Fundamental parameters of the highly reddened young open clusters Westerlund 1 and 2

Abstract

We study the compact open clusters Westerlund 1 (BH 197) and Westerlund 2. We present CCD integrated spectroscopy for both clusters, and CCD imaging in the V and I bands for the former one. So far, Westerlund 1 is possibly the most reddened open cluster studied in detail (Aυ ≈ 13.0). It has an age of 8 ± 3 Myr and a distance from the Sun of d⊙ ≈ 1.0 ± 0.4 kpc. For Westerlund 2 we derive a visual absorption AV ≈ 5.0 mag, an age of 2 - 3 Myr, and d⊙ = 5.7 ± 0.3 kpc. From luminosity and structural arguments we conclude that Westerlund 1, although young and compact, it is a massive cluster, in contrast to Westerlund 2.