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Astron. Astrophys. 343, 933-938 (1999)

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1. Introduction

Following the thermodynamical equilibrium calculations of Friedemann (1969a,b) and Gilman (1969) which suggested that silicon carbide (SiC) particles could form in the mass outflow of carbon stars, Hackwell (1972) and Treffers & Cohen (1974) performed infrared spectroscopy of such stars and thereby provided the empirical evidence for the presence of SiC particles in stellar envelopes. A broad infrared emission feature seen in the spectra of many carbon stars, peaking between 11.0 and 11.5 µm is therefore attributed to solid SiC particles and SiC is believed to be a significant constituent of the dust around carbon stars.

Presolar SiC grains have been identified in primitive meteorites (Bernatowicz et al. 1987). Based on isotopic measurements of the major and trace elements in the SiC grains and on models of stellar nucleosynthesis, it is established that a majority of the presolar SiC grains has their origin in the atmospheres of late-type carbon-rich stars (Gallino et al. 1990, 1994; Hoppe et al. 1994). For recent reviews see, e.g., Anders & Zinner (1993), Ott (1993) and Hoppe & Ott (1997). The grain sizes of presolar SiC from the Murchison meteorite have been found by Amari et al. (1994) to vary from less than 0.05 to 20 µm in equivalent spherical diameter, with about 95% (by mass) of the grains being between 0.3 and 3 µm. This distribution is coarser than for presolar SiC found in other meteorites (Russel et al. 1993, 1997; Huss & Lewis 1995; Gao et al. 1996). Therefore, Russel et al. (1997) have speculated that the finer grained SiC was lost through size sorting in the solar nebula prior to accretion of Murchison. In the carbonaceous chondrites only approximately 0.004% of the silicon is in the form of SiC (the remainder being in the form of silicates).

Silicon carbide occurs in a large variety of crystal types. The basic units are Si-C bilayers with a three-fold symmetry axis, in which the Si and C atoms are closely packed. It is the special stacking of these layers that determines the occurrence of the polytypes. The second bilayer is shifted in the [[FORMULA]]-direction by [FORMULA] of the Si-Si or C-C atomic distance in the layer. If a third and a fourth layer is stacked in an identical way, then the atoms in the fourth layer lie exactly above the ones in the first layer. Further repetition of this sequence results in a cubic crystal structure called [FORMULA]-SiC. If at least one bilayer is shifted in the opposite direction the resulting structure is hexagonal or rhombohedral (Mutschke et al. 1999). All the polytypes resulting from non-cubic stacking sequences are summarized in the term [FORMULA]-SiC.

Virag et al. (1992) have investigated the crystal structure of the large presolar SiC grains extracted by Amari et al. (1994) (LS and LU series). The authors investigated forty-one large (from [FORMULA] µm to [FORMULA] µm) grains from the Murchison meteorite by Raman spectroscopy. Thirty-two of these grains were found to have a cubic crystallographic structure ([FORMULA]-SiC), the remaining grains showed a non-cubic structure (hexagonal or rhombohedric; [FORMULA]-SiC). However, the [FORMULA]-SiC grains were also characterized by a normal isotopic composition, indicating that they might not be of presolar origin. Recently Daulton et al. (1998) investigated one of the finer grained samples extracted by Amari et al. (1994) (KJB grain size 0.3-0.7 µm) and found that for these smaller grains there seems to be even amounts of [FORMULA]- and [FORMULA]-SiC. This indicates that while the larger presolar grains seem to be dominated by the [FORMULA]-SiC type the smaller grains are a mix of the [FORMULA]- and [FORMULA]-SiC type, but the presolar nature of the small [FORMULA]-SiC grains still needs to be confirmed. Whether presolar SiC grains will turn out to be of predominately one or the other crystal type can place constraints on the formation parameters of the grains. Therefore, it has been attemted by several groups to derive the crystal type of circumstellar SiC grains from their observed IR emission spectra (e.g. Blanco et al. 1994, 1998; Groenewegen 1995; Speck et al. 1997). However, it is argued by Papoular et al. (1998) that the IR band profiles might not be sensitive to the crystal type but to other structural and morphological grain properties. To decide these questions, more laboratory studies are needed (Mutschke et al. 1999).

We have previously published the absorption coefficients of presolar diamonds (Mutschke et al. 1995; Andersen et al. 1998) and along this line of providing the spectral feature of "real star dust", we have now measured the spectral appearance of presolar SiC grains. These data are necessary in order to determine if the spectral emission (and in a few cases absorption (Jones et al. 1978; Speck et al. 1997)) feature found in circumstellar envelopes of carbon stars, is consistent with the meteoritic grains having originated in such stars. Agreement between the isotopic composition of the presolar SiC grains and those predicted in AGB star nucleosynthetic models, give strong belief that these meteoritic grains originated in carbon stars. It is our hope that comparison between the optical properties of presolar SiC and the appearance of the dust features in carbon star spectra will be able to impose further constraints. In this paper, we present results from measurements of the spectral properties of meteoritic SiC in the wavelength range between 2.5 and 16.5 µm.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999