond the first peak and up to Pb). However Travaglio et al. (2005) show that the s-process production in AGBs is strongly dependent on the metallicity (the AGB yields are metallicity-dependent). At solar metallicity large amount of the first s-process peak elements are produced (Sr, Y and Zr) because more nuclei capture neutrons. At lower metallicity more second s-process peak elements (Ba, La, Ce, Nd) are preferentially created because more

neutron per nuclei are available.

The main r-process dominates the production of heavy neutron capture elements for relatively metal-rich stars ([Ba/H] ≥ −2.5, this corresponds more or less to [Fe/H] ≥ −2.3 dex in the Galaxy). François et al. (2007) show that a weak r-process is required to explain the synthesis of light (first peak) neutron capture elements in metal-poor stars. From observations comparing abundances [light/heavy] elements versus [heavy/H] they suggest 3 different regimes : solar ratio for [Ba/H] ≥ −2.5 (corresponding to the main process), increasing over-abundance for decreasing [Ba/H] for −4.5 ≤[Ba/H] ≤ −2.5 (weak process), and again solar values for [Ba/H] ≤ −4.5 corresponding to [Fe/H] ' −3. The heavy neutron-capture elements (like Ba or Eu) are produced by the main r- and s-processes. The origin of the light neutron-capture elements (as Sr, Y, Zr) is more complex : at solar [Fe/H] they are formed through main s-process (Travaglio et al. 2004; Bisterzo et al. 2014) but at low [Fe/H] the are formed dominantly by weak r-process (François et al. 2007).

Some exceptions exist like Eu which is almost exclusively produced by r-process. Europium in the Sun is almost entirely produced via the r-process (Sneden et al. (2008) quote the value of 97%).