Carbon stars

Carbon nanotubes (CNTs) as well as fullerenes are splendid gift brought to the Earth from the red giant carbon stars in the long-distant universe through the spectroscopy. Moreover, those belong to new carbon allotropes of the mesoscopic scale with well-defined structures. In particular, CNTs are considered to be the materials appropriate to realise intriguing characteristics related to the mesoscopic system based on their size and physicochemical properties. 

Although, it is not expected these stars would contribute significantly to the galactic Li abundance, it is obvious that we cannot evaluate the real contribution of AGB (carbon) stars to the galactic Li until its production in AGB stars is fully understood. 

Abstract. We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxies permit us to test current models of stellar evolution and nucleosynthesis during the Asymptotic Giant Branch phase in low metallicity environments. 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

In order to test the current evolution and nucleosynthesis models predicting the formation and the yields of such carbon stars, we have collected high-resolution spectra of stars located in the SMC and the Sagittarius dwarf galaxy, extragalactic systems with low average metallicity and well known distances. 

The selected extragalactic carbon stars have been observed with the VLT/UVES instrument in service mode. The spectral resolution was around 40000 over the domains 420-500 nm and 670-900 nm. We used carbon-rich MARCS model atmospheres and specific linelists in order to derive the abundances of metals,... 

Table 1. Main characteristics of the observed carbon stars...

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

Fig. 1. Heavy over light s-elements ratio versus metallicity. Lines are theoretical predictions for a 1.5M AGB stars with C/0=1.1 and three different choices for the neutron exposure rate (see Busso et al., 2001, ApJ, 557, 802). Black dots are for galactic carbon stars analyzed in Abia et al. (2002, ApJ 578, 817).

After the flash, the HCS eventually merges with the convective envelope and the surface composition is enhanced in CNO elements (the star being now a carbon star, as X(C)/X(0) = 4.8). When the model evolves to the next helium flash, the temperature at the former OVHECS (between 0.5058 M0 and... 

Ba, Nd and Sm - as well as those in C, N, A1 and Si already described - in silicon carbide. Diamonds may come from supemovae and SiC and TiC grains probably from AGB carbon stars, while the broad 12C/13C distribution in graphite suggests a variety of sources. 

Carbon stars with spectral types R, N (or C) and S. R and N stars are cool giants with temperatures roughly corresponding to normal spectral types K and... 

The ejected material forms a cool molecular and dusty envelope which initially veils the star from optical observations as it goes through the stages of Mira variable followed by OH-infirared star or infrared carbon star later the star becomes hot enough to ionize part or all of the expanding gas-dust envelope forming a planetary nebula. 

Table 6.2 gives an overview of some of the stages of stellar evolution where carbon and/or s-process anomalies occur (see Fig. 3.37). The C/O ratio increases down the series. In addition to the types listed there, there are infrared carbon stars such as IRC +10216,1 proto-planetary nebulae and a whole zoo of peculiar carbon stars, including J stars (strong 13C as in the case of HD 52432 shown in Fig. 1.7) and hydrogen-deficient carbon stars which can be cool, e.g. R Cor Bor, RY Sag and HD 137613 shown in Fig. 1.7, or hot (when they look like extreme helium stars) such stars may have lost their envelopes by binary mass transfer, or they may be born-again AGB stars. 

Planetary nebulae are often even more rich in carbon than cool carbon stars, and those classified by M. Peimbert as Type I are rich in nitrogen, indicating effects of hot-bottom burning in intermediate-mass progenitor stars. The s-process elements are not normally detectable in PN or their central stars, but a remarkable case is that of FG Sagittae, the central star of a fossil planetary nebula, which has cooled in the course of the twentieth century from around 25 000 K to around 5000 K at constant bolometric luminosity. This star suddenly showed an enhancement of s-process elements in its atmosphere between 1965 and 1972 (see Jeffrey Schoenberner 2006, and references therein). 

Russell explains spectra of carbon stars (types R, N, S) as consequence of reversal of the usual C/O ratio. 

Many other fascinating cases are discussed in the previously mentioned classic clinical guide Uncommon Psychiatric Syndromes. For example, one woman said that the Earth, Sun, and other stars did not exist, and that she alone survived the Big Bang. She believed that she wandered an empty world in a form of a carbonized star. She also said that time ceased to exist and that she was condemned to wander for eternity. 

Two emission spectra of circumstellar shells are shown in Fig. 14.5, for an oxygen star in the bottom part of the figure and for a carbon star in the top. 

Dust around the carbon star shows an excess emission feature between about 10.2 and 11.6 jam, clearly distinguishable in both shape and position from the 9.7-jum feature of the oxygen star, which has been attributed to small SiC particles. These particles cannot be spherical, however. According to the discussion in Section 12.2, shape effects spread an absorption band in small particles of materials like SiC between the transverse (to,) and longitudinal (to,) optical mode frequencies these frequencies for SiC are indicated on the figure. This point was made by Treffers and Cohen (1974) using Gilra s unpublished calculations. To illustrate this further, calculations for a random distribution of... 

Figure 14.5 Emission spectra of dust shells around a carbon star (top) and around an oxygen star (bottom). From Treffers and Cohen (1974).

Emission band near 11 jim associated with carbon stars Shape broadened vibrational modes in SiC... 

Carbon isotopic compositions of silicon carbide grains from the Murchison meteorite compared with the carbon isotopic compositions of carbon stars (low- to intermediate-mass AGB stars). The composition of carbon in the solar system is indicated by the vertical line. Note the similarity in the distributions of compositions in the two plots. These data indicate that the silicon carbide in the Orgueil meteorite came from a population of carbon stars very similar to that in the galaxy today. 

Gallino, R., Raiteri, . M. and Busso, M. (1993) Carbon stars and isotopic Ba anomalies in meteoritic SiC grains. Astrophysical Journal, 410, 400 411. 

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