CEMP stars as peculiar metal-poor stars

2.2.2 Abundances and uncertainties

Once a high-resolution spectrum is acquired from a star, the abundances in term of numbers have to be determined (e.g. [X/Fe] ratios). This is a highly nontrivial task that required a realistic stellar atmosphere model. Stellar abundances are therefore notobservedbut deduced bymodeling the spectral lines. The stellar abundances are generally determined by inspection of the equiva-lent width of spectral absorption lines. The observational uncertainty varies as√

FWHM/ (S/N) where FWHM is the full width at half maximum of the line. A spectrograph with a high resolv-ing power and a high S/N minimizes the uncertainties, allowresolv-ing the detection of weaker features, which is primordial when observing stars with little metals, hence with weak lines.

A perfect atmosphere model would be a 3D model taking into account departures from local thermodynamic equilibrium (LTE) and where all the atomic and molecular physics (determining the absorption lines) is included. Such a model does not exist. Often, abundances are derived using a 1D LTE model.

Frebel et al. (2008), have estimated by how much the abundances are affected when deriving the abundances with either a 1D or a 3D model. They investigated the star HE 1327-2326 ([Fe/H]=

−5.7). They found a 3D-1D correction for C, N and O of about−0.7dex.

Ezzeddine et al. (2017) studied anew the stellar parameters of 20 stars with [Fe/H]<−4using a 1D NLTE (non LTE) atmosphere model instead of a 1D LTE model. They derived [Fe/H] cor-rections up to 1 dex compared to the 1D LTE case. These corcor-rections are larger at lower [Fe/H]:

at [Fe/H]= −4 and−7, the corrections are about 0.5 and 1 dex respectively. Lind et al. (2012) reported a similar trend: they derived NTLE corrections of. 0.1and. 0.5dex at solar and low metallicity, respectively. Also, these corrections apply only for the derivation of Fe abundances from neutral lines (FeI). The corrections are mostly insignificant if using FeIIlines. Determining the Fe abundance with accuracy is important since it is generally a prerequisite for the determina-tion of the abundance of other elements.

2.3 CEMP stars as peculiar metal-poor stars

This section aims at describing the characteristics of CEMP stars. Their possible origin is dis-cussed in the next section.

A natural guess could be that in metal-poor stars, all metals are scaled down compared to the Sun by roughly the same factor. However, observations progressively revealed that metal-poor stars generally have non-solar-like abundance patterns. In particular, the carbon to iron ratio is super-solar in many metal-poor stars. The first carbon stars were observed about 150 years ago by Angelo Secchi (Secchi 1868). At that time, he only reported a peculiar banding in the stellar spectra. These stars were later identified by Rufus (1916) as stars enriched in carbon. Since, many carbon-rich stars were discovered, particularly among the most iron-poor stars. Stars enriched in carbon and depleted in iron were called Carbon-Enhanced Metal-Poor (CEMP, Beers & Christlieb 2005). This name might seem somewhat contradictory since carbon is a metal. However, CEMP stars likely stay metal-poor compared to the Sun, even if they show overabundances in carbon or other metals (cf. discussion in Sect. 2.1). Beers & Christlieb (2005) proposed two criteria defining a CEMP star: [Fe/H]< −1.0and [C/Fe]>1.0. The criterium of [C/Fe]> 0.7 is also often used in the literature (first proposed by Aoki et al. 2007). For what follows, depending on the criterium used by the considered author, I use the names

• CEMP1.0if the condition is [C/Fe]>1.0.

• CEMP0.7if the condition is [C/Fe]>0.7.

CHAPTER 2. CEMP STARS: OBSERVATIONS AND ORIGINS

AA53CH16-Frebel ARI 29 July 2015 12:54

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[C/Fe], [N/Fe], and [O/Fe] are shown as a function of [Fe/H] (left) and [C/Fe] (right). Red and black symbols refer to C-rich (excluding CEMP-s, CEMP-r, and CEMP-r/s subclasses) and C-normal stars, respectively;

the circles and star symbols stand for objects with [Fe/H] above and below−4.5, respectively. The middle column contains generalized histograms pertaining to the abundances to the left (Gaussian kernel, with σ=0.30). The red dotted line in the upper-left panel is the boundary between C-rich and C-normal stars adopted in the present work. See Section 3.8.1 for discussion. The data are from Cayrel et al. (2004), Spite et al. (2005), Sivarani et al. (2006), Caffau et al. (2012), Cohen et al. (2013), Norris et al. (2013), Yong et al.

(2013a), Hansen et al. (2014), and Keller et al. (2014).

stars, and the open and filled circles denote the mixed and unmixed red giant stars, respectively, of Spite et al. (2005). The red symbols stand for C-rich stars (excluding CEMP-s, CEMP-r, and CEMP-r/s subclasses).

The leftmost panels show [C/Fe], [N/Fe], and [O/Fe] as a function of [Fe/H]; the middle panels present the generalized histograms of the abundances of these elements in the C-rich stars. On the right, [C/N], [N/Fe], and [O/Fe] are plotted as a function of [C/Fe].

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1.5

The relative abundances of the light elements Na, Mg, Al, Si, and Ca versus [Fe/H] (left) and [C/Fe] (middle) for C-rich and C-normal Galactic halo stars. Red and black symbols refer to C-rich (excluding CEMP-s, CEMP-r, and CEMP-r/s subclasses) and C-normal stars, respectively; the circles and star symbols stand for objects with [Fe/H] above and below−4.5, respectively (see leftmost panel). The right column contains generalized histograms pertaining to the abundances of the C-rich stars in the left panels (Gaussian kernel, withσ=0.15). See Section 3.8.2 for discussion. The data are from Cayrel et al. (2004), Spite et al. (2005), Sivarani et al. (2006), Caffau et al. (2012), Cohen et al. (2013), Norris et al. (2013), Yong et al. (2013a), Hansen et al. (2014), and Keller et al. (2014). Abbreviations: CEMP, carbon-enhanced metal-poor;

CEMP-s, process element enhancement; CEMP-r, r-process enhancement; CEMP-r/s, both r- and s-enhancements; C-normal, carbon-normal; C-rich, carbon-rich.

656 Frebel

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AA53CH16-Frebel ARI 29 July 2015 12:54

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The relative abundances of the light elements Na, Mg, Al, Si, and Ca versus [Fe/H] (left) and [C/Fe] (middle) for C-rich and C-normal Galactic halo stars. Red and black symbols refer to C-rich (excluding CEMP-s, CEMP-r, and CEMP-r/s subclasses) and C-normal stars, respectively; the circles and star symbols stand for objects with [Fe/H] above and below−4.5, respectively (see leftmost panel). The right column contains generalized histograms pertaining to the abundances of the C-rich stars in the left panels (Gaussian kernel, withσ=0.15). See Section 3.8.2 for discussion. The data are from Cayrel et al. (2004), Spite et al. (2005), Sivarani et al. (2006), Caffau et al. (2012), Cohen et al. (2013), Norris et al. (2013), Yong et al. (2013a), Hansen et al. (2014), and Keller et al. (2014). Abbreviations: CEMP, carbon-enhanced metal-poor;

CEMP-s, process element enhancement; CEMP-r, r-process enhancement; CEMP-r/s, both r- and s-enhancements; C-normal, carbon-normal; C-rich, carbon-rich.

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Figure 2.4: [X/Fe] ratios of the most iron-poor stars. Red symbols show C-rich stars (CEMP-s, CEMP-r, and CEMP-r/s stars are excluded). Black symbols show C-normal stars. Stars show objects with [Fe/H]<−4.5. Histograms of the abundances are shown on the right. (figure adapted from Frebel & Norris 2015).

Are CEMP stars enriched in other elements?

CEMP stars have various lithium abundances, from⁷A(Li)'2(i.e. close to the Spite plateau of 2.05±0.16, Spite & Spite 1982) to A(Li)<0.62for HE 1327-2326 with [Fe/H]=−5.7(Frebel et al.

2008). Some observational works have suggested a melt-down of the Spite plateau below [Fe/H]

' −3(Aoki et al. 2009; Sbordone et al. 2010; Bonifacio et al. 2012). The melt-down has nevertheless to be further confirmed since some iron-poor stars could have experienced severe surface Li de-pletion episodes (especially those who are giants). Korn et al. (2009) have computed models of the star HE 1327-2326 including the effects of atomic diffusion. They predict a maximal Li depletion of 1.2 dex from the birth of the star to the present day. It gives a maximal initial Li abundance of about 1.8, which is much closer to the Spite plateau. Other scenarios in the same work predict a milder Li depletion, of the order of 0.2 dex, which would give an initial Li well below the Spite plateau.

In any case, these initial Li abundances are not compatible with the WMAP-based primordial Li

7A(X) =log(X)=log(NX/NH) + 12, where X represents a given element.

2.3. CEMP stars as peculiar metal-poor stars

Table 2.2:Classification of CEMP stars, as defined in Beers & Christlieb (2005).

Term Conditions CEMP [C/Fe]>1.0

CEMP-r [C/Fe]>1.0 [Eu/Fe]>1.0

CEMP-s [C/Fe]>1.0 [Ba/Fe]>1.0 [Ba/Eu]>0.5 CEMP-r/s [C/Fe]>1.0 0.0<[Ba/Eu]<0.5

CEMP-no [C/Fe]>1.0 [Ba/Fe]<0

abundance of 2.63 predicted from primordial nucleosynthesis (Spergel et al. 2007).

Many CEMP stars have high N/Fe, O/Fe, Na/Fe or Mg/Fe ratios. For stars with [Fe/H]<−3, the scatter of the [X/Fe] ratios globally decreases from X=C to Ca (Frebel & Norris 2015). For instance, the [C/Fe], [O/Fe], [Na/Fe], [Mg/Fe] and [Ca/Fe] ratios span about 5, 4, 2.5, 1.5 and 0.5 dex, respectively (see Fig. 2.4).

A significant amount of CEMP stars are also enriched in elements heavier than iron. These elements are thought to be mostly synthesized through the slow and rapid neutron capture pro-cesses (also the intermediate process or i-process, see Sect. 2.5). Aoki et al. (2000) and Van Eck et al. (2001) discovered the first VMP stars enriched in Pb. Many other CEMP stars enriched in s-and/or r-elements were discovered, down to [Fe/H]∼ −3(Burris et al. 2000; Simmerer et al. 2004;

Sivarani et al. 2004; Lai et al. 2007; Placco et al. 2013). Below this threshold, enhancements in s-and r- elements are generally very modest (see Fig. 2.5).

Beers & Christlieb (2005) established a classification of CEMP stars based on their heavy el-ement abundances (see Table 2.2). One finds CEMP stars enriched in s-elel-ements (CEMP-s), r-elements (CEMP-r), both r- and s-r-elements (CEMP-r/s) and without significant enhancement in s-/r-elements (CEMP-no). As they note, this classification should be viewed as a first approxi-mation and used as a guideline for the future. Indeed, the location of the class boundaries are somewhat arbitrary and in some cases, it may exist a continuity between the classes rather that very distinct groups. Eu and Ba were chosen to define such classes because (1) they are generally readily measurable in a stellar spectrum and (2) they are expected to be produced in a different amount by the s- and r-process (Eu: mainly r-process, Ba: mainly s-process). It has been shown (especially Yoon et al. 2016) that CEMP-no stars are generally found on thelow carbon bandwith A(C)≤7.1 and CEMP-s/rs on thehigh carbon bandwith A(C)>7.1. It might be linked to the differ-ent formation channel for CEMP-no and CEMP-s/rs stars (Sect. 2.5). Interestingly, this separation into 2 bands, allows (with some level of confidence) to classify CEMP stars only based on their C abundance rather on e.g. Ba, whose abundance determination requires much higher resolution spectra. Fig. 2.5 shows that CEMP-s stars mainly lie at [Fe/H]> −3(green symbols). CEMP-no stars (blue symbols) are generally found at lower [Fe/H]. Some CEMP stars (magenta symbols) are still unclassified, according to the criteria of Table 2.2.

Are CEMP stars frequent?

Carbon et al. (1987) and Norris et al. (1997) first emitted the possibility of a higher frequency of C-rich stars with decreasing [Fe/H]. Marsteller et al. (2005) reported a possibly high fraction of about 50 % of CEMP1.0 stars⁸among [Fe/H]<−2stars selected in the HES survey. However, Cohen et al. (2005) showed that the [Fe/H] ratio of some stars in the HES survey was overestimated by∼0.5dex. Consequently, they derived a CEMP1.0fraction of14±4 % (instead of 50 %) among the stars with [Fe/H]<−2. In 240 stars with [Fe/H]≤ −2, Lucatello et al. (2006) found a CEMP1.0

fraction of 21±2%. Lee et al. (2013) considered a sample from SDSS/SEGUE of about 247000 stars

8I remind here that CEMP1.0means that the author used the condition [C/Fe]>1for a star to be CEMP. CEMP0.7

means that the author used the condition [C/Fe]>0.7.

CHAPTER 2. CEMP STARS: OBSERVATIONS AND ORIGINS

Figure 2.5:A(Li) (upper left) and [X/Fe] as a function of [Fe/H]. Green symbols are CEMP-s, blue are CEMP-no, according to criteria of Table 2.2. Magenta symbols are unclassified CEMP because of missing abundances (Ba, Eu). Light grey dots show carbon normal stars. Abundances with upper limits are not plotted. The typical uncertainty is±0.3dex. When abundances from different authors are available for one star, the most recent one is selected. The abundance data is taken from the SAGA database (Suda et al. 2008, last update on Sept. 2017).