68Zn
88Sr 138Ba
208Pb
Figure 5.7: Ejected mass (Eq. 3.15) of 4 isotopes as a function of the initial metallicity for 25M
models withυini/υcrit = 0.4. The mass cut is set according to Maeder (1992).
available among the heavy elements so that both massive stars and AGB star models can provide a reasonable solution.
Generally speaking, massive stars tend to produce larger light s-element (e.g. Sr) over heavy s-element (e.g. Ba, Pb) ratios than AGB stars. Another difference, as discussed in the previous letter, is the oxygen, that can be higher by1−2dex in massive stars compared to AGB stars. Some CEMP-s stars are O-rich, a characteristic that AGB models may not be able to account for. For instance CS 31080-095 has [O/Fe]= 2.35±0.12(Sivarani et al. 2006), LP625-44 has [O/Fe]= 1.8 (Aoki et al. 2002a), HE 2258-6358 has [O/Fe]= 1.8±0.1(Placco et al. 2013). Such high ratios do not exclude a contribution from an AGB companion but may indicate a contribution of one (or more) additional source (i.e. previous massive stars). By detailed comparisons between AGB and massive star models, it would probably be interesting to try to spot the elements like oxygen that could help probing the nature of the additional source(s).
5.4 The weak s-process at lower metallicities
With decreasing metallicity, less seed is available so that less s-elements are produced. In non-rotating models, the production of s-nuclei in massive stars becomes negligible below about Z = 10−4 (Prantzos et al. 1990). In rotating models, this metallicity threshold may be around Z = 10−7: Frischknecht et al. (2016) have shown that at Z = 10−7, rotation in a 25 M model has a modest effect on the abundances of s-elements. At very low metallicity, the lack of iron seed implies that even if the neutron source is boosted by rotation, there is no significant s-process boost. Also, it is worth mentioning that above an given metallicity threshold, the s-process boost induced by rotation becomes very small: atZ =Z, Frischknecht et al. (2016) have shown that the abundance patterns of rotating and non-rotating models are almost identical. This is because ro-tation does not provide much additional22Ne (neutron source) at this metallicity, as a result of the less efficient back-and-forth mixing process (Sect. 4.1). Indeed, at higher metallicity the distance between the He-core and the H-shell increases and the gradient ofΩis smaller so that the shear mixing between the He-core and the H-shell is less strong.
5.4. The weak s-process at lower metallicities
30 40 50 60 70 80
Atomic number (Z) 10
010
110
210
310
4Production factors
Fe Co
Ni Cu
Zn
GaGeAsSeBrKr Rb
SrY ZrNb Mo Ru
Rh PdAgCd
In Sn
SbTe
I XeCsBa
La CePr
Nd Sm
EuGdTbDy Ho
ErTmYbLuHfTa WReOs
Ir PtAu HgTl Pb
υ
ini/υ
crit=0υ
ini/υ
crit=0.
4υ
ini/υ
crit=0.
7,
17O( α,γ ) / 10
Figure 5.8: Production factors (Eq. 3.17) of 25M models withZ = 10−5 and with various initial rotation rates. For the fast rotating model, the rate of17O(α, γ) was divided by 10. The mass cut is set according to Maeder (1992).
5.4.1 A rotating 25Mmodel with various initial metallicities
The computation of grids of massive star models at lower metallicities (including s-process) is a work in progress. Only 25Mmodels are presented here. Fig. 5.7 shows the ejected mass (Eq. 3.15) of four isotopes for rotating 25Mmodels withZ = 10−3,2 × 10−4and10−5. As the metallicity decreases by 2 dex, the ejected mass of the considered isotopes decreases by2−4dex, depending on the chemical specie. The green and red patterns in Fig. 5.8 show the production factors (Eq. 3.17) of a non-rotating and a rotating 25Mmodel atZ = 10−5, respectively. Compared to their initial abundances, elements between Cu and Y are overproduced by1−2dex in the rotating model. The black pattern shows an extreme case, with fast rotation and a lower17O(α, γ) reaction rate (/10).
In this case, the production of elements between Fe and Eu are enhanced by 1 up to about 3 dex compared to their initial abundances. Overall, it shows that at very low metallicity, although the absolute yields are low (Fig. 5.7), rotation can strongly affect the distribution of heavy elements that are initially present in the star (Fig. 5.8). If rotation is not considered, the initial distribution of heavy elements is barely modified at this metallicity.
5.4.2 The origin of HE 1327-2326
The star HE 1327-2326 (Frebel et al. 2005a) is one of the most iron-poor star with [Fe/H]−5.7 and an excess of Sr compared to Fe. Red circles in Fig. 5.9 show its surface composition, according to a recent re-analysis (Ezzedine, priv. comm.). In particular, an overabundance of Zn was found ([Zn/Fe]= 0.84±30).
All possible chemical compositions one can obtain with the non-rotating 25 M model at Z = 10−5are shown by the purple area. This area shows the range of [X/Fe] ratios one can obtain when varying the mass cut from the final mass of the modelMfin = 24.5M(in this case only the material in the stellar winds is considered) to the remnant massMrem = 2.31M(according to the relation of Maeder 1992). We see that whatever the mass cut, N is underproduced by more than 2
CHAPTER 5. THE S-PROCESS IN CEMP SOURCE STARS pattern). The mass cut is set toMcut = 10.5M(it corresponds to the bottom of the H-envelope).
The purple shaded area stands for the same model without rotation. This area shows the range of yields one can obtain when varying the mass cut from the final mass of the model (only winds) to Mrem (see text for details). The dashed pattern shows the yields of the 20 M model with Z = 10−5 andυini/υcrit = 0.7presented in Sect. 4.3 (computed with a smaller network). In this case, Mcut = 7.9 M. D = 0 in all the cases (i.e. no dilution with ISM). Symbols show the abundances of the star HE 1327-2326 (light red sumbols denote upper limits, as shown by arrows).
dex and Sr by more than 1 dex.
The thick black pattern shows the [X/Fe] ratios of the fast rotating 25Mmodel (υini/υcrit= 0.7).
The mass cut is set to 10.5M, which is just below the H-rich envelope. No dilution with ISM was considered. In this case, the [Sr/Fe] ratio of HE 1327-2326 can be reproduced. The fit for light elements is also satisfactory. In particular, N is much closer to the observed value. It is however still underproduced by 0.5 dex. The dashed pattern, shows the yields of a model with same metal-licity and rotation but with an initial mass of 20Mand a small network (model of Sect. 4.3). This model can produce enough N, suggesting that a source star with a bit different initial mass may provide a better fit (at least for the light elements).
In Sect. 4.6, the source star model that best reproduced the abundance pattern (for light ele-ments, C to Si) of HE 1327-2326 was a fast rotating 60 M model without late mixing, without dilution with ISM and with a mass cut close to the bottom of the H-envelope (cf. Table 4.5). All these characteristics are consistent with the new results obtained here, except for the initial mass of the source star (20−25Mhere vs. 60Mbefore). The high [Sr/Fe] might favor rotating∼25M
instead of ∼60 M source stars because the s-process boost induced by rotation peaks around 20−40M(Choplin et al. 2018). A rotating 60Msource star may not produce enough Sr. In the future, the computation of additional source star models at this low metallicity but with different initial masses may help to further address this point.
The source star models presented here underproduce Ti, Ni and Zn by∼ 0.5 dex. Such ele-ments can be affected by explosive nucleosynthesis. Interestingly, it was shown that Ti and Zn are overproduced in jet-induced SNe (Tominaga 2009). This kind of asymmetric SN may be produced by fast rotation of the massive stellar core at the time of the explosion (Woosley 1993). Fast rota-tion of the core at the time of explosion may be due to fast rotarota-tion of the progenitor. The fact that HE 1327-2326 is enriched in both the product of rotation (e.g. N, Sr) and of jet-induced SNe (e.g.
Zn) may indicate that its source star was a massive rotating star that experienced a jet-induced SN.
5.5. Summary
In general, the determination of the abundances of both rotation products and jet-induced SNe products in very metal-poor stars might give clues on the impact of rotation on the explosion. An interesting perspective is to study these two families of elements in very metal-poor stars. How-ever, for some interesting elements like Zn, almost only upper limits are available at the moment since they are very challenging to detect.
5.5 Summary
By increasing the amount of available neutrons, rotation boosts the s-process in massive stars.
A new grid of non-rotating and rotating (υini/υcrit = 0.4) stellar models at [Fe/H]= −1.8with initial masses of 10, 15, 20, 25, 40, 60, 85, 120 and 150Mwas computed. Stellar yields are publicly available. Rotation has a strong impact on the production of s-elements (especially the first peak, e.g. Sr) for initial masses between about 20 and 60M. Two additional 25Mmodels were com-puted: one with faster rotation (υini/υcrit = 0.7) and another with faster rotation (υini/υcrit = 0.7) and lower17O(α, γ) rate (divided by 10, it reduces the poisoning effect of16O). In these two mod-els, the production of s-elements is boosted again. The fast rotating model with a lower17O(α, γ) rate is the model where the s-process boost is the strongest. In particular, a modest amount of heavy s-elements (e.g. Pb) is synthesized. In stellar models with initial massesM & 60M, the back-and-forth mixing is less efficient so that the production of s-elements is similar whether or not rotation is considered.
Some CEMP-s stars appear to be single stars, which may challenge the AGB binary scenario (for these specific stars). The yields of the fast rotating 25Mmodel of the computed grid provide a material able to fit the abundance patterns of 3 out of the 4 apparently single CEMP-s stars.
The s-process boost induced by rotation still exists in lower metallicity massive stellar models (probably down to aboutZ ∼ 10−7). HE 1327-2326, one of the most iron-poor star known, has [Sr/Fe] = 1.08. Its abundance pattern can be reproduced by a fast rotating very low metallicity 20−25Msource star. The fact that this CEMP star is also enriched in Zn, which is a product of jet-induced SNe, is consistent with a scenario proposing that its source star was a fast rotator that experienced a jet-induced SN.