Carbon enriched giant stars

It is interesting to note that all stars without exception are Li rich in the initial part of this cycle, characterizing an initial Li enrichment for all stars (see Figs. 7a and 7b in Drake et al. 2002). An important test of this scenario could be the observation of separated CS around RGB giants as is the case of the more evolved carbon asymptotic giant stars. 

Carbon rich giant stars are highly evolved stars that have a greater concentration of carbon than oxygen in their atmospheres. This superficial enrichment is the consequence of nuclear reactions that take place in their interiors and of the efficient convection that transports the nucleosynthesis products to the most external layers. Carbon stars have effective temperatures in the range 2,000-3,000 K and are some 10,000 times brighter than the Sun. They often present important mass loss... 

Fig. la shows the abundance ratio [Ba/Fe] for this sample as a function of [C/Fe]. Thirty stars (77% of the sample) have [Ba/Fe] > +0.7, while the others have [Ba/Fe] < 0.0. There is a clear gap in the Ba abundances between the two groups, suggesting at least two different origins of the carbon excesses. Ba-enhanced stars The Ba-enhanced stars exhibit a correlation between the Ba and C abundance ratios (Fig. la). This fact suggests that carbon was enriched in the same site as Ba. The Ba excesses in these objects presumably originated from the s-process, rather than the r-process, because (1) nine stars in this group for which detailed abundance analysis is available clearly show abundance patterns associated with the s-process [2], and (2) there is no evidence of an r-process excess in the other 21 objects. Hence, the carbon enrichment in these objects most likely arises from Asymptotic Giant Branch (AGB) stars, which are also the source of the s-process elements. 

Figure 2 - S-process enrichment vs. carbon enrichment in peculiar warm giants. The expected locus of the subgiant CH stars following the first dredge-up is indicated by open triangles. The barium stars, mild barium stars, subgiant CH stars, and CH stars all seem to follow a common relationship.

The CH stars and the CH-like stars are included in Table 1 for completeness, but we don t know much about them. The CH stars are metal-deficient, Population II giants enriched in carbon and s-process elements. They may be enriched in (Lambert 1985). They appear to be binaries (McClure 1985). The CH stars are probably Pop II barium stars, and to accept their designation as Pop II barium stars. Not much is known about the CH-like stars of Yamashita (1972, 1975), which are Pop I, carbon-rich giants. At low spectral resolution the 14554 line of Ba II is enhanced. These stars may be related to the barium or barium-carbon stars, but we need more information about their compositions, and especially about their binary status. 

Fullerenes and graphite may originate abundantly in stellar atmospheres rich in carbon like those of some giant stars and some progenitors of planetary nebulae (Fig. 1.5). These objects display for important mass loss rates and are therefore able to greatly enrich the interstellar medium. 

The compositions of several varieties of peculiar warm giants are also listed in Table 1. These stars differ from normal G and K giants in a variety of ways lithium is unusually high (or low) carbon is enriched through triple-a nucleosynthesis or depleted through CN-cycle processing and/or s-process elements... 

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