A photometric and spectroscopic study of the cluster IC 2944

The concentrated group of 0 stars surrounded by nebulosity and globules known as IC 2944 has been investigated photometrically and spectroscopically. Photoelectric results ( ) are presented for 6i stars down to 15 m. The nebulosity is too bright for photographic photometry of the faintest stars. Radial velocities (24 stars) and spectral classifications are available for the brighter stars. A mean colour excess = + , corresponding to visual absorption of m is found ; no marked variation in absorption across the cluster is found, but 8 stars of 0 type give an appreciably higher colour excess of +040. Among the stars with observed radial velocities, the percentage of binaries appears to be at least 50. There is no variation of radial velocity with either spectral type or magnitude exceeding a few kilometres per second, but a differential relativity shift between the 0 and early B stars of order 3 to 4 km/s is possible. The Cassegrain velocities for the K line differ systematically from the mean stellar velocities by -7 km/s. Coude' spectra of two of the brightest stars show a double K line with separation about I km/s; the shortward component probably arises in an expanding cloud directly associated with the group of 0 stars. Combination of photometry and spectroscopy gives an apparent distance modulus of + , and a true distance of kpc. Discussion of the colour-magnitude array is hampered by uncertainties regarding membership of faint stars, but the group must be dominated by bright stars (0 and B types). A few red stars appear to the right of the Hertzsprung gap ; of these, the most certain member is the brightest (HD 101712) with a peculiar composite spectrum (M2Ib pe + B?) including [Fe IIJ emission.