High-velocity interstellar clouds towards the M 15 globular cluster - I. Low resolution optical data

We present low resolution interstellar spectra of the Ca II K and Na I D li nes towards 12 stars in the M 15 globular cluster. These sightlines are use d to study the small scale structure of the interstellar medium, over scale s of a few arcseconds to a few arcminutes. A high-velocity cloud is detecte d in the Ca II and Na I lines at a LSR velocity of approximately similar to 70 km s(-1). Tentative velocity and equivalent width variations are observ ed on scales of a few arcseconds; however, higher resolution data are neces sary to quantify the magnitude of these variations. We discuss these observ ations and their importance for future studies of this high velocity gas at high spectral resolution.