Reverse dynamical evolution of Eta Chamaeleontis

Abstract: In the scope of the star formation process, it is unclear how the environment shapes the initial mass function (IMF). While observations of open clusters propose a universal picture for the IMF from the substellar domain up to a few solar masses, the young association eta Chamaeleontis presents an apparent lack of low mass objects (m<0.1 Msun). Another unusual feature of this cluster is the absence of wide binaries with a separation > 50 AU. We aim to test whether dynamical evolution alone can reproduce the peculiar properties of the association assuming a universal IMF. We use a pure N-body code to simulate the dynamical evolution of the cluster for 10 Myr, and compare the results with observations. A wide range of values for the initial parameters are tested in order to identify the initial state that would most likely lead to observations. In this context we also investigate the influence of the initial binary population on the dynamics and the possibility of having a discontinuous single IMF near the transition to the brown dwarf regime. We consider as an extreme case an IMF with no low mass systems (m<0.1 Msun). The initial configurations cover a wide range of initial density, from 10^2 to 10^8 stars/pc^3, in virialized, hot and cold dynamical state. We do not find any initial state that would evolve from a universal single IMF to fit the observations. Only when starting with a truncated IMF without any very low mass systems and no wide binaries, can we reproduce the cluster core properties with a success rate of 10% at best. Pure dynamical evolution alone cannot explain the observed properties of eta Cha from universal initial conditions. The lack of brown dwarfs and very low mass stars, and the peculiar binary properties (low binary fraction and lack of wide binaries), are probably the result of the star formation process in this association. (abridged)