1999A&A...345L..39S

We report the detection of the 30µm emission feature from the C-rich post-Asymptotic Giant Branch (post-AGB) star IRAS 04296+3429 based on ISO SWS observations. ISO data show that there is a clear substructure in the 30µm feature in the form of a plateau which extends from 26 to 27µm. This is confirmed by the ISO SWS data for another post-AGB object IRAS 22272+5435, which has the strongest known 30µm band accounting for about 24% of its total infrared (IR) emission or 12% of the bolometric luminosity. In the case of IRAS 04296+3429 we estimate that about 85% of the total energy is emitted in the infrared of which about 15-22% (depending on the estimated continuum level) is contributed by the 30µm band. The total energy emitted in the band is at least comparable for both sources, while the energy emitted in the 21µm band seems to be about 3 times larger in the case of IRAS 04296+3429, if they have the same bolometric luminosity. Grains of pure MgS could be responsible for the observed 30µm feature provided they have a broad distribution of shapes, but they cannot account for the observed plateau emission. Small graphite grains with relatively thick MgS coating could be responsible for the observed substructure, but we were not able to definitively solve the question of their contribution to the 30µm band.