We thank V. Bujarrabal and our referee, K. M. Menten, for their very useful comments concerning this paper. We also wish to thank the IRAM staff at Pico Veleta for their valuable assistance during the task of observations, and J. Keller from S.T./I.C.E. for revising this manuscript. This work has been partially supported by the Spanish DGICYT under grant PB90-408.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998