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Astron. Astrophys. 330, 1175-1179 (1998)

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1. Introduction

Comets are probably the most primitive objects of the Solar System (Yamamoto 1985). They were formed at the same time as planets and meteorites and are located at the periphery of the Solar System, in the Oort Cloud, at distances varying from 10 000 to 50 000 A.U. (Wyckoff 1991). At such distances the surface layers of cometary nuclei are modified by ultra-violet radiations, stellar winds (Festou et al. 1993; McSween & Weissman 1989) and cosmic radiations. In addition, the surface of periodic comets is warmed up when they reach their perihelion, leading to the formation of a crust of refractory materials (Mumma et al. 1993) mainly composed of carbon (Festou et al. 1993; Moreels et al. 1994; Wyckoff 1991). Carbon isotopic composition of comet Halley was measured and compared to the mean isotopic composition of the Solar System. Wyckoff (1991) gives a value of 12 C/13 C = [FORMULA] for comet Halley meanwhile Jaworski and Tatum (1991) rapport a value of 12 C/13 C = 89 [FORMULA] 17, markedly different from Wyckoff's estimate. The latter value determined on the organic gas phase CN, is clearly in favour of a Solar System origin (12 C/13 C = 89 [FORMULA] 2) for this comet.

Kissel et al. (1986a,b) and Sagdeev et al. (1986) have shown that the grains in the coma of Comet Halley are enriched in carbon compared to C1 carbonaceous chondrites and therefore their chemical compositions are closer to solar abundances. The 12 C/13 C ratio of meteorites exhibits extreme variations, i.e. SiC: 12 C/13 C = 2 to 200, with a mean value of [FORMULA] (Ott 1993; Zinner et al. 1989), graphite: 12 C/13 C = 3 to 4000 (Anders & Zinner 1993; Ott 1993) and diamond 88 [FORMULA] 0.3 (Russel et al. 1991). Recent measurements give a larger range of values for SiC and graphite (1.9 up to 2525 for SiC and 2 to 7300 for graphite - Hoppe et al. 1994, 1995). These isotopic variations are interpreted as the presence of preserved interstellar grains embedded in meteorites. The large variations in carbon isotopic material found in interstellar material grains is usually explained to originate from different stellar sources of these grains. For example, the carbon isotopic ratio of silicon carbide grains (12 C/13 C = 2 to 200) can be attributed to the evolution of carbon stars (Ott 1993). Other isotopic anomalies were found in interstellar diamonds and graphite (see review by Robert & Newton 1996). Such isotopic heterogeneities have also been found in the coma of comet Halley for which "in situ" analysis of dust revealed 12 C/13 C lying between 1 and 5000 (Jessberger & Kissel 1991; Jessberger et al. 1988).

In carbonaceous meteorites organic matter exhibits typical solar isotopic values. For example, Robert and Epstein (1982) measured carbon isotopic composition of organic macromolecules and found a value of 12 C/13 C = 90.5. In soluble organic compounds (amino acids, fatty acids, hydrocarbons etc...) Krishnamurthy et al. (1992) showed that the 13 C/12 C ratio was in the range 85.2 to 90.2. Therefore the situation in carbonaceous meteorites seems quite similar to comet Halley: the main carbon phase consists of organic compounds exhibiting typical solar isotopic values (12 C/13 C = 89 [FORMULA] 17) while an extremely small amount of refractory grains have preserved a presolar signature.

In this paper, carbon isotopic compositions of refractory organic polymers synthesised during irradiation experiments simulating the solar wind and/or cosmic ray bombardment on methane ice are presented.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998