We have obtained |10-μ m| spectra of 16 M supergiants, 15 of them in the h and |χ| Per association. All of the stars exhibit silicate emission features, but in addition seven of the stars show narrow UIR (unidentified infrared) band emission features, at |11.3\ μ m|, |8.65\ μ m| and other wavelengths, which are normally associated with carbon-rich media. Not only are these the coolest objects to have been found to exhibit UIR-band emission, but the outflows from these classical oxygen-rich stars should form only O-rich particles according to equilibrium condensation theory. We interpret our results in terms of the non-equilibrium chemistry model by Beck et al., whereby chromospheric UV radiation can liberate some atomic carbon via the photodissociation of CO molecules, enabling the formation of carbon-rich species as well as silicates. Such a chromospheric UV radiation field could also provide the photons needed to excite the observed UIR-band emission.
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Sylvester et al. (1994) studied this question.