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

Abstract

Intermediate-resolution (60000<=R<=120000) observations of interstellar Nai 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 Nai lines appear in the spectra, I obtain a distance of ~150+/-10pc to the Lupus molecular complex. While in agreement with a number of other independent estimates, this result is at odds with the value of 100pc 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+/-10pc 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.