The structure and properties of an intermediate-velocity cloud in the direction of the globular cluster M13

A study of an intermediate-velocity cloud (IVC; V-LSR approximate to -70 km s(-1)) detected in the foreground of the globular cluster M13 has been made using H I spectra obtained with the Lovell Telescope, Jodrell Bank (FWHP beamwidth 12') and H I spectral maps derived from Dominion Radio Astrophysical Observatory Synthesis Telescope observations (FWHP synthesised beamwidth approximate to 2.8' x 2.0'). The radio data complement the optical absorption line spectroscopy of M13 cluster stars reported by Bates et al. The H I data obtained with the Lovell Telescope show extended emission from a broad-velocity component (FWHM approximate to 30 km s(-1)) of H I column density greater than or similar to 7 x 10(18) cm(-2). Two cloudlets are revealed in the H I observations lying on either side of M13 and having a two-component structure; the broad component (FWHM approximate to 32 km s(-1)) has an H I column density of approximate to 5 x 10(19) cm(-2), while the narrow component (FWHM approximate to 8 km s(-1)) has a column density of approximate to 6 x 10(18) cm(-2) as seen in the Synthesis Telescope observations. The cloudlets have an angular size approximate to 2' and are only partially resolved in the synthesis telescope data. Fine-scale gas structure is also evident from the cluster star spectroscopy with variations in Na I column density of a factor of greater than or similar to 10 observed across the several arcminutes of the cluster face; there is also evidence for structure on an angular scale of several arcseconds. A column density ratio Na I/H I approximate to 4 x 10(-8) is measured for a cloudlet in the foreground of the cluster near the strongest Na I detection. Estimates are made of cloudlet gas density that are comparable with those reported for high-velocity cloud (HVC) concentrations. However, the present observations emphasize further the need for higher spatial resolution H I studies to provide firmer estimates of cloud properties.