THE STRANGE MIDINFRARED SPECTRUM OF M31 - ISOCAM OBSERVATIONS

We present observations of the mid-IR spectrum of 4 small regions of M 31. The spectrum of the central 3' is characterized by a strong, broa d unidentified emission band (UIB) at 11.3 mu m while the other usual UIBs at 6.2, 7.7 and 8.6 mu m are absent or very faint. We present a m ap of this area in the 11.3 mu m band and compare it with optical and H I images. The spectra of two other regions are similar; one of these regions is in the bulge, and the other one in a quiet region of the s tar-forming ring at 10 kpc radius. The spectrum of a more active regio n of this ring shows relatively stronger 6.2, 7.7 and 8.6 mu m bands. The profile of the 11.3 mu m band varies from field to field. We sugge st that we see in M31 the emission of hydrogenated amorphous carbon pa rticles as synthetized in the atmospheres of carbon stars. These parti cles have mid-infrared emission or absorption bands at 3.3-3.4 mu m an d 11.3 mu m only. They have to be graphitized by UV radiation to be ab le to emit the other UIBs. This can take place in planetary nebulae, b ut most particles escape this processing, and are only affected by the interstellar UV field. The interstellar radiation field in M 31 is ex ceptionally poor in UV, e.g, compared to that in the solar neighbourho od. It is insufficient to graphitize the particles, hence the faintnes s of the 6.2, 7.7 and 8.6 mu m UIBs.