Little et al. (1994) detected the HVC at a Local Standard of Rest
(LSR) velocity , using both Hi and optical absorption line observations. Further
high-resolution
optical spectroscopy of this sightline revealed the presence of multiple cloud
components at
(Ryans et al. 1996).
The HVC emission is weak (brightness temperature
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 (
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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