Infrared Space Observatory spectroscopy of extreme carbon stars

We present Infrared Space Observatory (ISO) SWS01 (2-43 mu m) and LWS01 (43 -195 mu m) observations of five highly evolved carbon stars with extremely thick circumstellar envelopes. The unidentified 30 mu m emission feature is detected in the SWS01 spectra of all five sources. This feature seems to b e resolved into two subfeatures at 25.5 and 28.3 mu m in two sources. These same two sources show a weak emission feature near 20 mu m which may be th e same as the 21 mu m feature seen in carbon-rich proto-planetary nebulae. Absorption features at 13.7 mu m attributed to C2H2 are also detected in fo ur sources. No dust feature is detected in the LWS spectra. The only line p resent is the 157.74 mu m line of C+, which could be interstellar in origin . We present radiative transfer models for the spectra of these five stars as suming that the dust is amorphous carbon. The derived optical depths at 11. 2 mu m range from 1.4 to 4.5, corresponding to optical depths at V = 190-60 0 making these stars among the asymptotic giant branch (AGB) stars with the most optically thick dust envelopes. From the model fittings, we found the 30 pm features to be optically thick with 5%-8% of the total stellar lumin osities being emitted in the feature. For four of the five stars, the model s also suggest that their mass-loss rates have been increasing rapidly over the last few 10(4) yr periods, with current mass-loss rates as high as 4 x 10(-4) (D/kpc) M.0 yr(-1). Such high mass-loss rates imply that these star s are in the final stages of AGE evolution. The observations of these extre me carbon stars therefore represent the best cases to study the transition from AGE to protoplanetary nebulae.