Electronographic photometry of star clusters in the Magellanic Clouds - III. The colour-magnitude diagram of NGC 2257.

Electronographic magnitudes and colours of stars in the cluster NGC 2257 in the Large Magellanic Cloud have been measured on electrographs taken with a Spectracon image-converter attached to the f/7.5 focus of the 6o-inch reflector at Cerro Tololo. The colour-magnitude diagram has been derived to V = and is basically similar to those of the classical `Population II' globular clusters in our Galaxy, although certain differences exist. The most striking of these is a gap in the giant branch at the magnitude level of the horizontal branch; the star density appears to be zero within this gap. The colour distribution of the horizontal branch stars and the colour and shape of the sub-giant branch are similar to those of the extremely metal-poor cluster M 92. However, the height of the red-giant branch at (B-V)0 = + I 4 is only A V = , as in the intermediate metal content globulars M 3 and M 13, although this value is somewhat uncertain owing to the very small number of red giants in the cluster. Thus, NGC 2257 may be interpreted either as: (I) a cluster with extremely low metal content like M 92 but with an abnormally low red-giant branch, or (2) a cluster of intermediate metal deficiency like M 3 or M 13 which shows the' M 13 anomaly ` in the distribution of its horizontal branch stars and in which the giants above the horizontal branch lie mostly on the asymptotic-giant branch. If the absolute visual magnitude of the horizontal branch at the blue edge of the variable star gap is Mv(BG) = + , then the apparent visual distance modulus of NGC 2257 is (rn - M)Av = I , in agreement with the distance of the main body of the Large Magellanic Cloud derived from the Cepheid variables. However, since it is possible that no unique value of the brightness of the horizontal branch exists, a definite determination of the distance modulus is not possible.