The Colour-Magnitude Diagram of the Globular Cluster NGC 6981
Open Access
- 1 May 1972
- journal article
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 157 (3) , 281-297
- https://doi.org/10.1093/mnras/157.3.281
Abstract
UBV photometry for stars in the globular cluster NGC 6981 = M72 is presented. Another paper describes results of BV photometry for 21 RR Lyrae variables in the cluster. The colour–magnitude diagram shows a horizontal branch populated on both sides of the variable star region. The two-colour (U–B/B–V) diagram for field stars is used to derive a reddening of E(B–V) = 0·07 ± 0·01 mag, and the two-colour diagram for cluster giants used to derive an ultra-violet excess of δ(U–B) = 0·15 ± 0·05 mag at (B–V)0 = 1·0. The latter value shows that the cluster is relatively metal rich, in agreement with the integrated spectral type of G3 but contrary to the Morgan II classification. A discussion of some problems associated with the interpretation of HR diagrams and other parameters of globular clusters in terms of current evolutionary theory is given.Keywords
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