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1 Introduction

This article is a continuation of studies using radio Hi, optical and ultraviolet observations (Little et al. 1994; Sembach 1995; Ryans et al. 1996; Kennedy et al. 1998) of a high-velocity cloud (HVC) observed towards both the M 15 globular cluster (Galactic co-ordinates $l,\,b ~ \sim ~65.0\hbox{$^\circ$},\, \\  -27.3\hbox{$^\circ$}$, $d \sim 10.4$ kpc; Durrel & Harris 1993), and the nearby ($2.5\hbox{$^\circ$}$ away) blue halo star HD 203664 ($ d \sim 3.2$ kpc; Little et al. 1994).

Little et al. (1994) detected the HVC at a Local Standard of Rest (LSR) velocity $v_{\rm LSR} \simeq 80 \,{\rm km\,s^{-1}}$, using both Hi and optical absorption line observations. Further high-resolution optical spectroscopy of this sightline revealed the presence of multiple cloud components at $v_{\rm LSR} \simeq 70,\,75,\,80 \,{\rm km\,s^{-1}}$ (Ryans et al. 1996). The HVC emission is weak (brightness temperature $T_{\rm b} \sim 0.07$ K), which makes detailed comparisons between the Hi emission and optical absorption line structure of the HVC difficult. However, much stronger HVC emission at similar velocities ($T_{\rm b} \sim 1.3$ K) is present across the M 15 globular cluster, and initial inspection of Nai absorption profiles towards stars in this direction show that the HVC gas lies in front of the cluster (Kennedy et al. 1998). Therefore, cluster members of M 15 provide an unprecedented opportunity to probe the properties within the HVC over small spatial scales.

Hot OB-type stars provide the best background objects to study interstellar features, since they possess clean spectra with few stellar lines, and display well defined continua. In this paper we report a survey to identify hot stars in M 15 that show strong HVC absorption in the Nai D1 & D2 and Caii K features. These stars are therefore suitable for further high resolution studies of this cloud.



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