Infrared space observatory spectroscopy of extreme carbon stars

Abstract

We present Infrared Space Observatory (ISO) SWS01 (2-43 μm) and LWS01 (43-195 μm) observations of five highly evolved carbon stars with extremely thick circumstellar envelopes. The unidentified 30 μm emission feature is detected in the SWS01 spectra of all five sources. This feature seems to be resolved into two subfeatures at 25.5 and 28.3 μm in two sources. These same two sources show a weak emission feature near 20 μm which may be the same as the 21 μm feature seen in carbon-rich proto-planetary nebulae. Absorption features at 13.7 μm attributed to C 2H 2 are also detected in four sources. No dust feature is detected in the LWS spectra. The only line present is the 157.74 μm line of C +, which could be interstellar in origin. We present radiative transfer models for the spectra of these five stars assuming that the dust is amorphous carbon. The derived optical depths at 11.2 μm range from 1.4 to 4.5, corresponding to optical depths at V = 190-600, 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 μm features to be optically thick with 5%-8% of the total stellar luminosities being emitted in the feature. For four of the five stars, the models 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 × 10 -4 (D/kpc) M ⊙ yr -1. Such high mass-loss rates imply that these stars are in the final stages of AGB evolution. The observations of these extreme carbon stars therefore represent the best cases to study the transition from AGB to proto-planetary nebulae. infrared: stars - radiative transfer - stars: carbon - stars: evolution -stars: late-type - stars: mass loss 1 Based on observations made with ISO, an ESA project with instruments funded by the ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) with the participation of ISAS and NASA.