eta Chameleontis: abnormal initial mass function or dynamical evolution?

TL;DR

This study investigates whether the unusual low-mass star deficit in Eta Chamaeleontis is due to its initial mass function or dynamical evolution, using N-body simulations to explore different initial conditions.

Contribution

The paper demonstrates that the peculiar mass function of Eta Chamaeleontis can be explained by dynamical evolution from a very compact initial state, challenging the idea of an abnormal IMF.

Findings

Reproduced Eta Cha's current mass function with a compact initial configuration.

High initial density explains the low number of wide binaries.

Dynamical evolution can account for the observed low-mass star deficit.

Abstract

Eta Chamaeleontis is a unique young (~9 Myr) association with 18 systems concentrated in a radius of ~35 arcmin, i.e. 1pc at the cluster distance of 97pc. 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 0.1Msun. 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. 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. We found that it is possible to reproduce eta Cha when starting with a very compact configuration (with Ninit=40 and R0=0.005pc) 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.