1994A&A...287..163G

We have searched for the 3.1µm absorption feature, a well-known characteristic of optical carbon stars, in a sample of sixteen candidate carbon stars, most of which have very red colors and some of which have no optical counterparts. The sample was selected on the basis of similarity of LRS spectra to those of known carbon stars. We detected the 3.1µm feature in eleven candidates, confirming them as carbon stars. There is a wide range in the strength of the feature. In general, the 3.1µm feature weakens with redder [K-L] color. However, two of the reddest stars (with [K-L]=5) show the strongest features. Models of the spectrum near the 3.1µm feature show that the absence of the 3.1µm feature in stars with [K-L] > 4 is expected, because dust emission fills in the feature, if the temperature of the dust at the inner radius is ∼1500K, equal to the typical condensation temperature of carbon-rich dust. The presence of a strong 3.1µm feature in stars with [K-L]≃5 can be explained if the dust temperature at the inner radius is λ700K. An alternative explanation is that in those stars there may be a circumstellar contribution to the 3.1µm feature.