Globular Clusters, II. The Spectral Types of Individual Stars and of the Integrated Light

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Abstract
The problem of classifying the spectra of individual stars and spectra taken in integrated cluster light is discussed. Types are given to the spectra of both kinds previously discussed in Paper I and a comparison is made with Mayall's and also Morgan's classification for spectra of lower dispersion. Mayall's types show small systematic differences from both the Radcliffe and Morgan types: a correction is therefore applied to Mayall's types to give integrated types for 63 clusters on a uniform system. The effect of the metal/hydrogen ratio on the metal line strengths in the spectra of the cluster giants is discussed and an attempt is made to calculate this ratio for different clusters from the present spectra. It is concluded that the metal/hydrogen ratio ranges from greater than 1/10 to less than 1/100 the solar value for different clusters. For a cluster such as NGC 5272, it is shown that if this ratio has ∼ 1/10 solar value then the cluster giants should have nearly normal spectra for their colour and luminosity as is observed but that the metal lines in the spectra of the subgiants should be weakened. An attempt is made to predict integrated spectral types for NGC 5272 and (with less accuracy) for five other clusters. Differences in the integrated spectra are largely due to the differences in the metal/hydrogen ratio and to the different relative populations of the horizontal branch. The predicted types agree with the observed types within the expected accuracy in support of the present analysis. A correlation is found between the integrated spectral type of a cluster and its concentration to the galactic plane. Clusters of the earliest spectral type whose stars have a very low metal abundance form an extended spherical distribution with little concentration to the plane, while those of latest type whose stars have the highest metal abundance show a strong concentration to the plane: clusters of intermediate type, which are the most numerous, show an intermediate distribution. The less definite correlation between spectral type and the properties of the RR Lyrae stars is also discussed.

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