Based on observations collected at the European Southern Observatory, La Silla, Chile within program ESO 60.E-0103, 62.L-0128, 64.L-0012 and 66.D-0027. Also based on observations with the IRAM telescope, Granada, Spain under programs 98-97, 141-97 and 010-99

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The complete Fig. 1 is only available in electronic form at http://www.edpsciences.org. The complete Table 3 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/501

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