Internal mixing processes in CEMP stars

and found that 12 % of [Fe/H]<−2stars are CEMP0.7.

One of the possible bias to these frequencies is that in giant stars, the first dredge-up has oc-curred. Amongst other, it changes the surface carbon abundance (cf. Sect. 2.4). For a sample of 505 stars with [Fe/H] < −2, Placco et al. (2014c) have estimated the effect of the first dredge-up on the surface carbon abundance. The recognized CEMP stars enriched in s- and/or r-elements were excluded from their analysis⁹. The stellar models they used predict that the correction on the [C/Fe] ratio induced by the first dredge-up is generally about 0.5 dex for the evolved stars. They recovered the initial [C/Fe] ratio of the 505 stars of their sample (the initial ratio is higher in case the star has experienced the first dredge-up). They finally found that 20, 43 and 81 % of stars with [Fe/H]<−2,<−3and−4are CEMP0.7, respectively. The first dredge-up effect corrected, it likely gives the fraction of stars which were born as CEMP0.7.

There is also a spatial variation of the CEMP frequency: Carollo et al. (2012) have shown that the CEMP0.7 frequency is increasing with the distance to the Galactic plane. Almost all CEMP0.7

stars belong to the halo and the fraction is higher in the outer than in the inner halo. Considering the most distant stars (more than 9 kpc from the Galactic plane, i.e. mainly in the outer halo), they derived a CEMP0.7frequency of 20 %. Carollo et al. (2014) reported a fraction of CEMP-no stars of 43 and 70 % in the inner and outer halo respectively. For the CEMP-s stars, they found 57 and 30 % in the inner and outer halo respectively. It suggests that the dominant source of CEMP stars in the two halo components were different. A recent study suggested that the CEMP fraction (among the [Fe/H]<−3stars and after excluding the likely CEMP-s and -r/s stars) in the Sculptor dwarf galaxy is∼36±8 %(Chiti et al. 2018), i.e. similar to the∼43 %of the Galactic halo (Placco et al.

2014c).

Are CEMP stars in binary systems?

Lucatello et al. (2005) and Starkenburg et al. (2014) showed that the whole sample of CEMP-s stars is consistent with the hypothesis of them all existing in binary systems. A careful monitoring of the radial velocity of 22 CEMP-s stars over several years has revealed a clear orbital motion for 18 stars (∼82 %) while 4 stars appeared to be single (Hansen et al. 2016b). The probability of finding one face-on system in their sample is about0.01 %, meaning that it is extremely unlikely that all the four apparently single CEMP-s stars are in fact binary systems that were seen face-on.

Apparently single stars might nevertheless have a companion with a long orbital period (about 10³−10⁴ days at minimum), which would prevent to detect radial motions of the CEMP-s stars.

A similar study was carried out for CEMP-r and CEMP-no stars. The binary frequency was found to be 18±6 %for CEMP-r (Hansen et al. 2015b) and 17±9 %for CEMP-no stars (Hansen et al.

2016a), i.e. much lower than for CEMP-s stars. It suggests that the CEMP-r and CEMP-no stars are likely disconnected from a binary origin.

2.4 Internal mixing processes in CEMP stars

CEMP stars formed with the material ejected by previous stars (at least some of it). One may see the nucleosynthetic signature from these previous stellar generations in the CEMP star surface chemical composition. However, the CEMP star surface composition can be altered during the life of the CEMP star itself. If this alteration is important enough, it could erase the chemical imprint let by the previous star(s). Two different categories of processes can affect the surface abundances of CEMP stars: external and internal processes.

• External processes can be accretion of interstellar material or accretion of material from a binary companion. While accretion of interstellar material was found to not have a

signif-9Note that this does not mean that all stars they consider are CEMP-no. Some CEMP stars in the sample without a

determined Ba abundance might appear to be CEMP-s, -r/s or -r in the future.

CHAPTER 2. CEMP STARS: OBSERVATIONS AND ORIGINS

Figure 2.6: Evolutionary tracks of 0.85Mmodels atZ = 0.0001without thermohaline and with-out rotation (top panels), with thermohaline and with rotation (bottom panels, models from La-garde et al. 2012). The color shows the surface¹²C/¹³C (left panels) and [C/N] ratios (right panels).

Symbols show CEMP stars with [C/Fe]>1and [Fe/H]<−3(recognized CEMP-s, -r and -r/s are not plotted). The triangle shows HE 1029-0546 with¹²C/¹³C= 9and [C/N]=−0.26. The square shows CS 29528-041 with [C/N]=−1.47(¹²C/¹³C is unknown).

icant effect on metal-poor stars (Frebel et al. 2009; Johnson & Khochfar 2011), the accretion of material ejected by a companion is certainly an important process, especially for CEMP-s stars (cf. Sect. 2.5.1).

• Internal processes happen in the CEMP star itself. There are for instance the first dredge-up or thermohaline mixing.

Below it is discussed the main (known) internal mixing processes that can happen in CEMP stars and possibly alter their surface composition. A sample of CEMP stars is considered and compared with low-mass stellar models. The sample comprises the CEMP stars with [C/Fe]>1 and [Fe/H]<−3. The CEMP stars enriched in s- and/or r-elements are excluded¹⁰.

The first dredge-up. Stellar models predict that as a low-mass star runs out of hydrogen, its envelope expands, the surface temperature decreases, making the star evolving to the red giant

10This likely excludes most of the CEMP stars whose surface abundances were modified because of the accretion of

material from a companion (especially the CEMP-s stars, see Sect. 2.5.1).

2.4. Internal mixing processes in CEMP stars

branch (RGB). During that stage, the outer convective envelope expands inward and penetrates hotter regions, where the CN-cycle is active. Some CN-processed material is consequently brought to the surface and alters the star’s surface light element abundances. This mixing episode is called the first dredge-up (Iben 1964). Since the CN-cycle mainly transforms¹²C into¹⁴N and to a lesser extent into¹³C, the effect of the first dredge-up is to decrease the surface C/N and¹²C/¹³C ratios.

Charbonnel (1994) has shown that if starting with¹²C/¹³C'65and¹²C/¹⁴N'3.7(equivalent to [C/N= 0]) in a 1M model atZ = 10⁻³, the first dredge-up decreases¹²C/¹³C and¹²C/¹⁴N to about 25 and 2 (equivalent to [C/N]∼ −0.3) respectively. In the top panels of Fig. 2.6 the surface

12C/¹³C and [C/N] ratios along the evolution of a standard low metallicity 0.85M model are shown. The first dredge-up occurs at log(T_eff) ∼ 3.7 and logg ∼ 3 and decreases the surface

12C/¹³C from about 70 to 45. The [C/N] ratio is barely affected by the first dredge-up (top right panel). Dredge-up events generally cannot explain CEMP stars since (1) they tend to deplete the carbon while we look for the opposite and (2) some CEMP stars are still unevolved (Fig. 2.6) meaning that they did not experienced any dredge-up episode. Quantifying this process for each CEMP star is nevertheless important in order to correct the surface abundances of evolved stars and recover their initial surface abundances, that reflect more directly the abundances in their natal cloud. This was done in Placco et al. (2014c) for the carbon abundance. Their CEMP stellar models suggest that the first dredge-up correction on the surface C abundance is about 0.5 dex (0.8 dex in the most extreme case).

Thermohaline mixing and rotation. Thermohaline mixing occurs after the first dredge-up so that unevolved CEMP stars are not affected by this process (provided they do not accrete heavy material from a companion, cf. Sect. 2.5.1). Thermohaline mixing can occur in giant stars because of an inversion of the molecular weight around the top of the H-burning shell, just below the convective envelope. The negative∇_µ(µ, the mean molecular weight, is growing outward) results in a mixing event, following the first dredge-up, and decreasing again the C/N and¹²C/¹³C ratios (e.g. Charbonnel & Zahn 2007; Eggleton et al. 2008). Rotation transports the H-burning products outwards. It likely adds another mechanism to decrease the surface C/N and¹²C/¹³C ratios. The bottom panels of Fig. 2.6 show the surface¹²C/¹³C and [C/N] ratios during the evolution of a low metallicity 0.85 Mmodel including thermohaline mixing and rotation. In this case, the surface ratios are significantly reduced, particularly near the end of the evolution, where¹²C/¹³C∼8and [C/N] ∼ −1.6. According to these models, some of the evolved CEMP stars shown in Fig. 2.6 may have experienced an important modification of their surface C and N abundances (cf. also the work of Stancliffe et al. 2009). Among the unevolved CEMP stars, some have low ¹²C/¹³C ratios, like HE 1029-0546 with¹²C/¹³C= 9(triangle in Fig. 2.6, Hansen et al. 2015a). Some other unevolved stars have low [C/N] ratios like CS 29528-041 with [C/N]=−1.47(square in Fig. 2.6, Sivarani et al. 2006). These low ratios likely cannot be explained by internal processes. Such ratios probably reflect some processes at work in external sources (e.g. previous massive stars).

Atomic diffusion. Such a process has the effect of separating the elements. It groups together the effects of gravitational settling, thermal diffusion and radiative acceleration (Michaud et al. 2015).

Richard et al. (2002) predicted that atomic diffusion can alter the surface composition of metal-poor stars by∼0.1−1dex. It depends on the chemical species considered, effective temperature and the evolutionary stage. For C, O, Na, Mg, Al and Si, the alteration does not exceed 0.5 dex, except in some hot models (Teff &6300 K). Richard et al. (2002) have also shown that the impact of atomic diffusion becomes very small (about ∼ 0.1 dex) if including an additional turbulence effect that is required to account for the chemical anomalies of some stars (the AmFm stars, Richer et al. 2000).

Overall, although internal processes in CEMP stars can modify their surface abundances, these processes likely cannot account for the unevolved or relatively unevolved CEMP stars. For the

CHAPTER 2. CEMP STARS: OBSERVATIONS AND ORIGINS

Time

> 1 M

~ 0.8 M ^CEMP-s

AGB: s-process, surface enrichment

White dwarf Mass loss

Accretion

(C, s-elements…)

Birth of the system Today

☉

☉

Figure 2.7: Schematic view of the binary scenario to explain the peculiar abundance pattern of CEMP-s stars. In a binary system composed of a0.8Mand a>1Mstars, the more massive star goes through the Asymptotic Giant Branch (AGB) phase and loses mass. This lost material is en-riched, among other, in carbon and s-elements. Some of this material is accreted by the secondary, that becomes a CEMP-s star. The primary ends its life as a white dwarf (see text for more details).

most evolved CEMP stars, internal mixing processes might have erased the initial surface chemical composition (hence the signature of previous stellar generations) for some elements like C or N.

An important work would be to compute a grid of CEMP stellar models including various mixing processes in order to quantify the degree of mixing experienced by each observed evolved CEMP star. The aim would be to recover the initial surface abundances of all elements. Such a correction would give the chemical composition of the CEMP natal cloud. It will establish a more direct link between the evolved CEMP stars and the ejecta of the previous generation of stars.