Silicate and hydrocarbon emission from Galactic M supergiants

Following our discovery of unidentified infrared (UIR) band emission in a number of M supergiants in h and chi Per, we have obtained 10-mu m spectra of a sample of 60 galactic M supergiants. Only three new sources, V1749 Cyg, UWAq1 and IRC+40427, appear to show the UIR bands; the others show the expected silicate emission or a featureless continuum. The occurrence of UIR-band emission in M supergiants is therefore much higher in the h and chi Per cluster than in the Galaxy as a whole. Possible explanations for the origin and distribution of UIR bands in oxygen-rich supergiants are discussed. We use our spectra to derive mass-loss rates ranging from 10(-8) to 10(-4) M. yr(-1) for the new sample, based on the power emitted in the silicate feature. The relationship between mass-loss rate and luminosity for M supergiants is discussed, and correlations are explored between their mid-infrared emission properties.