Abstract
We have obtained 10-mu m spectra of 16 M supergiants, 15 of them in the h and chi Per association. Ah 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 mu m, 8.65 mu 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 held could also provide the photons needed to excite the observed UIR-band emission.
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