The Effect of Stellar Evolution on SiC Dust Grain Sizes

Abstract

Stars on the asymptotic giant branch (AGB) produce dust in their circumstellar shells. The nature of the dust-forming environment is influenced by the evolution of the stars, in terms of both chemistry and density, leading to an evolution in the nature of the dust that is produced. Carbon-rich AGB stars are known to produce silicon carbide (SiC). Furthermore, observations of the ∼11µm SiC feature show that the spectral features change in a sequence that correlates with stellar evolution. We present new infrared spectra of amorphous SiC and show that the ∼ 9µm feature seen in both emission and absorption, and correlated with trends in the ∼11µm feature, may be due to either amorphous SiC or to nano-crystalline diamond with a high proportion of Si substituting for C. Furthermore, we identify SiC absorption in three ISO spectra of extreme carbon stars, in addition to the four presented by Speck et al. (1997). An accurate description of the sequence in the IR spectra of carbon stars requires accounting for both SiC emission and absorption features. This level of detail is needed to infer the role of dust in evolution of carbon stars. Previous attempts to find a sequence in the infrared spectra of carbon stars considered SiC emission features, while neglecting SiC absorption features, leading to an interpretation of the sequence inadequately describes the role of dust. We show that the evolutionary sequence in carbon star spectra is consistent with a grain size evolution, such that dust grains get progressively smaller as the star evolves. The evolution of the grain sizes provides a natural explanation for the shift of the ∼11µm SiC feature in emission and in absorption. Further evidence for this scenario is seen in both post-AGB star spectra and in meteoritic studies of presolar grains.

Mid‐Infrared Spectroscopy of Carbon Stars in the Small Magellanic Cloud

“…The wavelength decreases from colors of 0.0-0.5, then increases steadily to almost 11.6 m for the reddest source in our sample. The increase in apparent central wavelength for the red sources is consistent with the stronger self-absorption at 10.8 m described by Speck et al (2005). Line segments fitted to the data on either side of ½6:4 À ½9:3 ¼ 0:45 are shown as dotted lines.…”

“…As the SiC grains increase in size, the LO increases with respect to the TO, shifting the emission feature to the blue. Speck et al (2005) explain the absorption at 10.8 m as the result of different grain size distributions, with the cool absorbing component containing larger grains with an enhanced LO and the warmer emitting component containing smaller grains with an enhanced TO, shifting the feature from 11.3 m to the red. If this scenario is valid, we can use wavelength to track the optical depth of the SiC feature.…”

“…While the emission feature typically appears at 11.3 m, it can be seen out to 11.7 m. In extreme carbon stars, the feature can appear in absorption, usually at $10.8 m. Speck et al (2005) have presented recent laboratory measurements to explain these wavelength shifts. SiC grains emit with two components, a longitudinal optic ( LO) at 10.8 m and a transverse optic (TO) to the red.…”

The shape and composition of interstellar silicate grains

“…However, the position and shape of the SiC resonance in the 10 µm region is very sensitive to the particle shape (see e.g. Andersen et al 2006) and size (see Speck et al 2005).…”

Optical Properties and Applications of Silicon Carbide in Astrophysics

“…Whereas Cstars are expected to have circumstellar shells dominated by amorphous or graphitic carbon, SiC is also expected to form; its IR spectrum provides a diagnostic tool not available from the carbonaceous grains. Therefore, SiC has been of greatest interest to astrophysicists seeking to understand the evolution of dust shells and infrared features of C-stars (Baron et al, 1987;Chan & Kwok, 1990;Goebel et al, 1995;Speck et al, 1997;Sloan et al, 1998;Speck et al, 2005Speck et al, , 2006; Thompson et al, 2006). For Galactic sources, the majority of C-stars should first condense TiC, then C, then SiC, as supported by meteoritic evidence (e.g., Bernatowicz et al, 2005).…”