A global comparison

Let us first mention thatZ = 0source star models may face more difficulties in reproducing the abundance patterns of CEMP-no stars than very low metallicity source star models: the 20M

Pop III model in Fig. 4.16 either produces enough C, N, O but not enough Na, Mg, Al (left panel) or it produces enough Na, Mg, Al but then C, O and¹²C/¹³C are overestimated (right panel). Also [Si/H] is underestimated by several dex.

Considering all the other source star models discussed in Sect. 4.3.3, we see that a global match can be found with the abundances of CEMP-no stars. Source star ejecta with various initial rotation rates, mass cuts and dilution factors allow to reproduce all or nearly all the range of CEMP-no star abundances. However, a global match does not mean that individual CEMP-no stars can be well reproduced. The next step is to compare source star models with individual CEMP-no stars. As it will be discussed thereafter (especially in Sect. 4.4.2), a more in-depth comparison reveals some difficulties for models to reproduce some of the abundances of individual CEMP-no stars.

Interestingly, we also note that the CNO pattern of the material ejected by the source star has generally either a∧-shape (H-rich ejecta, Fig. 4.9) or a∨-shape (H-rich + He-rich ejecta, Fig. 4.10).

It is nevertheless not always the case (e.g. Fig. 4.12). In Table 4.2, very few stars have C, N and O abundances available together, without upper limits. CS 29498-043 has C,N and O abundances available and it has a clear ∨-shape. By contrast, HE 1327-2326 has a clear ∧-shape. Another example is the CNO pattern of HE 0107-5240, which has a different shape (it as much N as O).

When considering the entire sample of Table 4.2, about 70 % is compatible with a∧-shape for the CNO pattern. Similarly, about 70 % is compatible with a∨-shape. About the same fraction (70 %) of the sample is compatible with an almost flat CNO pattern. Detailed comparisons between source star models and CEMP stars are likely required to obtain more informations on the CEMP source stars (Sect. 4.5 and 4.6).

7With the exception of G77-61, withTeff = 4000K andlogg = 5.05that is considered as a main-sequence star,

following Plez & Cohen (2005) and Beers et al. (2007).

CHAPTER 4. MIXING IN CEMP-NO SOURCE STARS

Table4.2:Starswith[Fe/H]<−3and[C/Fe]>1.[C/Fe]iscorrectedfrom∆[C/Fe](alsoreportedinthetable),thataccountsfortheeffectof thefirstdredge-up(Placcoetal.2014c).RecognizedCEMP-s,-r/sand-rstarsareexcluded.AbundancedataisfromtheSAGAdatabase(Suda etal.2008).Whenmultipleabundancesexistforastar,theabundancefromthemostrecentliteraturesourceisselected. StarTefflogg[Fe/H]A(Li)[C/Fe][N/Fe][O/Fe][Na/Fe][Mg/Fe][Al/Fe][Si/Fe]12 C/13 C∆[C/Fe]Ref BD+44_49354303.4-3.80.641.23<2.21.540.30.89-0.440.48>15.00.01,2,3 CS22877-00147901.45-3.240.661.54-0.081.22-0.38-0.73-35.00.442 CS22891-20044900.5-3.88<0.131.261.38<1.670.290.82-0.371.05>6.00.732,4 CS22897-00845500.7-3.730.771.050.05<1.86-0.050.6-0.770.46>20.00.455,6,7 CS22949-03746300.95-4.2<0.131.92.67<2.391.891.560.261.52>4.00.745,8,2 CS22950-04643800.5-3.64<-0.051.131.13<2.39-0.180.58-0.631.08-0.525,4 CS22953-03761503.7-3.051.971.0<2.18<0.5-0.55-0.560.21-0.05 CS22957-02752202.65-3.0<0.862.631.69<1.20.980.08-0.12-6.00.022 CS22960-05348601.65-3.33<0.551.423.41--0.770.51--0.025 CS29498-04344400.5-3.85<-0.053.061.742.371.031.780.751.088.00.312 CS29502-09248201.5-3.2<0.451.451.11.13-0.020.5-0.6-12.00.392 CS29527-01562404.0-3.552.0911.29---0.190.410.070.18-0.09,10,11,12,13,14 CS29528-04161704.0-3.061.711.573.04<1.380.670.38-0.38-0.16-0.012,15 G77-6140005.1-4.0<3.313.212.21.820.770.56--5.00.016,17 HE0015+004846000.9-3.07<0.921.29---0.66-0.370.5-0.6718 HE0017-434661983.8-3.07-3.11--1.510.870.03--0.08 HE0102-063360123.7-3.1-1.08--<1.060.44-0.270.32-0.08 HE0107-524051002.2-5.4<1.123.882.542.541.060.25<-0.08<0.48>50.00.0619,20 HE0134-151955003.2-4.01.271.0<1.0<2.9-0.240.25-0.380.05>4.00.021,22 HE0146-154846361.7-3.46-1.57--1.160.870.140.5-0.7323,24 HE0233-034361003.4-4.71.773.48<2.8<4.0<0.50.59<0.030.05>5.00.021 HE0251-321657503.7-3.15-2.53--0.970.610.05-0.6-0.08 HE0450-490263004.5-3.1<1.982.032.0<3.50.230.53-0.780.0--21 HE0557-484049002.2-4.8<0.71.77<0.962.3-0.250.16-0.71--0.0125,26 HE1005-143950001.9-3.2-2.511.79-1.170.59---0.0227 HE1012-154052302.65-3.76<0.752.41.15<2.181.651.810.690.65>30.00.02,8 HE1029-054666504.3-3.3<2.02.642.9<3.7--0.03<-0.42-0.039.0-21 HE1150-042852002.5-3.21-2.512.62-1.440.35---0.028 HE1201-151257254.2-3.89-1.14<1.23--0.350.2-0.73--0.023,24 HE1249-312153733.4-3.23-1.91---0.24-0.79--0.028,29 HE1300+015755503.3-3.491.061.34<0.81.78-0.130.33-0.150.57-0.08,30 HE1300-064153082.96-3.14-1.25---0.02-1.19--0.0128,29 HE1305-033160814.22-3.26-1.09-----0.7--0.031,28 HE1310-053650001.9-4.2<0.82.443.2<2.80.190.42-0.390.83.00.0821 HE1327-232661803.7-5.66<0.74.184.673.862.461.651.16->5.00.032,33 HE1338-005258563.7-3.0-1.53---0.44-0.160.39-0.08 HE1351-104952042.85-3.46-1.74---0.28-0.75--0.0128,29

4.4. Comparison with CEMP-no stars

Table4.2–continued StarTefflogg[Fe/H]A(Li)[C/Fe][N/Fe][O/Fe][Na/Fe][Mg/Fe][Al/Fe][Si/Fe]12 C/13 C∆[C/Fe]Ref HE1410-000456053.5-3.02<1.322.24-1.280.610.53---0.048,34 HE1413-195463023.8-3.52.0351.67-------0.010,29 HE1456+023056642.2-3.32-2.373.03-0.30.29-0.090.54-0.028 HE1506-011350162.4-3.54-1.490.61-1.650.89-0.530.5-0.0223,24 HE2123-032947251.15-3.22<0.581.06---0.58-0.570.56-0.6618 HE2139-543254162.2-4.02-2.612.09-2.151.610.361.0-0.0123,24 HE2318-162148461.4-3.67-1.041.24-0.710.2-0.58--0.535 HE2331-715549001.5-3.7<0.371.342.57<1.70.461.2-0.38<0.255.0-21 LAMOSTJ125346.0960303.65-4.021.81.59---0.20.24----36 LAMOSTJ131331.1847501.6-4.12<0.681.83---0.060.34-0.45--36 LAMOSTJ1626+172159303.6-3.2-1.07---0.020.5-0.330.46--37 LAMOSTJ1709+161657803.5-3.71-1.58---0.28-0.18--37 SDSSJ0002+292861504.0-3.26-2.63--0.990.36----38 SDSSJ0126+060769004.0-3.01<2.23.08--0.860.66----38,39 SDSSJ0212+013763334.0-3.592.042.28<2.661.6-0.52-0.550.23--40 SDSSJ0351+102654503.6-3.18-1.55---0.150.71----38 SDSSJ0723+363751502.2-3.32-1.79---0.10.23----38 SDSSJ1035+064162624.0<-5.07<1.13.54---<-0.06----40 SDSSJ1114+182862004.0-3.35-3.32.2----->60.0-41 SDSSJ1143+202062404.0-3.15-2.82.48-----20.0-41 SDSSJ1245-073861102.5-3.21-3.45--1.290.68-0.120.02--40 SDSSJ131326.8952002.6-5.0<0.82.963.46-0.370.44-0.11<0.21--42 SDSSJ1349-022962004.0-3.24-3.01--1.870.73-->30.0-38,43 SDSSJ1422+003152002.2-3.03-1.7--0.360.77----38 SDSSJ1613+530953502.1-3.33-2.09--0.760.92----38 SDSSJ161956+17053961914.0-3.57-2.34---0.02--0.27--44 SDSSJ1646+282461004.0-3.05-2.52---0.71----38 SDSSJ1742+253163454.0-4.8-3.63-<3.03<0.7<0.27-<0.34--40 SDSSJ1746+245553502.6-3.17-1.24--0.480.69----38 SDSSJ2209-002864404.0-3.96-2.61--------41 SMSSJ005953.9854132.95-3.942.01.2--1.990.61-0.250.73--45 SMSSJ031300.3651252.3<-7.30.74.9<3.8<5.0<1.83.0<1.1<3.0--46 References.1-Placcoetal.(2014a);2-Roedereretal.(2014a);3-Itoetal.(2013);4-Vennetal.(2004);5-Roedereretal.(2014b);6-Cayreletal.(2004);7-Spiteetal.(2006); 8-Cohenetal.(2013);9-Spiteetal.(2012);10-Sbordoneetal.(2010);11-Bonifacioetal.(2009);12-Andrievskyetal.(2007);13-Andrievskyetal.(2010);14-Andrievsky etal.(2008);15-Sivaranietal.(2006);16-Beersetal.(2007);17-Plez&Cohen(2005);18-Holleketal.(2011);19-Besselletal.(2004);20-Christliebetal.(2004);21-Hansen etal.(2015a);22-Hansenetal.(2014);23-Norrisetal.(2013);24-Yongetal.(2013);25-Norrisetal.(2012);26-Norrisetal.(2007);27-Aokietal.(2007);28-Barklemetal. (2005);29-Zhangetal.(2011);30-Frebeletal.(2007b);31-Renetal.(2012);32-Frebeletal.(2008);33-Aokietal.(2006);34-Cohenetal.(2006);35-Placcoetal.(2014b);36- Lietal.(2015);37-Lietal.(2015);38-Aokietal.(2013);39-Aokietal.(2008);40-Bonifacioetal.(2015);41-Spiteetal.(2013);42-Frebel&Norris(2015);43-Beharaetal. (2010);44-Caffauetal.(2013);45-Jacobsonetal.(2015);46-Kelleretal.(2014)

CHAPTER 4. MIXING IN CEMP-NO SOURCE STARS

Constraints from the¹²C/¹³C ratio

The CEMP-no stars with a measured¹²C/¹³C ratio have3<¹²C/¹³C<35. Two stars are still dwarfs, with¹²C/¹³C∼6. As mentioned previously (Sect. 4.2), the¹²C/¹³C ratio at the surface of an unevolved CEMP-no star is probably similar to the¹²C/¹³C ratio in the cloud in which the star formed, hence similar to the¹²C/¹³C ratio in the source star ejecta.

The¹²C/¹³C ratios of the CEMP-no star sample are well reproduced by the H-rich ejecta of the source star models (Fig. 4.9). By contrast, they are largely overestimated by the H-rich + He-rich ejecta (Fig. 4.10, cf. also Choplin et al. 2016, Sect. 5.1, page 176 of this thesis). Of course one could imagine to dilute the H-rich + He-rich ejecta, having a high¹²C/¹³C, with an ISM having a very low¹²C/¹³C (as discussed in Sect. 3.3.4, here I take¹²C/¹³C = 300 in the ISM but a lower value may also be chosen). The final mixture may present a low¹²C/¹³C ratio, consistent with observations.

In this case however, it just pushes back the problem: the need for a source producing a large amount of¹³C in the early Universe remains and has to be explained by some mechanism.

It is worth mentioning here that Galactic evolution models predict that a standard population of very low metallicity massive source stars will lead to a¹²C/¹³C ratio of4500−31000in the (al-most) primordial ISM (Chiappini et al. 2008). If instead, this population is dominated by massive fast rotators, Galactic evolution models predict30 <¹²C/¹³C< 300. Rotation in massive stars is indeed a way to produce large amounts of¹³C (e.g. Fig. 4.6). The fact that it exists CEMP-no stars (especially unevolved stars) with a¹²C/¹³C ratio even lower that 30 suggests that a very special material is required to form them, maybe coming from the relatively external layers of one specific rotating massive source star.

As a remark, let us mention that the fact that there is no CEMP star with a¹²C/¹³C ratio below the CNO-equilibrium value (which is about 4) suggests that the dominant source of¹³C in the early Universe comes from CNO burning. If there was another important source of ¹³C, we should observe CEMP-no stars with lower ¹²C/¹³C ratios. Moreover if such stars exist, they should be observable since low¹²C/¹³C ratios are easier to detect: if the ratio is low, ¹³C is abundant and then¹³C lines are stronger. This last point also shows that it can exist a bias towards low¹²C/¹³C.

Some CEMP-no stars have a lower limit for¹²C/¹³C (e.g. HE 1201-1512 and HE 1327-2326 with

12C/¹³C>20and>5respectively, Aoki et al. 2006; Norris et al. 2013). Future observations may reveal a population of stars with higher¹²C/¹³C ratios.

Can the H-rich material of massive source stars explain the abundances of CEMP-no stars?

In the previous discussion and in Sect. 4.2, it was proposed that CEMP-no stars could have formed with only the H-rich material of the source star. However, if considering only the H-rich source star ejecta, several issues arise (see Fig. 4.9):

1. The ranges of Mg/H, Al/H and Si/H ratios are not covered by source star models (also N and Na to a smaller extent).

2. The predicted [Al/H] ratios are too high (also true if considering the H- + He-rich ejecta, Fig. 4.10).

3. Such an ejecta shows a clear CNO processed signature with a very characteristic CNO pattern (∧-shape), while some CEMP-no stars are not compatible with this pattern.

4. The most C-rich CEMP-no stars, with [C/H]&−1.5cannot be explained.

First, some scatter in the CEMP star abundances is probably induced by the fact that the abun-dance data is not homogeneous, together with the possible 3D/NLTE corrections on abunabun-dances (cf. Sect. 2.2.2 and 4.2). This could alleviate the issue 1.

Second, some nuclear reaction rates of the Ne-Na and Mg-Al chains are uncertain (cf. Sect. 4.2).

Changing these rates will change the predicted Na/H, Mg/H, Al/H ratios but in a similar way

4.4. Comparison with CEMP-no stars

for all the models, i.e. the predicted scatter will not change. I computed again theυ_ini/υ_crit = 0.4 model with the nuclear rate of²⁷Al(p, γ)²⁸Si from Cyburt et al. (2010) instead of Iliadis et al. (2001).

This is an extreme case, favoring Al destruction, since the rate of Cyburt et al. (2010) is the highest one below 100 MK. AtT = 50MK, the Cyburt et al. (2010) rate is∼100times larger than the Iliadis et al. (2001) rate. The orange dashed line in Fig. 4.9 shows that the [Al/H] ratio in the ejecta of the model with the Cyburt et al. (2010) decreases by∼1dex compared to the standard case. We note that the [Si/H] is barely modified. Nuclear rate uncertainties can lead to significant differences in the predicted yields and make the [Al/H] consistent with the bulk of observed abundances. This helps with the issue 2.

The issues 3 and 4 can be solved if considering the H-rich + He-rich ejecta (Fig. 4.10), which gives much more carbon and reverses the CNO pattern. Some dilution with the ISM might provide abundance patterns able to reproduce the observations (right panel of Fig. 4.13). However, in this case, the predicted ¹²C/¹³C is too high. The difficulty here is to get a high enough C/N ratio together with a low¹²C/¹³C ratio, as it is observed on many CEMP-no stars (Fig. 4.17). This issue is discussed in the next section.

4.4.2 The C/N−¹²C/¹³C puzzle