A study of interstellar Na Ι D absorption lines towards the Lupus molecular clouds

Intermediate-resolution (60 00 less than or equal to R less than or equal to 120 000) observations of interstellar Na I lines towards 29 stars in the general direction of the Lupus molecular clouds (330 degrees less than or similar to l less than or similar to 350 degrees; 0 degrees less than or similar to b less than or similar to 25 degrees) are presented. Previously published spectra towards an additional seven stars are also included. Based on the Hipparcos distances to these stars, and the minimum distance at which strong interstellar Na I lines appear in the spectra, I obtain a distance of similar to 150 +/- 10 pc to the Lupus molecular complex. While in agreement with a number of other independent estimates, this result is at odds with the value of 100 pc recently obtained by Knude & Hog from a Hipparcos-based study of interstellar extinction. A possible explanation for this discrepancy is discussed, and it is concluded that the value of 150 +/- 10 pc obtained here is to be preferred. In addition, these observations have some other implications for the structure of the interstellar medium in this direction, and these are briefly considered.