Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...473..849s&link_type=abstract
Astrophysical Journal v.473, p.849
Astronomy and Astrophysics
Astronomy
20
Galaxy: Globular Clusters: Individual Messier Number: M13, Ism: Abundances, Ism: Structure, Ism: Clouds, Radio Lines: Ism
Scientific paper
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 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 30 km s-1) of H I column density≥ 7 x 1O18. 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 H I column density of 5 x 1O19 cm-2, while the narrow component (FWHM ≍ 8 km s-1) has a column density of ≍6 x 1O18 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.
Bates Brandon
Davies Rhodri D.
Keenan Francis P.
Kemp Simon N.
Roger Robert S.
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