The region of Collinder 121

Abstract

The distribution of bright B-type stars in a field with a radius of 5° centred at the Galactic open cluster Cr 121 is studied utilizing Strömgren and Hβ photometry. All PPM stars earlier than spectral type A0 are used, revealing a loose nearby structure at a distance of 660-730pc, and a compact more distant group, which appears to be a genuine cluster: Cr 121. Based on similar coordinates, distances and positions on the colour-magnitude (CM) and Hertzsprung-Russell (HR) diagrams, 11 photometric cluster members are selected at a mean distance of 1085(+/-41 standard error) pc. The results are discussed in the light of both classical and Hipparcos points of view.