In this chapter, the origin of the CEMP stars with [Fe/H]< −3 is investigated. The recog-nized CEMP-s, -r/s and -r stars are not considered. For convenience, the stars considered here are referred to as CEMP-no stars, even if strictly speaking, some of them are just CEMP (because of missing abundance data to classify them). Sections 4.2 and 4.5 include the results published in the first and second articles mentioned below, respectively (the full papers can be found page 159 and 176 of this thesis).
Constraints on CEMP-no progenitors from nuclear astrophysics A. Choplin, A. Maeder, G. Meynet, & C. Chiappini, 2016 A&A593, 36 Pre-supernova mixing in CEMP-no source stars
A. Choplin, S. Ekström, G. Meynet, A. Maeder, C. Georgy, & R. Hirschi, 2017 A&A605, 63
4.1 The back-and-forth mixing process
In the previous chapter, some aspects regarding the nucleosynthesis and the physics of rotation were discussed. The interplay between rotation and nucleosynthesis in rotating massive stars is now discussed.
During the core H-burning and He-burning phase, the mixing induced by rotation changes the distribution of the chemical elements inside the star. In advanced stages (C-burning and after), the burning timescale becomes small compared to the rotational mixing timescale so that rota-tion barely affects the distriburota-tion of chemical elements. During the core He-burning phase, two different burning regions exist in the star (He-burning core and H-burning shell). The rotational mixing triggers exchanges of material between the convective He-burning core and the convec-tive H-burning shell: He-burning products are transported to the H-burning shell, processed by H-burning, transported back to the He-burning core, etc... The main steps of this mixing process are (see Fig. 4.1 for a schematic view):
1. In the He-burning core, the triple alpha process synthesizes12C.16O is formed by12C(α, γ)16O.
2. 12C and16O are mixed into the H-burning shell. It boosts the CNO cycle and creates primary CNO elements, especially14N (and13C to a smaller extent).
3. The products of the H-burning shell (among them primary 13C and 14N) are mixed back into the He-core. From the primary14N, the reaction chain14N(α, γ)18F(e+νe)18O(α, γ)22Ne allows the synthesis of primary22Ne. The reactions22Ne(α, n) and22Ne(α, γ) make25Mg and
4.2. Nucleosynthesis in a box
14N
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r
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27Al
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r
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r
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Early He-b Mid He-b End He-b
25,26Mg convective zone radiative zone
Figure 4.1: Schematic view of the back-and-forth mixing process at work in a fast rotating massive star (for clarity, only some chemical species are represented). The small vertical arrows denote some of the elements whose abundance is increased by the arrival of the products of the other burning zone.
26Mg respectively. The neutrons released by the22Ne(α, n) reaction produce19F,23Na,24Mg and27Al by14N(n, γ)15N(α, γ)19F,22Ne(n, γ)23Ne(e−ν¯e)23Na,23Na(n, γ)24Na(e−ν¯e)24Mg and
26Mg(n, γ)27Mg(e−ν¯e)27Al, respectively. Free neutrons can also be captured by heavier seeds like 56Fe and boost the s-process (cf. Sect. 3.1.2). This point is investigated in details in Chapter 5.
4. The newly formed elements in the He-burning core can be mixed again into the H-burning shell. It boosts the Ne-Na and Mg-Al chains: additional Na and Al are produced.
A very fast rotator will go through all the steps while a slow rotator only through the first one.
An important effect (not shown in Fig. 4.1) is the growth of the convective He-burning core that helps reaching layers that had been previously enriched in H-burning products (e.g. 13C,14N).
Both the growing of the convective He core and the backward diffusion of chemical elements impact the nucleosynthesis in the He-burning core of rotating models.
Fig. 4.2 shows the results of this mixing process in a 20Mstellar model, when the central4He mass fraction is about 0.2. The 14N and 13C peaks can be seen and to a smaller extent, the23Na peak (atMr∼8M). In the He-core,22Ne has been enhanced because of the ingestion of the extra
14N. Complete stellar models are discussed in more details in Sect. 4.3.
4.2 Nucleosynthesis in a box
Maeder et al. (2015) suggested that the wide range of abundances covered by CEMP-no stars could come from a material ejected by massive source stars having experienced various degree of rotational mixing. In this scenario, the variety of CEMP-no star abundances are mainly explained by the interplay between rotation and nucleosynthesis at work during the core He-burning phase of the massive source star (described in Sect. 4.1). Maeder & Meynet (2015) built a new classifica-tion scheme for CEMP-no stars by considering the successive steps in the back-and-forth mixing process. Five classes were proposed, the first one showing a complete absence of mixing and the fifth one a high degree of mixing (see Fig. 4.3). For instance, HE 1327-2326, with [Fe/H]= −5.7 (Frebel et al. 2008) and showing strong overabundances in light elements (e.g. CNO) as well as in
CHAPTER 4. MIXING IN CEMP-NO SOURCE STARS
Figure 4.2: Abundance profile of a fast rotating 20M model withυini/υcrit = 0.7atZ = 10−5. The model is near the end of the core He-burning stage. The thick black lines on the top labelled He-core and H-shell show the location of the convective He-burning core and H-burning shell respectively.
strontium, is belonging to class 4. In Maeder & Meynet (2015), they considered 46 stars, 4 appeared to be of class 2, 17 of class 3, 9 of class 4 and 16 unclassified because of missing abundance data.
No star belonging to class 0 or 1 were found.
In Choplin et al. (2016), we aimed at investigating quantitatively the effect of the back-and-forth mixing process using a one-zone (or box) nucleosynthesis code I developed. This code allows the injection of chemical species in the box while nucleosynthesis is calculated. It mimics the effect of rotational mixing at work in complete rotating stellar models. In the paper, I used the one-zone code to mimic the hydrogen burning shell of a massive rotating star in which12C,16O,22Ne, and
26Mg are injected (these species are supposed to come from the He-burning core of the rotating massive star). We studied the nucleosynthesis of the CNO cycle and the Ne-Na Mg-Al chains at different temperatures, densities, and with different nuclear reaction rates while injecting chemical species.
Initial setup
For the initial composition of the box, a rotating 60M model is used, withZ = 10−5 (corre-sponding to [Fe/H]=−3.8) and computed with anα−enhanced mixture. The initial abundances in the box are taken from the H-burning shell of this stellar model, when it starts the core He-burning stage (central4He mass fraction equals to 0.98). The corresponding initial [X/Fe] ratios in the box are shown in Fig. 4.4 by the patterns with squares. The initial composition of the box is different than the initial composition of the stellar model since some nuclear burning has already operated in the stellar model during the H-burning phase. In particular, CNO burning has oper-ated (that mainly transforms C and O into N) explaining the high initial [N/Fe] and lower [C/Fe]
and [O/Fe] in the box. The nuclear reaction rates of the CNO cycle are from Angulo et al. (1999)
4.2. Nucleosynthesis in a box
Figure 4.3:Five different possible chemical compositions of the CEMP-no source star at the end of the core He-burning phase. Each diagram corresponds to a different degree of mixing (top left: no mixing, bottom right: highest degree of mixing). The corresponding classes of CEMP-no stars are indicated, following the classification (simplified) of Maeder & Meynet (2015). Ejectadenotes the material that should be used to form the CEMP-no star.
except for 14N(p, γ)15O which is from Mukhamedzhanov et al. (2003) ifT ≤ 0.1GK and Angulo et al. (1999) otherwise. The rates related to the Ne-Na and Mg-Al chains are from Iliadis et al.
(2001). Only20Ne(p, γ)21Na and22Ne(p, γ)23Na are taken from Angulo et al. (1999) and Hale et al.
(2002), respectively. For20−60 M stellar models at such metallicity,T ∼ 50 MK andρ ∼ 1 g cm−3in the H-burning shell. There are the default temperature and density taken in the box. The box simulations are stopped either at H-exhaustion (1H mass fraction is below10−8) or when the time exceeds 10 Myr.
Four separate injection experiments were carried out: (1) no injection, (2) injection of12C and
16O, (3) injection of12C,16O and22Ne and (4) injection of12C,16O,22Ne and26Mg. The species are injected at a constant rate. During a time∆t, a mass∆mi =Ri∆tunder the form of the isotopei is injected in the burning box. Riis the injection rate of the isotopeiexpressed inMyr−1.Riwas calibrated using results from complete stellar models. Details on this calibration are now given (see also the appendix of Choplin et al. 2016, page 176 of this thesis).
CHAPTER 4. MIXING IN CEMP-NO SOURCE STARS
6 7 8 9 10 1112 13 14 4
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