9 Summary and concluding remarks

We present a large sample of 63 ISO/SWS spectra of carbon-rich evolved objects that exhibit a "30'' $\mu$m feature. The feature is detected along the complete evolutionary track from low mass loss C-stars and extreme C-stars via post AGB objects to PNe.

We present a simple approach to determine the continuum. We study the profile of the "30'' $\mu$m feature after continuum subtraction. We find large systematic variations in the appearance of the "30'' $\mu$m feature. We firmly identify the carrier of the feature with MgS in a CDE shape distribution. The profile of the observed feature is consistent with MgS provided that the temperature of the MgS can be different from the bulk of the dust. This approach allows us to model, in a unified way, the profiles of the "30'' $\mu$m feature in a wide range of objects even when the feature can appear extremely different.

We find an additional component at 26 $\mu$m in $\sim$25 sources. We argue that this component is due to differences in the distribution over shapes of the MgS grains; specifically, it requires a distribution more weighted to spherical MgS grains. We find no clear correlations of this excess with other properties of the sources. The self-absorbed "30'' $\mu$m features of RAFGL 618 and RAFGL 2688 reflect the high optical depth in the "30'' $\mu$m feature in these sources.

We find that the typical profile of the "30'' $\mu$m feature in the PNe is narrower than predicted by our model. We consider several possible explanations. We find that flattened MgS grains provide a better spectral match to the "30'' $\mu$m feature in the PNe. However it is presently unclear why in the PNe environment the "30'' $\mu$m feature is dominated by flattened grains.

We find that the temperature of the MgS grains is different from the bulk of the dust. Therefore, they cannot be in thermal contact with the other dust species but must exist as separate grains. In the C-stars MgS is colder than the other dust while in the post-AGB objects MgS is warmer. Likely, this is because the MgS is efficiently heated by mid-IR radiation which is less important in the C-stars.

The behaviour of the temperature of the MgS for the hottest C-stars is enigmatic. The hottest sources have very cold MgS grains. We propose two explanations for this phenomenon. First, the "30'' $\mu$m feature in these sources may be due to a previous mass-loss phase and the MgS is thus located far from the source and cold. Second, the MgS grains absorb very inefficiently in the optical and near-IR and therefore the hottest C-stars do not heat the MgS grains well.

We examine the feature over continuum ratio to study the relative proportion of MgS to the other dust components. We find no evidence for rapid destruction of MgS grains during the PNe phase and possibly MgS grains may survive to be incorporated into the ISM.

We would like to emphasise the need for further measurements of the optical properties of astrophysically relevant materials at all relevant wavelengths. This study has put forwards some very interesting questions in relation to the absorption properties of MgS in the optical and near-IR and also concerning low temperature effects. Unfortunately, no published laboratory measurements are available to test some of our scenarios and conclusions.

Acknowledgements

SH and LBFMW acknowledge financial support from NWO Pionier grant 616-78-333. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of NASA's Astrophysics Data System Bibliographic Services. IA³ is a joint development of the SWS consortium. Contributing institutes are SRON, MPE, KUL and the ESA Astrophysics Division.