Based on observations collected at the European Southern Observatory (ESO) using the Swedish-ESO Submillimetre Telescope (SEST), La Silla, Chile.

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Figure 2 and Tables 1, and 2 are only available in electronic form at http://www.edpsciences.org

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Present address: Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028, Barcelona, Catalunya, Spain.

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MSX images have been taken from the on-line MSX database http://www.ipac.caltech.edu/ipac/msx/msx.html

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