Colour Magnitude Arrays for Two Associations in the Large Magellanic Cloud

Abstract

The paper is concerned with colour-magnitude studies of stars in the range 11 < V < 15 (−8 < M υ < −4) in two adjacent star concentrations in Constellation III of the Large Magellanic Cloud, NGC 1955 and NGC 1968–1974 and their immediate surroundings. UBV colours of selected OB stars ( M υ = −6.5) show; (1) that interstellar absorption is either very small or absent; (2) that the ( B–V ) 0 colours derived from the observed values of ( B–V ) and ( U–B ) are in excellent agreement with those derived from the spectral data; (3) that the intrinsic colours of the OB supergiants of luminosity class Ia are comparable to those of the Class Ia supergiants in our galaxy. The stars in the two colour-magnitude arrays are either in the colour interval −0.4 < B–V < −0.1 or in the interval +0.4 < B–V < +0.8, with the very blue stars outnumbering those with positive colour indices by a factor of 3 or 4 to 1. The stars with positive colour indices are probably mainly foreground stars or Magellanic Cloud field stars. There are very few stars with colour indices between −0.1 and +0.4, or greater than +0.8.