The IMF of open star clusters with Tycho – 2

We studied the fields of nine nearby open star clusters based on the Tycho–2 catalogue. We determined membership probabilities for the stars in the cluster fields from the stellar proper motions and used the Tycho–2 photometry to compute the initial mass function (IMF) of the clusters from the main sequence turn-off point down to approx. 1M⊙. We found IMF slopes ranging from Γ = −0.69 down to Γ = −2.27 (when the Salpeter (1955) value would be Γ = −1.35). We also studied the membership of individual stars of special astrophysical interest. In some cases previous results had to be revised. As a by-product, we investigated some general properties of the Tycho–2 catalogue; we confirmed that the Tycho–2 proper motions show only marginal deviations from the Hipparcos catalogue. On the other hand, in some regions the completeness of the catalogue seems to decrease at magnitudes brighter than claimed by Høg et al. (2000).