Volume 552, April 2013
|Number of page(s)||13|
|Section||Galactic structure, stellar clusters and populations|
|Published online||21 March 2013|
Reverse dynamical evolution of η Chamaeleontis
UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de
Grenoble (IPAG) UMR 5274, 38041
2 Astronomy Department, University of California Berkeley, HFA B-20 3411, Berkeley CA 94720-3411, USA
3 School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, Canberra ACT 2600, Australia
Received: 9 March 2012
Accepted: 24 January 2013
Context. 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 η Chamaeleontis presents an apparent lack of low mass objects (m < 0.1 M⊙). Another unusual feature of this cluster is the absence of wide binaries with a separation >50 AU.
Aims. We aim to test whether dynamical evolution alone can reproduce the peculiar properties of the association under the assumption of a universal IMF.
Methods. 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 (number of systems, typical radius of the density distribution and virial ratio) 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 M⊙).
Results. The initial configurations cover a wide range of initial density, from 102 to 108 stars/pc3, 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.
Conclusions. Pure dynamical evolution alone cannot explain the observed properties of η Chamaeleontis 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.
Key words: binaries: general / stars: luminosity function, mass function / stars: kinematics and dynamics / methods: numerical / open clusters and associations: individual:ηChamaeleontis
© ESO, 2013
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