Neutral Hydrogen in Galactic Star Clusters: I. A Search for Neutral and Ionized Hydrogen in Five Galactic Clusters

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Abstract
The 21 cm line emission from neutral hydrogen was investigated at the position of the following galactic clusters: the Pleiades, NGC 1502, the Trapezium–Orion cluster, NGC 2244 and NGC 6910. The measurements were made using a six-channel receiver with a parametric amplifier first stage in conjunction with the 250 ft radio telescope. Neutral hydrogen was found at the position of all the clusters except NGC 1502. Arguments are presented which indicate that it is doubtful whether the neutral hydrogen now at the position of the older Pleiades cluster is the remainder of the same mass of gas from which the stars were formed. A variation of 50: 1 was found in the ratio of gaseous to stellar mass in the four O-type clusters; the results indicate that the more massive clusters retain a greater proportion of their gas. The neutral hydrogen in the Trapezium–Orion cluster, NGC 2244 and NGC 6910 was found in expanding shells surrounding the ionized hydrogen regions associated with the cluster stars. The recession velocities were similar to those of the clusters and the expansion velocities of the first two are similar to those found optically in the ionized hydrogen regions.

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