IRAS Low-Resolution Spectra of Cool Carbon Stars - Part Three - Stars with Thick Circumstellar Shells

Details IRAS Low-Resolution Spectra of Cool Carbon Stars - Part Three - Stars with Thick Circumstellar Shells IRAS Low-Resolution Spectra of Cool Carbon Stars - Part Three - Stars with Thick Circumstellar Shells

Sep 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988a%26a...203...65w&link_type=abstract

Astronomy and Astrophysics, Vol. 203, NO. 1/SEPI, P. 65, 1988

Astronomy and Astrophysics

Astrophysics

21

Scientific paper

IRAS low resolution spectra and broad-band data and near-infrared photometry are presented of 15 carbon stars with near-infrared color temperatures significantly smaller than 2000 K, indicating that the stellar photosphere between 1 and 4, μm is masked by a circumstellar dust shell. All low resolution spectra exhibit the SiC emission feature, with a peak intensity near 11.3 μm. Amorphous carbon dust contributes to the continuum opacity in the wavelength region of interest (7.5 - 23 μm). In some spectra C2H2 absorption bands centered at 7.5 and 14 μm partly filled in with dust emission are observed. The limited range of the SiC index values indicates that only a small fraction of the circumstellar dust is in the form of SiC. The IRAS broad-band data for the 15 sources in the sample cluster around the 800-1000 K black body points in the log (S12/S25) versus log(S25/S60) color-color plot. In view of the v1 proportionality of the amorphous carbon opacity, the slope of the far-infrared flux, which varies with v2, can be best explained by the existence of a well-defined outer boundary of the dust shell. The temperature at this outer boundary is between 100 and 300 K, which implies that the onset of the present high mass loss rate (M ≡ 10-6 - 10-5 Msun yr-1) happened about 102 - 103 yr ago. The difference in spectral appearance of the 15 stars in the sample compared to extreme carbon stars like CW Leo can be explained by the fact that the extreme carbon stars have mass loss rates in excess of 10-6 Msun yr-1 for at least 104 yr. These results suggest that the stars in this sample will eventually evolve into extreme carbon stars.

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