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Article Contents
1 Introduction
2 Observations
3 Characteristics of the extragalactic sources
4 3.4 ${\mu }$m absorption feature
5 The 3.4 ${\mu }$m hydrocarbons/10 ${\mu }$m silicates ratio
6 Galactic center and extragalactic mid infrared spectra
7 Constraints on the nature of the hydrocarbons
8 Conclusions
Appendix: Integrated absorbances of ketones and alcanes at 10 K

List of tables
List of figures

Copyright ESO 2004
Published by EDP Sciences