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

4. A possible detection of FeSi

4.1. Extinction properties of FeSi

Above K the extinction by FeSi is rather smooth with wavelength (cf. Fig. 3) which makes it unlikely to detect FeSi in warm circumstellar dust shells, especially if at the same time there is emission from silicate dust from preceding mass-loss phases with normal element composition. For K some sharp phonon modes become visible. Especially two modes at and m become very strong. These two features may be identified in emission spectra if most of the dust is rather cold, i.e., for objects where most of the emission in the far IR comes from a detached shell.

In order to search for possible emission features from FeSi in IR spectra of evolved stars we have fitted the optical reflectivity data of FeSi measured at K from Degiorgi et al. (1994) with a Drude-Lorentz model

(Bohren & Huffman 1983) and determined optical constants for FeSi in the far IR. From this we calculated the extinction efficiency for 0.1 µm particles with K in the small particle limit (see Fig. 4, bottom).

4.2. Comparison with AFGL 4106

The high abundances of Fe and Si suggests that FeSi, if it exists, should be identifiable by its two strong spectral features (see Fig. 3) at m, which are accessible to the ISO satellite. The shorter wavelength band unfortunately is blended by a strong silicate feature, but the second one is located in a wavelength region free of strong silicate features and may be detectable.

We have searched for published ISO spectra of highly evolved objects since these seem to be the best candidates for looking for the spectral features of FeSi. In the far-IR spectrum of AFGL 4106 published by Molster et al. (1999) there is a strong band at m which is possibly also seen in some other highly evolved objects (Barlow 1998, Sylvester et al. 1999). This band is clearly seen in Fig. 4 which shows a sector from the spectrum of AFGL 4106 where the two strongest bands of FeSi should be visible. The band cannot be attributed to any other known dust species so far known to exist in circumstellar dust shells. A weak feature at m may also be due to FeSi.

AFGL 4106 is a double star with a late-A or early F-type member and an early M-type member. Both are massive stars (van Loon et al. 1999) and the earlier star is likely to be in the post red supergiant stage of evolution towards a WR star. The enhanced N abundance found in the spectrum (van Loon et al. 1999) indicates that CNO processed material is exposed by mass-loss. Temperatures of dust grains in the detached shell are estimated by Molster et al. (1999) from a radiative transfer model to be between 120 and 160 K, depending on the grain material. This makes AFGL 4106 a possible candidate (i) for the chemical peculiarity required for FeSi formation and (ii) for the low dust temperature required for visibility of the FeSi bands. We propose that the carrier of the 47.5 µm emission band in AFGL 4106 might be grains of FeSi.

The simultaneous presence of ice bands around 60 µm and of FeSi in AFGL 4106 shows that the detached shell also contains material from the preceding oxygen rich evolutionary phase. This requires a change of the stellar surface abundances during shell ejection. Whether this is likely to happen depends on the details of the mass-loss process of red supergiants which are only badly known.

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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