Little-Marenin et al. 2000, IAU, Classification of ... carbon stars

Classification of dust emission features in carbon stars

Irene R. Little-Marenin (Wellesley), G.C. Sloan (NASA Ames), S.D. Price (Phillips Lab./GPOB)

2000, in IAU Symposium 177: The Carbon Star Phenomenon, ed. R.F. Wing, 559.

We have cross-referenced the IRAS Point Source Catalog with the General Catalog of Variable Stars, searching for carbon-rich variables associated with the asymptotic giant branch. We have found 99 sources which are brighter than 28 Jy at 12 µm. For each of these sources, we have classified the spectrum from the IRAS Low-Resolution Spectrometer after removing an estimated stellar contribution. The majority of our sources fall into two categories: spectra with the classic SiC emission feature peaking around 11.2-11.5 µm (we will call this class SiC), and spectra where the SiC feature appears along with an additional component peaking ~8.5-9.0 µm (class SiC+). In a few stars the ~8.5-9.0 µm feature rivals or exceeds the SiC feature in strength (class SiC++). Our sample also contains several unusual and low-contrast dust spectra which are difficult to classify. Five sources show emission from oxygen-rich dust, including two well-known silicate-carbon stars (BM Gem and V778 Cyg), and three S or SC stars which have been classified at various times as carbon stars (S Lyr, ST Cam and NP Pup).

The classic SiC class consists mostly of Mira variables, while the SiC+ and SiC++ classes contain mostly semi-regular and irregular variables. Classic SiC sources tend to have redder [12]-[25] colors and slightly lower temperatures in the optical than the SiC+ and SiC++ sources. The classic SiC feature appears to be superimposed on a featureless continuum most likely due to graphitic material. The C/O ratio increases along the sequence SiC — SiC+ — SiC++. We will also discuss our results in terms of previous classification efforts and the evolutionary scenarios by Willems (1998a, A&A, 203, 51; 1988b, A&A, 203, 65), Willems & de Jong (1988, A&A, 196, 173), and Goebel et al. (1995, ApJ, 449, 246).

This abstract was actually presented at the conference in 1996, but it took a little longer than usual for the proceedings to be published. In the meantime, the refereed paper appeared as Sloan, Little-Marenin, and Price (1998, AJ, 115, 809).

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