This work has been partially supported by the Spanish CICYT under grant number PB96-0104 and CICYT-PNIE under grant number 1FD1997-1442. We thank the IRAM staff from the Plateau de Bure and from Grenoble for carrying the observations and help provided during the data reduction. We also thank our referee, Christian Henkel, for his careful reading and comments on the manuscript.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000