η Chamaeleontis: abnormal initial mass function or dynamical evolution?

Free access article

Issue		A&A Volume 473, Number 1, October I 2007


Page(s)		163 - 170
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20066847

A&A 473, 163-170 (2007)
DOI: 10.1051/0004-6361:20066847

$\eta$ Chamaeleontis: abnormal initial mass function or dynamical evolution?

E. Moraux¹, W. A. Lawson², and C. Clarke³

¹ Laboratoire d'Astrophysique, Observatoire de Grenoble (LAOG), BP 53, 38041 Grenoble Cedex 9, France
e-mail: Estelle.Moraux@obs.ujf-grenoble.fr
² School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, Canberra ACT 2600, Australia
³ Institute of Astronomy, University of Cambridge Madingley Road, Cambridge, CB3 0HA, UK

(Received 30 November 2006 / Accepted 13 July 2007 )

Abstract
Context. $\eta$ Chamaeleontis is a unique young (~9 Myr) association with 18 systems concentrated in a radius of $\approx$ 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 $M_{\odot}$ .
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 $\eta$ Cha when starting with a very compact configuration (with $N_{\rm init}=40$ 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.

Key words: stars: low-mass, brown dwarfs -- stars: luminosity function, mass function -- methods: N-body simulations -- open clusters and associations: individual: $\eta$ Chamaeleontis

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