Optical and H I observations of the low-velocity and intermediate-velocity gas toward the globular cluster M13

Abstract

High-resolution spectra of interstellar lines toward 11 stars in the globular cluster M13 and toward three foreground stars which lie within 3 deg from the cluster are reported. The optical spectra are compared with an H I profile of the gas recorded in the direction of the cluster. The principal interstellar components have LSR radial velocities centered near +10 km/s and -4km/s, but there is a considerable variation in the velocities and the column densities of both components across the face of the cluster. The positive velocity gas has a high Na I/H I column density ratio, and it lies beyond the foreground stars at a distance approximately greater than 200 pc; this gas may be associated with the receding part of the Hercules shell discussed in detail by Lilienthal et al. The negative velocity gas is detected toward the foreground stars, and the observed Na I/Ca II ratios suggest an association with gas at the approaching side of the Hercules shell. Gas at a velocity approximately equal -80 km/s has been previously reported in H I emission in the M13 direction and in UV spectra of the post-AGB cluster star, Barnard 29. This intermediate-velocity clouds in the lower halo. We discuss also evidence for the detection of this gas in Na I and show that it is most likely located at a distance.