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Astron. Astrophys. 345, 233-243 (1999)

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1. Introduction

A group of carbon stars can be distinguished for the enormous strength of the absorption due to 13C-bearing molecules, and they were designated as J-type by Bouigue (1954). Such enormous enhancements of 13C-bearing molecules are not observed for N- and SC-type carbon stars. Though SC-type carbon stars were mostly classified as J-type by the earlier classification, Ohnaka & Tsuji (1996, hereafter Paper I) reveal that [FORMULA] ratios in 15 SC-type carbon stars are mostly larger than 10.

The enormous strength of the absorption of 13C-bearing molecules in J-type carbon stars implies very low [FORMULA] ratios. Besides, unlike N- and SC-type carbon stars, J-type carbon stars do not exhibit the enhancement of the s -process elements (Utsumi 1985). These observational facts cannot be interpreted by the scenario considered for the formation of N- and SC-type carbon stars. In the formation of N- and SC-type carbon stars, the thermal pulse and the third dredge-up at the asymptotic giant branch (AGB) are considered to play a crucial role (e.g. Iben 1981, Vassiliadis & Wood 1993, Straniero et al. 1997). Namely, 12C freshly synthesized in the thermal pulse is mixed to the stellar surface by the third dredge-up, resulting in the increases of C/O and [FORMULA] ratios. The enhancements of the s -process elements observed in N- and SC-type carbon stars (e.g. Utsumi 1985) also indicate the operation of the thermal pulse, where neutrons are expected to be supplied via 13C([FORMULA],n)16O. However, this scenario cannot reproduce the enrichment of 13C and the lack of the s -process enhancements observed in J-type carbon stars, and the formation of J-type carbon stars still remains unclear. The mixing of CN-cycled material is one of the scenarios considered for the formation of J-type carbon stars, since [FORMULA] ratio is lowered to [FORMULA] at the equilibrium of the CN-cycle. In fact, the operation of the CN-cycle at the bottom of the convective envelope and extra mixing processes have been intensively investigated, but no definitive answer has been given so far.

Carbon stars with the silicate emission at 9.8 µm, which were discovered by Willems & de Jong (1986) and Little-Marenin (1986) in the IRAS Low Resolution Spectra (LRS), are also associated with J-type. Since their discovery, silicate carbon stars have been identified as J-type by Willems & de Jong (1986), Lloyd-Evans (1990), Lambert et al. (1990), and Chan (1993). However, [FORMULA] ratios in silicate carbon stars have not well quantitatively been determined yet. The detection of silicate emission suggests that the circumstellar envelopes of these stars should be oxygen-rich, on the contrary to the photospheric chemical compositions characterized by C/O [FORMULA] 1. The detections of OH and H2O masers (Nakada et al. 1987, 1988; Little-Marenin et al. 1988, 1994) also suggest the existence of oxygen-rich material in the circumstellar envelopes.

[FORMULA] ratios in J-type carbon stars reflect nuclear reactions and mixing processes in the stellar interior. A quantitative analysis of 13C enrichments gives us a clue to clarify the nuclear reactions responsible for the formation of J-type carbon stars. As for silicate carbon stars, some peculiar [FORMULA] ratios which would be associated with the presence of silicate emission may be found. Thus, we have carried out a quantitative analysis of [FORMULA] ratios, using lines of the CN red system located around 8000 Å, for 26 J-type carbon stars including 5 silicate carbon stars. Our main purpose is to find the distribution of [FORMULA] ratios in J-type carbon stars. We will also discuss a comparison of the resulting [FORMULA] ratios with those previously derived by other authors, and possible mechanisms responsible for small [FORMULA] ratios in J-type carbon stars.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999