Astron. Astrophys. 357, L13-L16 (2000)

5. Concluding remarks

From thermodynamic equilibrium calculations of condensation sequences we have found that FeSi is the first condensate of the refractory abundant elements in element mixtures which can be characterised as being oxygen and carbon poor. Such environments occur during evolution of massive stars and at the transition to the PN-stage for medium-mass stars.

FeSi has two strong characteristic spectral features at and m which due to the temperature dependence of the optical conductivity of FeSi are visible only if the dust grains are cooler than K. The m feature is strongly blended by an enstatite feature and cannot be detected if silicate dust from earlier mass loss phases contributes significantly to the far IR emission. The m feature appears in a window free from strong features of other possible dust materials and should easely be detectable, if present. A resonance which occurs in small particles at at m falls on the shoulder of a strong enstatite band, but may also be detectable.

An inspection of a few recently published spectra of evolved stars taken with the LWS aboard the ISO satellite showed in one case (AFGL 4106) a rather strong feature at m and a second weaker feature at m on the shoulder of the strong m enstatite band. We propose that these two features in the spectrum of AFGL 4106 are due to FeSi grains. In some other cases there are also indications for the presence of the m feature of FeSi.

If this identification is true, it is for the first time that a material is found in a circumstellar dust shell with strongly temperature dependent spectral features, which can serve to some extent as thermometers for their environment.

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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