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Astron. Astrophys. 337, L35-L38 (1998)


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Letter to the Editor

The strange mid-infrared spectrum of M 31: ISOCAM observations *

D. Cesarsky 1, J. Lequeux 2, L. Pagani 2, C. Ryter 3, L. Loinard 4 and M. Sauvage 3

1 Institut d'Astrophysique Spatiale, Bat. 121, Université Paris XI, F-91450 Orsay Cedex, France
2 DEMIRM, Observatoire de Paris, 61 Avenue de l'Observatoire, F-75014 Paris, France
3 SAp/DAPNIA/DSM, CEA-Saclay, F-91191 Gif sur Yvette Cedex, France
4 IRAM, 300 Rue de la Piscine, Domaine Universitaire, F-38406 St Martin d'Hères Cedex, France

Received 10 June 1998 / Accepted 3 August 1998

Abstract

We present observations of the mid-IR spectrum of 4 small regions of M 31. The spectrum of the central [FORMULA] is characterized by a strong, broad unidentified emission band (UIB) at 11.3 µm while the other usual UIBs at 6.2, 7.7 and 8.6 µm are absent or very faint. We present a map of this area in the 11.3 µm 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 star-forming ring at 10 kpc radius. The spectrum of a more active region of this ring shows relatively stronger 6.2, 7.7 and 8.6 µm bands. The profile of the 11.3 µm band varies from field to field. We suggest that we see in M 31 the emission of hydrogenated amorphous carbon particles as synthetized in the atmospheres of carbon stars. These particles have mid-infrared emission or absorption bands at 3.3-3.4 µm and 11.3 µm only. They have to be graphitized by UV radiation to be able to emit the other UIBs. This can take place in planetary nebulae, but most particles escape this processing, and are only affected by the interstellar UV field. The interstellar radiation field in M 31 is exceptionally poor in UV, e.g. compared to that in the solar neighbourhood. It is insufficient to graphitize the particles, hence the faintness of the 6.2, 7.7 and 8.6 µm UIBs.

Key words: galaxies: M 31 – galaxies: ISM – dust, extinction – infrared: ISM: lines and bands – infrared: stars – stars: carbon

* Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Send offprint requests to: james.lequeux@obspm.fr

© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998

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