Massive source stars: nucleosynthesis and models with rotation
3.1 Nucleosynthesis processes in massive stars
3.1.1 Stellar fusion
Core hydrogen burning. The first burning stage, lasting for∼1Myr to several tens of Myr, is the fusion of hydrogen to helium. The pp-chains and the CNO-cycle both contribute to transform H into He. Above 17 MK (corresponding the H-burning temperatures of a∼1.2Mstar), the CNO cycle becomes more efficient than the pp-chain. Provided a little bit of CNO elements are avail-able, this is the CNO-cycle that dominates the production of energy during the H-burning stage of massive stars. The main CNO loop (CNOI) is shown by the colored loop in Fig. 3.1. One loop transforms four protons into one4He. As shown by the colors of the arrows,14N(p, γ) is the slowest reaction, followed by12C(p, γ) and13C(p, γ). After a timescale determined by14N(p, γ) (the slow-est reaction) the CNOI cycle reaches an equilibrium where 12C/14N= 0.025and12C/13C= 3.3.
Because14N(p, γ) is the bottleneck reaction, the main effect of CNOI is to transform the initial C nuclei into14N. The branching point at15N is the starting point of other CNO loops that take place at higher temperatures and that allow the synthesis of other isotopes, particularly16O.
During this first stage, the Ne-Na and Mg-Al chains (Fig. 3.2) can also be activated for temper-atures higher than∼35MK for Ne-Na and∼50MK for Mg-Al. The rates of the reactions at work in these chains are more uncertain than the reactions rates of the CNO-cycle. At the branching point23Na, the23Na(p, α)/23Na(p, γ) ratio equals 159 and 7 at 40 and 70 MK respectively accord-ing to Iliadis et al. (2010). For the rates of Cyburt et al. (2010), these ratios are 0.3 and 0.6 (still at 40 and 70 MK). We see that depending on the literature source used and of the temperature, the Ne-Na and Mg-Al chains can either be considered as closed loops or not. They are closed loops if the23Na(p, α)/23Na(p, γ) ratio is high.
Generally, the production of Ne, Na, Mg and Al in massive stars can be significantly affected if considering nuclear reaction rates from different sources. Decressin et al. (2007) have shown that multiplying the rate of24Mg(p, γ) by103around 50 MK changes the production of Mg-Al isotopes during the main sequence of low metallicity massive stars by0.5−1dex.
3.1. Nucleosynthesis processes in massive stars
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Approx.
𝝉
(at T = 25 MK) 105 yr 1 min(
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Figure 3.1: Illustration of the CNO cycle. The colored hexagon shows the main loop (CNOI). The other loops are shown by dashed lines. The colored arrows show the approximate timescales for the associated reactions (adapted from Maeder 2009).
Core helium burning. When running out of hydrogen, the core contracts until the central tem-perature is hot enough (about 100 MK) for the3αprocess to start. The second burning stage is the core helium burning phase. It lasts for∼ 0.1−1Myr, which is about 1/10 of the core hydrogen burning phase. At the beginning of core He-burning, the14N synthesized during core H-burning through the CNO cycle (cf. previous discussion) is quickly transformed into22Ne via the chain
14N(α, γ)18F(e+νe)18O(α, γ)22Ne. The22Ne(α, n)25Mg reaction, activated atT ∼ 220MK releases free neutrons that can be captured by seeds like iron and heavier elements. This is the slow neutron capture process (s-process) which is discussed below in Sect. 3.1.2.
During core He-burning, a hydrogen shell burns above the helium core. The H-shell burns at a slightly higher temperature than the H-core. It induces a somewhat different nucleosynthesis in the H-shell. For instance, the Ne-Na and Mg-Al cycles are more active in the H-shell. Rotational mixing can transport elements from the He-burning core to the H-burning shell (and vice-versa) and trigger a rich and varied nucleosynthesis (Sect. 4.1).
Advanced burning stages. At the end of the core He-burning stage, the most abundant species in the core are12C and16O. Since the12C +12C reaction has the lowest coulomb barrier, the next burning stage is core carbon burning. After carbon burning comes neon photodisintegration, oxy-gen and then silicon burning. The advanced stages last for10−1000yr (C),0.1−1yr (Ne),0.1−1yr (O) and0.1−10days (Si, e.g. Heger et al. 2000; Hirschi et al. 2004). During these stages, most of the energy goes out from the star in the form of neutrinos. To compensate for this strong energy loss, the rates of nuclear reactions increase. This leads to a quick burning of chemical species, explain-ing the short duration of the advanced stages. Durexplain-ing these stages, the core is decoupled from the
CHAPTER 3. MASSIVE SOURCE STARS: NUCLEOSYNTHESIS AND MODELS WITH ROTATION
21
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Figure 3.2:Ne-Na and Mg-Al chains (or cycles) (adapted from Maeder 2009).
rest of the star. It implies that the star does not move anymore in the Hertzsprung-Russell (HR) diagram. The main products of C-burning are 20Ne,23Na and24Mg. Neon photodisintegration produces mainly16O,24Mg and28Si, oxygen burning mostly28Si and32S and silicon burning56Ni (cf. Woosley & Weaver 1995, especially their Table 19). At the end of each core burning phase, the burning continues in a shell. As evolution proceeds, more and more different burning regions are present in the massive star.
3.1.2 The weak s-process
Massive stars are generally associated to the weak s-process, mainly responsible for the ele-ments withA <90. Cameron (1960) identified that at the beginning of the core He-burning phase of massive stars, the secondary1 14N (synthesized during the main sequence thanks to the CNO-cycle) is converted into22Ne by successiveα−captures and can provide a source of neutrons with the22Ne(α, n) reaction. This reaction is efficiently activated atT >220MK. It was later recognized that the s-process in massive stars occurs principally in the He-burning core of massive stars (Pe-ters 1968; Couch et al. 1974; Lamb et al. 1977; Langer et al. 1989; Prantzos et al. 1990; Raiteri et al.
1991a). In solar metallicity stars withMini &30M, some22Ne is left at the end of core He-burning phase so that s-process can occur during later stages (Couch et al. 1974). The carbon shell burning is the second efficient s-process production site inside massive stars (it contributes to ∼ 20 % at solar metallicity, Raiteri et al. 1991b; The et al. 2007). Neutrons are released by 22Ne(α, n) with theα particles provided by the12C(12C,α)20Ne reaction. The neutron density is typically∼ 1011 cm−3, which is∼4dex higher than in the He-burning core. He-burning shell and C-burning core do not contribute significantly in producing s-elements (Arcoragi et al. 1991). At low metallic-ity, the13C(α, n)16O reaction provides a small burst of neutrons at the very beginning of the core He-burning phase that can synthesize some light s-elements (A.85, Baraffe et al. 1992).
1An isotope is synthesized through thesecondarychannel if it comes from the initial metal content of the star. It is
formed through theprimarychannel if produced from the initial hydrogen and helium content of the star.