ηChamaeleontis: abnormal initial mass function or dynamical evolution?

Abstract

Context. η Chamaeleontis is a unique young (∼9 Myr) association with 18 systems concentrated in a radius of ≈35 arcmin, i.e. 1 pc at the cluster distance of 97 pc. No other members have been found up to 1.5 degrees from the cluster centre. The cluster mass function is consistent with the IMF of other rich young open clusters in the higher mass range but shows a clear deficit of low mass stars and brown dwarfs with no objects below 0.1 M� . Aims. The aim of this paper is to test whether this peculiar mass function could result from dynamical evolution despite the young age of the cluster. Methods. We performed N-body numerical calculations starting with a log-normal IMF and different initial conditions in terms of number of systems and cluster radius using the code NBODY3. We simulated the cluster dynamical evolution over 10 Myr and compared the results to the observations. Results. We found that it is possible to reproduce η Cha when starting with a very compact configuration (with Ninit = 40 and R0 = 0.005 pc) which suggests that the IMF of the association might not be abnormal. The high initial density might also explain the deficit of wide binaries that is observed in the cluster.