The carrier of the ``30'' mu m emission feature in evolved stars. A simple model using magnesium sulfide

Abstract

We present 2-45 mu m spectra of a large sample of carbon-rich evolved stars in order to study the ``30'' mu m feature. We find the ``30'' mu m feature in a wide range of sources: low mass loss carbon stars, extreme carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles from the sources by using a simple systematic approach to model the continuum. We find large variations in the wavelength and width of the extracted profiles of the ``30'' mu m feature. We modelled the whole range of profiles in a simple way by using magnesium sulfide (MgS) dust grains with a MgS grain temperature different from the continuum temperature. The systematic change in peak positions can be explained by cooling of MgS grains as the star evolves off the AGB. In several sources we find that a residual emission excess at ~ 26 mu m can also be fitted using MgS grains but with a different grains shape distribution. The profiles of the ``30'' mu m feature in planetary nebulae are narrower than our simple MgS model predicts. We discuss the possible reasons for this difference. We find a sample of warm carbon-stars with very cold MgS grains. We discuss possible causes for this phenomenon. We find no evidence for rapid destruction of MgS during the planetary nebula phase and conclude that the MgS may survive to be incorporated in the ISM. Based on observations obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) with the participation of ISAS and NASA. Appendix A (Figs. A.1 and A.2) is only available in electronic form at http://www.edpsciences.org