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

Intermediate-resolution (60 000 ≤ R ≤ 120 000) observations of interstellar Na i lines towards 29 stars in the general direction of the Lupus molecular clouds (330° ≲ l ≲ 350°; 0° ≲ b ≲ 25°) 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 ∼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 & Høg 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.