The Chemical Composition of Carbon Stars. II. The J‐Type Stars

Abstract

Abundances of lithium, heavy elements and carbon isotope ratios have been measured in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance 7800Å Rb I line, are surprisingly low, probably due to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and 13 C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. (1986) and Harris et al. (1987) are used to discuss the origin of J-stars. The luminosity and variability class of the stars studied would indicate that they are low mass (M 2 − 3 M ⊙ ), less evolved objects than the normal carbon stars, although the presence of some luminous (M bol < −5.5) J-stars in our galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J-stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra-mixing mechanism which transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario. Subject headings: stars: abundances -stars: carbon -stars: evolutionnucleosynthesis 12 C/ 13 C ratios can be obtained in current AGB star models of M≥ 4 M ⊙ if hot H-burning takes place at the bottom of the convective envelope (the so-called hot bottom burning, HBB)(Lattanzio 1999; Sackmann & Boothroyd 1992). However, the performance of the CN-cycle at the same time destroys 12 C and, in consequence, the C/O ratio in the envelope is reduced and the star again becomes O-rich. Thus, a fine-tuning of the parameters of the AGB models (mass, mixing-length, mass-loss rate, metallicity, etc.), that determine the chemistry of the envelope, seems to be required to obtain a J-star. Mixing at the He-core flash has also been proposed as an alternative scenario to form J-stars. In this event an -4injection of carbon-rich material from the core into the hydrogen-rich shell may occur. The introduction of core material ( 12 C and 4 He) into a proton-rich region yields enhanced 12 C and 13 C, with perhaps a small enhancement of 14 N (Deupree & Cole 1983). The presence of strong λ6708Å Li I lines is frequent in J-stars. In a Li survey of galactic C-stars, Boffin et al. (1993) found that among 30 Li-rich stars in a sample of 250 C-stars ∼ 50% are of J-type. This figure increases up to ∼ 70% if the Li-rich phenomenon is c...

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

“…J-type carbon stars are solar metallicity stars characterized by strong enrichments in 13 C leading to low 12 C/ 13 C ratios (<15) and enrichments in Li. Abundances of s-process elements in J-type carbon star photospheres are nearly normal (e.g., Abia & Isern 2000). Since there is no enrichment of s-process elements, we speculate that there is also no enrichment in 22 Ne, since the latter is also a product of He-shell nucleosynthesis which would be dredged up to the surface.…”

“…Since there is no enrichment of s-process elements, we speculate that there is also no enrichment in 22 Ne, since the latter is also a product of He-shell nucleosynthesis which would be dredged up to the surface. The characteristics of J-type carbon stars cannot be explained by standard AGB star models, and extra mixing processes have to be invoked, depending on stellar mass (Abia & Isern 2000). Extra mixing and processing of envelope material can also occur in AGB stars (cool bottom processing, CBP) and can lead to low 12 C/ 13 C ratios.…”

Analysis of the infrared spectra of the peculiar post-AGB stars EP Lyrae and HD 52961

“…3), a silicate J-type carbon AGB star (Molster et al 2001;García-Hernández et al 2006). Silicate J-type carbon stars have surprisingly low 12 C/ 13 C ratios and do not show the typical s-process overabundances seen in N-type carbon stars (Abia & Isern 2000). The infrared spectrum of these stars shows features of both carbon-and oxygen-rich dust species.…”

The chemical composition of carbon stars. The R-type stars

“…This is a further step in the study of the chemical composition of field giant carbon stars in the Galaxy. In Paper I (Abia & Isern 2000) we studied J-type stars and in Papers II and III (Abia et al 2001(Abia et al , 2002) the N-type stars. In the next section we describe the main characteristics of the sample stars and the observations.…”