INFRARED-SPECTRA AND CIRCUMSTELLAR EMISSION OF LATE-TYPE STARS .2. IRAS CARBON STARS

We carried out near-infrared photometry of the J, H, K, L, and 3.1-mu-m bands for 67 IRAS sources which have a 11.3-mu-m emission feature due to silicon carbide dust in their IRAS low-resolution spectra. Since these stars have no optical counterpart, most of them are considered to be carbon stars surrounded, and thus obscured, by thick circumstellar dust envelopes. We examined the statistical characteristics of the infrared spectra of carbon stars by comparing them with those of oxygen-rich stars, including stars with thick circumstellar dust envelopes, on the basis of near-infrared photometric data, IRAS photometric data, and IR-AS low-resolution spectra. It is shown that near-infrared colors are a good indicator of the thickness of the circumstellar dust envelopes on the basis of the dilution effect of the 3-mu-m absorption in carbon stars and the change of the 10-mu-m silicate feature in oxygen-rich stars. Three diagrams (L-[12] versus K-L, [12]-[25] versus K-L, and 25-mu-m excess in IRAS data versus K-L) are used to discriminate stars with carbon-rich dust envelopes separately from oxygen-rich stars. In these diagrams oxygen-rich stars and carbon stars form separate sequences depending on K-L. The differences between oxygen-rich stars and carbon stars are due to differences in their infrared spectra of circumstellar dust emission.