The carrier of the “30” μm emission feature in evolved stars ^*,**

A simple model using magnesium sulfide

¹ Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
³ SRON Laboratory for Space Research Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Kapteyn Astronomical Institute PO Box 800, 9700 AV Groningen, The Netherlands

Corresponding author: S. Hony, hony@astro.uva.nl

Received: 4 March 2002
Accepted: 16 April 2002

Abstract

We present 2-45 μm spectra of a large sample of carbon-rich evolved stars in order to study the “30” μm feature. We find the “30” μm 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” μm 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 μm can also be fitted using MgS grains but with a different grains shape distribution. The profiles of the “30” μm 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.