Electronographic Photometry of Star Clusters in the Magellanic Clouds-IV THE COLOUR-MAGNITUDE DIAGRAM OF NGC 419*

Abstract

Electronographic magnitudes and colours of 146 stars in and near the cluster NGC 419 in the Small Magellanic Cloud have been measured to V = 20·9 on electrographs taken with a Spectracon image-converter attached to the f/7·5 focus of the Cerro Tololo 60-in. reflector. Comparison with the photographic photometry of Arp indicates that at distances of r >110″ from the centre of the cluster, Arp's photometric calibration is quite good over the range |$15\cdot5\,\leq\,B,\,V\,\leq\,20\cdot1$| . However, for r ≤110″ the photographic measures are affected by background light from the faint, unresolved members of the cluster. The upper portion of the colour–magnitude diagram of NGC 419 consists of a red-giant branch which extends from V = 15·9 and B–V = + 2·0 to V = 19·8 and B-V = +o·7 and resembles that in Kron 3. However, fainter than V ≃ 19·8 the diagram is confused by the presence in the field of many early-type main-sequence stars belonging to the main body of the Small Magellanic Cloud. Consequently, it is not clear whether NGC 419 is: (1) a cluster like Kron 3 but in which there is a gap in the giant-branch between about V = 19·8 and V = 20·3, or (2) is a cluster more like NGC 2209 with a red-giant branch, a small Hertzsprung gap, and an evolved mainsequence whose upper end appears at about V = 20·2 and B–V = +0·25. One extremely red star was measured which is probably a carbon star belonging to the general population of the Small Magellanic Cloud.