The Structure and Properties of an Intermediate‐Velocity Cloud in the Direction of the Globular Cluster M13

Abstract

A study of an intermediate-velocity cloud (IVC; VLSR ≈- 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 ≈2'.8 × 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 ≈ 30 km s–1) of Hi column density 7 × 1018 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 ≈ 32 km s–1) has an Hi column density of ≈ 5 × 1019 cm–2, while the narrow component (FWHM ≈ 8 km s–1) has a column density of ≈ 6 × 1018 cm–2 as seen in the Synthesis Telescope observations. The cloudlets have an angular size ≈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 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 ≈ 4 × 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.